source: project/release/4/ugarit/trunk/README.txt @ 25570

Last change on this file since 25570 was 25570, checked in by Alaric Snell-Pym, 9 years ago

ugarit: Version 2 of the backend protocol, supporting better reporting back to the user, and administrative interfaces. Backends outfitted with admin interfaces, and a ugarit-archive-admin tool added to drive them.

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1# Introduction
2
3Ugarit is a backup/archival system based around content-addressible storage.
4
5This allows it to upload incremental backups to a remote server or a
6local filesystem such as an NFS share or a removable hard disk, yet
7have the archive instantly able to produce a full snapshot on demand
8rather than needing to download a full snapshot plus all the
9incrementals since. The content-addressible storage technique means
10that the incrementals can be applied to a snapshot on various kinds of
11storage without needing intelligence in the storage itself - so the
12snapshots can live within Amazon S3 or on a removable hard disk.
13
14Also, the same storage can be shared between multiple systems that all
15back up to it - and the incremental upload algorithm will mean that
16any files shared between the servers will only need to be uploaded
17once. If you back up a complete server, than go and back up another
18that is running the same distribution, then all the files in `/bin`
19and so on that are already in the storage will not need to be backed
20up again; the system will automatically spot that they're already
21there, and not upload them again.
22
23## So what's that mean in practice?
24
25You can run Ugarit to back up any number of filesystems to a shared
26archive, and on every backup, Ugarit will only upload files or parts
27of files that aren't already in the archive - be they from the
28previous snapshot, earlier snapshots, snapshot of entirely unrelated
29filesystems, etc. Every time you do a snapshot, Ugarit builds an
30entire complete directory tree of the snapshot in the archive - but
31reusing any parts of files, files, or entire directories that already
32exist anywhere in the archive, and only uploading what doesn't already
33exist.
34
35The support for parts of files means that, in many cases, gigantic
36files like database tables and virtual disks for virtual machines will
37not need to be uploaded entirely every time they change, as the
38changed sections will be identified and uploaded.
39
40Because a complete directory tree exists in the archive for any
41snapshot, the extraction algorithm is incredibly simple - and,
42therefore, incredibly reliable and fast. Simple, reliable, and fast
43are just what you need when you're trying to reconstruct the
44filesystem of a live server.
45
46Also, it means that you can do lots of small snapshots. If you run a
47snapshot every hour, then only a megabyte or two might have changed in
48your filesystem, so you only upload a megabyte or two - yet you end up
49with a complete history of your filesystem at hourly intervals in the
50archive.
51
52Conventional backup systems usually either store a full backup then
53incrementals to their archives, meaning that doing a restore involves
54reading the full backup then reading every incremental since and
55applying them - so to do a restore, you have to download *every
56version* of the filesystem you've ever uploaded, or you have to do
57periodic full backups (even though most of your filesystem won't have
58changed since the last full backup) to reduce the number of
59incrementals required for a restore. Better results are had from
60systems that use a special backup server to look after the archive
61storage, which accept incremental backups and apply them to the
62snapshot they keep in order to maintain a most-recent snapshot that
63can be downloaded in a single run; but they then restrict you to using
64dedicated servers as your archive stores, ruling out cheaply scalable
65solutions like Amazon S3, or just backing up to a removable USB or
66eSATA disk you attach to your system whenever you do a backup. And
67dedicated backup servers are complex pieces of software; can you rely
68on something complex for the fundamental foundation of your data
69security system?
70
71## System Requirements
72
73Ugarit should run on any POSIX-compliant system that can run [Chicken
74Scheme](http://www.call-with-current-continuation.org/). It stores and
75restores all the file attributes reported by the `stat` system call -
76POSIX mode permissions, UID, GID, mtime, and optionally atime and
77ctime (although the ctime cannot be restored due to POSIX
78restrictions). Ugarit will store files, directories, device and
79character special files, symlinks, and FIFOs.
80
81Support for extended filesystem attributes - ACLs, alternative
82streams, forks and other metadata - is possible, due to the extensible
83directory entry format; support for such metadata will be added as
84required.
85
86Currently, only local filesystem-based archive storage backends are
87complete: these are suitable for backing up to a removable hard disk
88or a filesystem shared via NFS or other protocols. However, the
89backend can be accessed via an SSH tunnel, so a remote server you are
90able to install Ugarit on to run the backends can be used as a remote
91archive.
92
93However, the next backend to be implemented will be one for Amazon S3,
94and an SFTP backend for storing archives anywhere you can ssh
95to. Other backends will be implemented on demand; an archive can, in
96principle, be stored on anything that can store files by name, report
97on whether a file already exists, and efficiently download a file by
98name. This rules out magnetic tapes due to their requirement for
99sequential access.
100
101Although we need to trust that a backend won't lose data (for now), we
102don't need to trust the backend not to snoop on us, as Ugarit
103optionally encrypts everything sent to the archive.
104
105## Terminology
106
107A Ugarit backend is the software module that handles backend
108storage. An archive is an actual storage system storing actual data,
109accessed through the appropriate backend for that archive. The backend
110may run locally under Ugarit itself, or via an SSH tunnel, on a remote
111server where it is installed.
112
113For example, if you use the recommended "splitlog" filesystem backend,
114your archive might be `/mnt/bigdisk` on the server `prometheus`. The
115backend (which is compiled along with the other filesystem backends in
116the `backend-fs` binary) must be installed on `prometheus`, and Ugarit
117clients all over the place may then use it via ssh to
118`prometheus`. However, even with the filesystem backends, the actual
119storage might not be on `prometheus` where the backend runs -
120`/mnt/bigdisk` might be an NFS mount, or a mount from a storage-area
121network. This ability to delegate via SSH is particularly useful with
122the "cache" backend, which reduces latency by storing a cache of what
123blocks exist in a backend, thereby making it quicker to identify
124already-stored files; a cluster of servers all sharing the same
125archive might all use SSH tunnels to access an instance of the "cache"
126backend on one of them (using some local disk to store the cache),
127which proxies the actual archive storage to an archive on the other
128end of a high-latency Internet link, again via an SSH tunnel.
129
130## What's in an archive?
131
132An Ugarit archive contains a load of blocks, each up to a maximum size
133(usually 1MiB, although other backends might impose smaller
134limits). Each block is identified by the hash of its contents; this is
135how Ugarit avoids ever uploading the same data twice, by checking to
136see if the data to be uploaded already exists in the archive by
137looking up the hash. The contents of the blocks are compressed and
138then encrypted before upload.
139
140Every file uploaded is, unless it's small enough to fit in a single
141block, chopped into blocks, and each block uploaded. This way, the
142entire contents of your filesystem can be uploaded - or, at least,
143only the parts of it that aren't already there! The blocks are then
144tied together to create a snapshot by uploading blocks full of the
145hashes of the data blocks, and directory blocks are uploaded listing
146the names and attributes of files in directories, along with the
147hashes of the blocks that contain the files' contents. Even the blocks
148that contain lists of hashes of other blocks are subject to checking
149for pre-existence in the archive; if only a few MiB of your
150hundred-GiB filesystem has changed, then even the index blocks and
151directory blocks are re-used from previous snapshots.
152
153Once uploaded, a block in the archive is never again changed. After
154all, if its contents changed, its hash would change, so it would no
155longer be the same block! However, every block has a reference count,
156tracking the number of index blocks that refer to it. This means that
157the archive knows which blocks are shared between multiple snapshots
158(or shared *within* a snapshot - if a filesystem has more than one
159copy of the same file, still only one copy is uploaded), so that if a
160given snapshot is deleted, then the blocks that only that snapshot is
161using can be deleted to free up space, without corrupting other
162snapshots by deleting blocks they share. Keep in mind, however, that
163not all storage backends may support this - there are certain
164advantages to being an append-only archive. For a start, you can't
165delete something by accident! The supplied fs backend supports
166deletion, while the splitlog backend does not yet. However, the actual
167snapshot deletion command hasn't been implemented yet either, so it's
168a moot point for now...
169
170Finally, the archive contains objects called tags. Unlike the blocks,
171the tags contents can change, and they have meaningful names rather
172than being identified by hash. Tags identify the top-level blocks of
173snapshots within the system, from which (by following the chain of
174hashes down through the index blocks) the entire contents of a
175snapshot may be found. Unless you happen to have recorded the hash of
176a snapshot somewhere, the tags are where you find snapshots from when
177you want to do a restore!
178
179Whenever a snapshot is taken, as soon as Ugarit has uploaded all the
180files, directories, and index blocks required, it looks up the tag you
181have identified as the target of the snapshot. If the tag already
182exists, then the snapshot it currently points to is recorded in the
183new snapshot as the "previous snapshot"; then the snapshot header
184containing the previous snapshot hash, along with the date and time
185and any comments you provide for the snapshot, and is uploaded (as
186another block, identified by its hash). The tag is then updated to
187point to the new snapshot.
188
189This way, each tag actually identifies a chronological chain of
190snapshots. Normally, you would use a tag to identify a filesystem
191being backed up; you'd keep snapshotting the filesystem to the same
192tag, resulting in all the snapshots of that filesystem hanging from
193the tag. But if you wanted to remember any particular snapshot
194(perhaps if it's the snapshot you take before a big upgrade or other
195risky operation), you can duplicate the tag, in effect 'forking' the
196chain of snapshots much like a branch in a version control system.
197
198# Using Ugarit
199
200## Installation
201
202Install [Chicken Scheme](http://www.call-with-current-continuation.org/) using their [installation instructions](http://chicken.wiki.br/Getting%20started#Installing%20Chicken).
203
204Ugarit can then be installed by typing (as root):
205
206    chicken-install ugarit
207
208See the [chicken-install manual](http://wiki.call-cc.org/manual/Extensions#chicken-install-reference) for details if you have any trouble, or wish to install into your home directory.
209
210## Setting up an archive
211
212Firstly, you need to know the archive identifier for the place you'll
213be storing your archives. This depends on your backend. The archive
214identifier is actually the command line used to invoke the backend for
215a particular archive; communication with the archive is via standard
216input and output, which is how it's easy to tunnel via ssh.
217
218### Local filesystem backends
219
220These backends use the local filesystem to store the archives. Of
221course, the "local filesystem" on a given server might be an NFS mount
222or mounted from a storage-area network.
223
224#### Logfile backend
225
226The logfile backend works much like the original Venti system. It's
227append-only - you won't be able to delete old snapshots from a logfile
228archive, even when I implement deletion. It stores the archive in two
229sets of files; one is a log of data blocks, split at a specified
230maximum size, and the other is the metadata: an sqlite database used
231to track the location of blocks in the log files, the contents of
232tags, and a count of the logs so a filename can be chosen for a new one.
233
234To set up a new logfile archive, just choose where to put the two
235parts. It would be nice to put the metadata file on a different
236physical disk to the logs directory, to reduce seeking. If you only
237have one disk, you can put the metadata file in the log directory
238("metadata" is a good name).
239
240You can then refer to it using the following archive identifier:
241
242      "backend-fs splitlog ...log directory... ...metadata file... max-logfile-size"
243
244For most platforms, a max-logfile-size of 900000000 (900 MB) should
245suffice. For now, don't go much bigger than that on 32-bit systems
246until Chicken's `file-position` function is fixed to work with files
247more than 1GB in size.
248
249#### Filesystem backend
250
251The filesystem backend creates archives by storing each block or tag
252in its own file, in a directory. To keep the objects-per-directory
253count down, it'll split the files into subdirectories. Because of
254this, it uses a stupendous number of inodes (more than the filesystem
255being backed up). Only use it if you don't mind that; splitlog is much
256more efficient.
257
258To set up a new filesystem-backend archive, just create an empty
259directory that Ugarit will have write access to when it runs. It will
260probably run as root in order to be able to access the contents of
261files that aren't world-readable (although that's up to you), so be
262careful of NFS mounts that have `maproot=nobody` set!
263
264You can then refer to it using the following archive identifier:
265
266      "backend-fs fs ...path to directory..."
267
268### Proxying backends
269
270These backends wrap another archive identifier which the actual
271storage task is delegated to, but add some value along the way.
272
273### SSH tunnelling
274
275It's easy to access an archive stored on a remote server. The caveat
276is that the backend then needs to be installed on the remote server!
277Since archives are accessed by running the supplied command, and then
278talking to them via stdin and stdout, the archive identified needs
279only be:
280
281      "ssh ...hostname... '...remote archive identifier...'"
282
283### Cache backend
284
285The cache backend is used to cache a list of what blocks exist in the
286proxied backend, so that it can answer queries as to the existance of
287a block rapidly, even when the proxied backend is on the end of a
288high-latency link (eg, the Internet). This should speed up snapshots,
289as existing files are identified by asking the backend if the archive
290already has them.
291
292The cache backend works by storing the cache in a local sqlite
293file. Given a place for it to store that file, usage is simple:
294
295      "backend-cache ...path to cachefile... '...proxied archive identifier...'"
296
297The cache file will be automatically created if it doesn't already
298exist, so make sure there's write access to the containing directory.
299
300 - WARNING - WARNING - WARNING - WARNING - WARNING - WARNING -
301
302If you use a cache on an archive shared between servers, make sure
303that you either:
304
305 * Never delete things from the archive
306
307or
308
309 * Make sure all access to the archive is via the same cache
310
311If a block is deleted from an archive, and a cache on that archive is
312not aware of the deletion (as it did not go "through" the caching
313proxy), then the cache will record that the block exists in the
314archive when it does not. This will mean that if a snapshot is made
315through the cache that would use that block, then it will be assumed
316that the block already exists in the archive when it does
317not. Therefore, the block will not be uploaded, and a dangling
318reference will result!
319
320Some setups which *are* safe:
321
322 * A single server using an archive via a cache, not sharing it with
323   anyone else.
324
325 * A pool of servers using an archive via the same cache.
326
327 * A pool of servers using an archive via one or more caches, and
328   maybe some not via the cache, where nothing is ever deleted from
329   the archive.
330
331 * A pool of servers using an archive via one cache, and maybe some
332   not via the cache, where deletions are only performed on servers
333   using the cache, so the cache is always aware.
334
335## Writing a ugarit.conf
336
337`ugarit.conf` should look something like this:
338
339      (storage <archive identifier>)
340      (hash tiger "<salt>")
341      [double-check]
342      [(compression [deflate|lzma])]
343      [(encryption aes <key>)]
344      [(file-cache "<path>")]
345      [(rule ...)]
346
347The hash line chooses a hash algorithm. Currently Tiger-192 (`tiger`),
348SHA-256 (`sha256`), SHA-384 (`sha384`) and SHA-512 (`sha512`) are
349supported; if you omit the line then Tiger will still be used, but it
350will be a simple hash of the block with the block type appended, which
351reveals to attackers what blocks you have (as the hash is of the
352unencrypted block, and the hash is not encrypted). This is useful for
353development and testing or for use with trusted archives, but not
354advised for use with archives that attackers may snoop at. Providing a
355salt string produces a hash function that hashes the block, the type
356of block, and the salt string, producing hashes that attackers who can
357snoop the archive cannot use to find known blocks (see the "Security
358model" section below for more details).
359
360I would recommend that you create a salt string from a secure entropy
361source, such as:
362
363   dd if=/dev/random bs=1 count=64 | base64 -w 0
364
365Whichever hash function you use, you will need to install the required
366Chicken egg with one of the following commands:
367
368    chicken-install -s tiger-hash  # for tiger
369    chicken-install -s sha2        # for the SHA hashes
370
371`double-check`, if present, causes Ugarit to perform extra internal
372consistency checks during backups, which will detect bugs but may slow
373things down.
374
375`lzma` is the recommended compression option for low-bandwidth
376backends or when space is tight, but it's very slow to compress;
377deflate or no compression at all are better for fast local
378archives. To have no compression at all, just remove the `(compression
379...)` line entirely. Likewise, to use compression, you need to install
380a Chicken egg:
381
382       chicken-install -s z3       # for deflate
383       chicken-install -s lzma     # for lzma
384
385Likewise, the `(encryption ...)` line may be omitted to have no
386encryption; the only currently supported algorithm is aes (in CBC
387mode) with a key given in hex, as a passphrase (hashed to get a key),
388or a passphrase read from the terminal on every run. The key may be
38916, 24, or 32 bytes for 128-bit, 192-bit or 256-bit AES. To specify a
390hex key, just supply it as a string, like so:
391
392      (encryption aes "00112233445566778899AABBCCDDEEFF")
393
394...for 128-bit AES,
395
396      (encryption aes "00112233445566778899AABBCCDDEEFF0011223344556677")
397
398...for 192-bit AES, or
399
400      (encryption aes "00112233445566778899AABBCCDDEEFF00112233445566778899AABBCCDDEEFF")
401
402...for 256-bit AES.
403
404Alternatively, you can provide a passphrase, and specify how large a
405key you want it turned into, like so:
406
407      (encryption aes ([16|24|32] "We three kings of Orient are, one in a taxi one in a car, one on a scooter honking his hooter and smoking a fat cigar. Oh, star of wonder, star of light; star with royal dynamite"))
408
409I would recommend that you generate a long passphrase from a secure
410entropy source, such as:
411
412   dd if=/dev/random bs=1 count=64 | base64 -w 0
413
414Finally, the extra-paranoid can request that Ugarit prompt for a
415passphrase on every run and hash it into a key of the specified
416length, like so:
417
418      (encryption aes ([16|24|32] prompt))
419
420(note the lack of quotes around `prompt`, distinguishing it from a passphrase)
421
422Please read the "Security model" section below for details on the
423implications of different encryption setups.
424
425Again, as it is an optional feature, to use encryption, you must
426install the appropriate Chicken egg:
427
428       chicken-install -s aes
429
430A file cache, if enabled, significantly speeds up subsequent snapshots
431of a filesystem tree. The file cache is a file (which Ugarit will
432create if it doesn't already exist) mapping filenames to
433(mtime,size,hash) tuples; as it scans the filesystem, if it finds a
434file in the cache and the mtime and size have not changed, it will
435assume it is already archived under the specified hash. This saves it
436from having to read the entire file to hash it and then check if the
437hash is present in the archive. In other words, if only a few files
438have changed since the last snapshot, then snapshotting a directory
439tree becomes an O(N) operation, where N is the number of files, rather
440than an O(M) operation, where M is the total size of files involved.
441
442For example:
443
444      (storage "ssh ugarit@spiderman 'backend-fs splitlog /mnt/ugarit-data /mnt/ugarit-metadata/metadata 900000000'")
445      (hash tiger "i3HO7JeLCSa6Wa55uqTRqp4jppUYbXoxme7YpcHPnuoA+11ez9iOIA6B6eBIhZ0MbdLvvFZZWnRgJAzY8K2JBQ")
446      (encryption aes (32 "FN9m34J4bbD3vhPqh6+4BjjXDSPYpuyskJX73T1t60PP0rPdC3AxlrjVn4YDyaFSbx5WRAn4JBr7SBn2PLyxJw"))
447      (compression lzma)
448      (file-cache "/var/ugarit/cache")
449
450Be careful to put a set of parentheses around each configuration
451entry. White space isn't significant, so feel free to indent things
452and wrap them over lines if you want.
453
454Keep copies of this file safe - you'll need it to do extractions!
455Print a copy out and lock it in your fire safe! Ok, currently, you
456might be able to recreate it if you remember where you put the
457storage, but encryption keys and hash salts are harder to remember...
458
459## Your first backup
460
461Think of a tag to identify the filesystem you're backing up. If it's
462`/home` on the server `gandalf`, you might call it `gandalf-home`. If
463it's the entire filesystem of the server `bilbo`, you might just call
464it `bilbo`.
465
466Then from your shell, run (as root):
467
468      # ugarit snapshot <ugarit.conf> [-c] [-a] <tag> <path to root of filesystem>
469
470For example, if we have a `ugarit.conf` in the current directory:
471
472      # ugarit snapshot ugarit.conf -c localhost-etc /etc
473
474Specify the `-c` flag if you want to store ctimes in the archive;
475since it's impossible to restore ctimes when extracting from an
476archive, doing this is useful only for informational purposes, so it's
477not done by default. Similarly, atimes aren't stored in the archive
478unless you specify `-a`, because otherwise, there will be a lot of
479directory blocks uploaded on every snapshot, as the atime of every
480file will have been changed by the previous snapshot - so with `-a`
481specified, on every snapshot, every directory in your filesystem will
482be uploaded! Ugarit will happily restore atimes if they are found in
483an archive; their storage is made optional simply because uploading
484them is costly and rarely useful.
485
486## Exploring the archive
487
488Now you have a backup, you can explore the contents of the
489archive. This need not be done as root, as long as you can read
490`ugarit.conf`; however, if you want to extract files, run it as root
491so the uids and gids can be set.
492
493      $ ugarit explore <ugarit.conf>
494
495This will put you into an interactive shell exploring a virtual
496filesystem. The root directory contains an entry for every tag; if you
497type `ls` you should see your tag listed, and within that tag, you'll
498find a list of snapshots, in descending date order, with a special
499entry `current` for the most recent snapshot. Within a snapshot,
500you'll find the root directory of your snapshot, and will be able to
501`cd` into subdirectories, and so on:
502
503      > ls
504      Test <tag>
505      > cd Test
506      /Test> ls
507      2009-01-24 10:28:16 <snapshot>
508      2009-01-24 10:28:16 <snapshot>
509      current <snapshot>
510      /Test> cd current
511      /Test/current> ls
512      README.txt <file>
513      LICENCE.txt <symlink>
514      subdir <dir>
515      .svn <dir>
516      FIFO <fifo>
517      chardev <character-device>
518      blockdev <block-device>
519      /Test/current> ls -ll LICENCE.txt
520      lrwxr-xr-x 1000 100 2009-01-15 03:02:49 LICENCE.txt -> subdir/LICENCE.txt
521      target: subdir/LICENCE.txt
522      ctime: 1231988569.0
523
524As well as exploring around, you can also extract files or directories
525(or entire snapshots) by using the `get` command. Ugarit will do its
526best to restore the metadata of files, subject to the rights of the
527user you run it as.
528
529Type `help` to get help in the interactive shell.
530
531## Duplicating tags
532
533As mentioned above, you can duplicate a tag, creating two tags that
534refer to the same snapshot and its history but that can then have
535their own subsequent history of snapshots applied to each
536independently, with the following command:
537
538      $ ugarit fork <ugarit.conf> <existing tag> <new tag>
539
540## Archive administration
541
542Each backend offers a number of administrative commands for
543administering archives. These are accessible via the
544`ugarit-archive-admin` command line interface.
545
546To use it, run it with the following command:
547
548      $ ugarit-archive-admin '<archive identifier>'
549
550The available commands differ between backends, but all backends
551support the `info` and `help` commands, which give basic information
552about the archive, and list all available commands, respectively. Some
553offer a `stats` command that examines the archive state to give
554interesting statistics, but which may be a time-consuming operation.
555
556### Administering `splitlog` archives
557
558The splitlog backend offers a wide selection of administrative
559commands. See the `help` command on a splitlog archive for
560details. The following facilities are available:
561
562 * Configuring the block size of the archive (this will affect new
563   blocks written to the archive, and leave existing blocks untouched,
564   even if they are larger than the new block size)
565
566 * Configuring the size at which a log file is finished and a new one
567   started (likewise, existing log files will be untouched; this will
568   only affect new log files)
569
570 * Configuring the frequency of automatic synching of the archive
571   state to disk. Lowering this harms performance when writing to the
572   archive, but decreases the number of in-progress block writes that
573   can fail in a crash.
574
575 * Enable or disable write protection of the archive
576
577 * Reindex the archive, rebuilding the block and tag state from the
578   contents of the log. If the metadata file is damaged or lost,
579   reindexing can rebuild it (although any configuration changes made
580   via other admin commands will need manually repeating as they are
581   not logged).
582
583## `.ugarit` files
584
585By default, Ugarit will archive everything it finds in the filesystem
586tree you tell it to snapshot. However, this might not always be
587desired; so we provide the facility to override this with `.ugarit`
588files, or global rules in your `.conf` file.
589
590Note: The syntax of these files is provisional, as I want to
591experiment with usability, as the current syntax is ugly. So please
592don't be surprised if the format changes in incompatible ways in
593subsequent versions!
594
595In quick summary, if you want to ignore all files or directories
596matching a glob in the current directory and below, put the following
597in a `.ugarit` file in that directory:
598
599      (* (glob "*~") exclude)
600
601You can write quite complex expressions as well as just globs. The
602full set of rules is:
603
604* `(glob "`*pattern*`")` matches files and directories whose names
605  match the glob pattern
606
607* `(name "`*name*`")` matches files and directories with exactly that
608  name (useful for files called `*`...)
609
610* `(modified-within ` *number* ` seconds)` matches files and
611  directories modified within the given number of seconds
612
613* `(modified-within ` *number* ` minutes)` matches files and
614  directories modified within the given number of minutes
615
616* `(modified-within ` *number* ` hours)` matches files and directories
617  modified within the given number of hours
618
619* `(modified-within ` *number* ` days)` matches files and directories
620  modified within the given number of days
621
622* `(not ` *rule*`)` matches files and directories that do not match
623  the given rule
624
625* `(and ` *rule* *rule...*`)` matches files and directories that match
626  all the given rules
627
628* `(or ` *rule* *rule...*`)` matches files and directories that match
629  any of the given rules
630
631Also, you can override a previous exclusion with an explicit include
632in a lower-level directory:
633
634    (* (glob "*~") include)
635
636You can bind rules to specific directories, rather than to "this
637directory and all beneath it", by specifying an absolute or relative
638path instead of the `*`:
639
640    ("/etc" (name "passwd") exclude)
641
642If you use a relative path, it's taken relative to the directory of
643the `.ugarit` file.
644
645You can also put some rules in your `.conf` file, although relative
646paths are illegal there, by adding lines of this form to the file:
647
648    (rule * (glob "*~") exclude)
649
650# Questions and Answers
651
652## What happens if a snapshot is interrupted?
653
654Nothing! Whatever blocks have been uploaded will be uploaded, but the
655snapshot is only added to the tag once the entire filesystem has been
656snapshotted. So just start the snapshot again. Any files that have
657already be uploaded will then not need to be uploaded again, so the
658second snapshot should proceed quickly to the point where it failed
659before, and continue from there.
660
661Unless the archive ends up with a partially-uploaded corrupted block
662due to being interrupted during upload, you'll be fine. The filesystem
663backend has been written to avoid this by writing the block to a file
664with the wrong name, then renaming it to the correct name when it's
665entirely uploaded.
666
667Actually, there is *one* caveat: blocks that were uploaded, but never
668make it into a finished snapshot, will be marked as "referenced" but
669there's no snapshot to delete to un-reference them, so they'll never
670be removed when you delete snapshots. (Not that snapshot deletion is
671implemented yet, mind). If this becomes a problem for people, we could
672write a "garbage collect" tool that regenerates the reference counts
673in an archive, leading to unused blocks (with a zero refcount) being
674unlinked.
675
676## Should I share a single large archive between all my filesystems?
677
678I think so. Using a single large archive means that blocks shared
679between servers - eg, software installed from packages and that sort
680of thing - will only ever need to be uploaded once, saving storage
681space and upload bandwidth. However, do not share an archive between
682servers that do not mutually trust each other, as they can all update
683the same tags, so can meddle with each other's snapshots - and read
684each other's snapshots.
685
686# Security model
687
688I have designed and implemented Ugarit to be able to handle cases
689where the actual archive storage is not entirely trusted.
690
691However, security involves tradeoffs, and Ugarit is configurable in
692ways that affect its resistance to different kinds of attacks. Here I
693will list different kinds of attack and explain how Ugarit can deal
694with them, and how you need to configure it to gain that
695protection.
696
697## Archive snoopers
698
699This might be somebody who can intercept Ugarit's communication with
700the archive at any point, or who can read the archive itself at their
701leisure.
702
703Ugarit's splitlog backend creates files with "rw-------" permissions
704out of the box to try and prevent this. This is a pain for people who
705want to share archives between UIDs, but we can add a configuration
706option to override this if that becomes a problem.
707
708### Reading your data
709
710If you enable encryption, then all the blocks sent to the archive are
711encrypted using a secret key stored in your Ugarit configuration
712file. As long as that configuration file is kept safe, and the AES
713algorithm is secure, then attackers who can snoop the archive cannot
714decode your data blocks. Enabling compression will also help, as the
715blocks are compressed before encrypting, which is thought to make
716cryptographic analysis harder.
717
718Recommendations: Use compression and encryption when there is a risk
719of archive snooping. Keep your Ugarit configuration file safe using
720UNIX file permissions (make it readable only by root), and maybe store
721it on a removable device that's only plugged in when
722required. Alternatively, use the "prompt" passphrase option, and be
723prompted for a passphrase every time you run Ugarit, so it isn't
724stored on disk anywhere.
725
726### Looking for known hashes
727
728A block is identified by the hash of its content (before compression
729and encryption). If an attacker was trying to find people who own a
730particular file (perhaps a piece of subversive literature), they could
731search Ugarit archives for its hash.
732
733However, Ugarit has the option to "key" the hash with a "salt" stored
734in the Ugarit configuration file. This means that the hashes used are
735actually a hash of the block's contents *and* the salt you supply. If
736you do this with a random salt that you keep secret, then attackers
737can't check your archive for known content just by comparing the hashes.
738
739Recommendations: Provide a secret string to your hash function in your
740Ugarit configuration file. Keep the Ugarit configuration file safe, as
741per the advice in the previous point.
742
743## Archive modifiers
744
745These folks can modify Ugarit's writes into the archive, its reads
746back from the archive, or can modify the archive itself at their leisure.
747
748Modifying an encrypted block without knowing the encryption key can at
749worst be a denial of service, corrupting the block in an unknown
750way. An attacker who knows the encryption key could replace a block
751with valid-seeming but incorrect content. In the worst case, this
752could exploit a bug in the decompression engine, causing a crash or
753even an exploit of the Ugarit process itself (thereby gaining the
754powers of a process inspector, as documented below). We can but hope
755that the decompression engine is robust. Exploits of the decryption
756engine, or other parts of Ugarit, are less likely due to the nature of
757the operations performed upon them.
758
759However, if a block is modified, then when Ugarit reads it back, the
760hash will no longer match the hash Ugarit requested, which will be
761detected and an error reported. The hash is checked after
762decryption and decompression, so this check does not protect us
763against exploits of the decompression engine.
764
765This protection is only afforded when the hash Ugarit asks for is not
766tampered with. Most hashes are obtained from within other blocks,
767which are therefore safe unless that block has been tampered with; the
768nature of the hash tree conveys the trust in the hashes up to the
769root. The root hashes are stored in the archive as "tags", which an
770archive modifier could alter at will. Therefore, the tags cannot be
771trusted if somebody might modify the archive. This is why Ugarit
772prints out the snapshot hash and the root directory hash after
773performing a snapshot, so you can record them securely outside of the
774archive.
775
776The most likely threat posed by archive modifiers is that they could
777simply corrupt or delete all of your archive, without needing to know
778any encryption keys.
779
780Recommendations: Secure your archives against modifiers, by whatever
781means possible. If archive modifiers are still a potential threat,
782write down a log of your root directory hashes from each snapshot, and keep
783it safe. When extracting your backups, use the `ls -ll` command in the
784interface to check the "contents" hash of your snapshots, and check
785they match the root directory hash you expect.
786
787## Process inspectors
788
789These folks can attach debuggers or similar tools to running
790processes, such as Ugarit itself.
791
792Ugarit backend processes only see encrypted data, so people who can
793attach to that process gain the powers of archive snoopers and
794modifiers, and the same conditions apply.
795
796People who can attach to the Ugarit process itself, however, will see
797the original unencrypted content of your filesystem, and will have
798full access to the encryption keys and hashing keys stored in your
799Ugarit configuration. When Ugarit is running with sufficient
800permissions to restore backups, they will be able to intercept and
801modify the data as it comes out, and probably gain total write access
802to your entire filesystem in the process.
803
804Recommendations: Ensure that Ugarit does not run under the same user
805ID as untrusted software. In many cases it will need to run as root in
806order to gain unfettered access to read the filesystems it is backing
807up, or to restore the ownership of files. However, when all the files
808it backs up are world-readable, it could run as an untrusted user for
809backups, and where file ownership is trivially reconstructible, it can
810do restores as a limited user, too.
811
812## Attackers in the source filesystem
813
814These folks create files that Ugarit will back up one day. By having
815write access to your filesystem, they already have some level of
816power, and standard Unix security practices such as storage quotas
817should be used to control them. They may be people with logins on your
818box, or more subtly, people who can cause servers to writes files;
819somebody who sends an email to your mailserver will probably cause
820that message to be written to queue files, as will people who can
821upload files via any means.
822
823Such attackers might use up your available storage by creating large
824files. This creates a problem in the actual filesystem, but that
825problem can be fixed by deleting the files. If those files get
826archived into Ugarit, then they are a part of that snapshot. If you
827are using a backend that supports deletion, then (when I implement
828snapshot deletion in the user interface) you could delete that entire
829snapshot to recover the wasted space, but that is a rather serious
830operation.
831
832More insidiously, such attackers might attempt to abuse a hash
833collision in order to fool the archive. If they have a way of creating
834a file that, for instance, has the same hash as your shadow password
835file, then Ugarit will think that it already has that file when it
836attempts to snapshot it, and store a reference to the existing
837file. If that snapshot is restored, then they will receive a copy of
838your shadow password file. Similarly, if they can predict a future
839hash of your shadow password file, and create a shadow password file
840of their own (perhaps one giving them a root account with a known
841password) with that hash, they can then wait for the real shadow
842password file to have that hash. If the system is later restored from
843that snapshot, then their chosen content will appear in the shadow
844password file. However, doing this requires a very fundamental break
845of the hash function being used.
846
847Recommendations: Think carefully about who has write access to your
848filesystems, directly or indirectly via a network service that stores
849received data to disk. Enforce quotas where appropriate, and consider
850not backing up "queue directories" where untrusted content might
851appear; migrate incoming content that passes acceptance tests to an
852area that is backed up. If necessary, the queue might be backed up to
853a non-snapshotting system, such as rsyncing to another server, so that
854any excessive files that appear in there are removed from the backup
855in due course, while still affording protection.
856
857# Future Directions
858
859Here's a list of planned developments, in approximate priority order:
860
861## General
862
863* More checks with `double-check` mode activated. Perhaps read blocks
864  back from the archive to check it matches the blocks sent, to detect
865  hash collisions. Maybe have levels of double-check-ness.
866
867* Migrate the source repo to Fossil (when there's a
868  kitten-technologies.co.uk migration to Fossil), and update the egg
869  locations thingy. Migrate all these Future Directions items to
870  actual tickets.
871
872* Profile the system. As of 1.0.1, having done the periodic SQLite
873  commits improvement, Ugarit is doing around 250KiB/sec on my home
874  fileserver, but using 87% CPU in the ugarit procesa and 25% in the
875  backend-fs process, when dealing with large files (so full 1MiB
876  blocks are being processed). This suggests that the main
877  block-handling loop in `store-file!` is less than efficient; reading
878  via `current-input-port` rather than using the POSIX egg `file-read`
879  functions may be a mistake, and there is probably more copying afoot
880  than we need.
881
882## Backends
883
884* Carefully document backend API for other backend authors: in
885  particular note behaviour in crash situations - we assume that after
886  a succesful flush! all previous blocks are safe, but after a flush,
887  if some blocks make it, then all previous blocks must have. Eg,
888  writes are done in order and periodically auto-flushed, in
889  effect. This invariant is required for the file-cache to be safe
890  (see v1.0.2).
891
892* Lock the archive for writing in backend-splitlog, so that two
893  snapshots to the same archive don't collide. Do we lock per `put!`
894  to allow interleaving, or is that too inefficient? In which case, we
895  need to hold a lock that persists for a while, and release it
896  periodically to allow other writers to the same archive to have a
897  chance.
898
899* Make backend-splitlog write the current log file offset as well as
900  number into the metadata on each flush, and on startup, either
901  truncate the file to that position (to remove anything written but
902  not flushed to the metadata) or scan the log onwards from that point
903  to find (complete) blocks that did not get flushed to the metadata.
904
905* Support for unlinking in backend-splitlog, by marking byte ranges as
906  unused in the metadata (and by touching the headers in the log so we
907  maintain the invariant that the metadata is a reconstructible cache)
908  and removing the entries for the unlinked blocks, perhaps provide an
909  option to attempt to re-use existing holes to put blocks in for
910  online reuse, and provide an offline compaction operation. Keep
911  stats in the index of how many byte ranges are unused, and how many
912  bytes unused, in each file, and report them in the info admin
913  interface, along with the option to compact any or all files. We'll
914  need to store refcounts in the backend metadata (should we log
915  reuses, then, so the metadata can always be reconstructed, or just
916  set them to NULL on a reconstruct); when this is enabled on an
917  existing archive with no refcounts, default them to NULL, and treat
918  a NULL refcount as "infinity".
919
920* For people doing remote backups who want to not hog resources, write
921  a proxy backend that throttles bandwidth usage. Make it record the
922  time it last sent a request to the backend, and the number of bytes
923  read and written; then when a new request comes in, delay it until
924  at least the largest of (write bandwidth quota * bytes written) and
925  (read bandwidth quota * bytes read) seconds has passed since the
926  last request was sent. NOTE: Start the clock when SENDING, so the
927  time spent handling the request is already counting towards
928  bandwidth quotas, or it won't be fair.
929
930* Support for SFTP as a storage backend. Store one file per block, as
931  per `backend-fs`, but remotely. See
932  http://tools.ietf.org/html/draft-ietf-secsh-filexfer-13 for sftp
933  protocol specs; popen an `ssh -p sftp` connection to the server then
934  talk that simple binary protocol. Tada! Ideally make an sftp egg,
935  then a "ugarit-backend-sftp" egg to keep the dependencies optional.
936
937* Support for S3 as a storage backend. There is now an S3 egg! Make an
938  "ugarit-backend-s3" egg to keep the dependencies optional.
939
940* Support for replicated archives. This will involve a special storage
941  backend that can wrap any number of other archives, each tagged with
942  a trust percentage and read and write load weightings. Each block
943  will be uploaded to enough archives to make the total trust be at
944  least 100%, by randomly picking the archives weighted by their write
945  load weighting. A read-only archive automatically gets its write
946  load weighting set to zero, and a warning issued if it was
947  configured otherwise. A local cache will be kept of which backends
948  carry which blocks, and reads will be serviced by picking the
949  archive that carries it and has the highest read load weighting. If
950  that archive is unavailable or has lost the block, then they will be
951  tried in read load order; and if none of them have it, an exhaustive
952  search of all available archives will be performed before giving up,
953  and the cache updated with the results if the block is found. In
954  order to correctly handle archives that were unavailable during
955  this, we might need to log an "unknown" for that block key / archive
956  pair, rather than assuming the block is not there, and check it
957  later. Users will be given an admin command to notify the backend of
958  an archive going missing forever, which will cause it to be removed
959  from the cache. Affected blocks should be examined and re-replicated
960  if their replication count is now too low. Another command should be
961  available to warn of impending deliberate removal, which will again
962  remove the archive from the cluster and re-replicate, the difference
963  being that the disappearing archive is usable for re-replicating
964  FROM, so this is a safe operation for blocks that are only on that
965  one archive. The individual physical archives that we put
966  replication on top of won't be "valid" archives unless they are 100%
967  replicated, as they'll contain references to blocks that are on
968  other archives. It might be a good idea to mark them as such with a
969  special tag to avoid people trying to restore directly from them;
970  the frontend should complain if you attempt to directly use an
971  archive with the special tag in place. A copy of the replication
972  configuration could be stored under a special tag to mark this fact,
973  and to enable easy finding of the proper replicated archive to work
974  from. There should be a configurable option to snapshot the cache to
975  the archives whenever the replicated archive is closed, too. The
976  command line to the backend, "backend-replicated", should point to
977  an sqlite file for the configuration and cache, and users should use
978  admin commands to add/remove/modify archives in the cluster.
979
980## Core
981
982* Add the option to append hash signatures to the post-encryption
983  blocks in the archive, to protect against people who tamper with
984  blocks in order to try and exploit vulnerabilities in the
985  decompression or decryption code (and to more quickly detect
986  tampering in the pipeline, to reduce the DoS effect of all that
987  wasted decryption and decompression, potentially including things
988  that decrypt to giant amounts of RAM).
989
990* More stats. Log bytes written AFTER compression and encryption in
991  `archive-put!`. Log snapshot start and end times in the snapshot
992  object.
993
994* Clarify what characters are legal in tag names sent to backends, and
995  what are legal in human-supplied tag names, and check that
996  human-supplied tag names match a regular expression. Leave space for
997  system-only tag names for storing archive metadata; suggest making a
998  hash sign illegal in tag names.
999
1000* Clarify what characters are legal in block keys. Ugarit will only
1001  issue [a-zA-Z0-9] for normal blocks, but may use other characters
1002  (hash?) for special metadata blocks; establish a contract of what
1003  backends must support (a-z, A-Z, 0-9, hash?)
1004
1005* API documentation for the modules we export
1006
1007* Encrypt tags, with a hash inside to check it's decrypted
1008  correctly. Add a special "#ugarit-archive-format" tag that records a
1009  format version number, to note that this change has been
1010  applied. Provide an upgrade tool. Don't do auto-upgrades, or
1011  attackers will be able to drop in plaintext tags.
1012
1013* Store a test block in the archive that is used to check the same
1014  encryption and hash settings are used for an archive, consistently
1015  (changing compression setting is supported, but changing encryption
1016  or hash will lead to confusion). Encrypt the hash of the passphrase
1017  and store it in the test block, which should have a name that cannot
1018  clash with any actual hash (eg, use non-hex characters in its
1019  name). When the block does not exist, create it; when it does exist,
1020  check it against the current encryption and hashing settings to see
1021  if it matches. When creating a new block, if the "prompt" passphrase
1022  specification mechanism is in use, prompt again to confirm the
1023  passphrase. If no encryption is in use, check the hash algorithm
1024  doesn't change by storing the hash of a constant string,
1025  unencrypted. To make brute-forcing the passphrase or hash-salt
1026  harder, consider applying the hash a large number of times, to
1027  increase the compute cost of checking it. Thanks to Andy Bennett for
1028  this idea.
1029
1030* More `.ugarit` actions. Right now we just have exclude and include;
1031  we might specify less-safe operations such as commands to run before
1032  and after snapshotting certain subtrees, or filters (don't send this
1033  SVN repository; instead send the output of `svnadmin dump`),
1034  etc. Running arbitrary commands is a security risk if random users
1035  write their own `.ugarit` files - so we'd need some trust-based
1036  mechanism; they'd need to be explicitly enabled in `ugarit.conf`,
1037  then a `.ugarit` option could disable all unsafe operations in a
1038  subtree.
1039
1040* `.ugarit` rules for file sizes. In particular, a rule to exclude
1041  files above a certain size. Thanks to Andy Bennett for this idea.
1042
1043* Support for FFS flags, Mac OS X extended filesystem attributes, NTFS
1044  ACLs/streams, FAT attributes, etc... Ben says to look at Box Backup
1045  for some code to do that sort of thing.
1046
1047* Deletion support - letting you remove snapshots. Perhaps you might
1048  want to remove all snapshots older than a given number of days on a
1049  given tag. Or just remove X out of Y snapshots older than a given
1050  number of days on a given tag. We have the core support for this;
1051  just find a snapshot and `unlink-directory!` its contents, leaving a
1052  dangling pointer from the snapshot, and write the snapshot handling
1053  code to expect this. Again, check Box Backup for that.
1054
1055* Option, when backing up, to not cross mountpoints
1056
1057* Option, when backing up, to store inode number and mountpoint path
1058  in directory entries, and then when extracting, keeping a dictionary
1059  of this unique identifier to pathname, so that if a file to be
1060  extracted is already in the dictionary and the hash is the same, a
1061  hardlink can be created.
1062
1063* Archival mode as well as snapshot mode. Whereas a snapshot record
1064  takes a filesystem tree and adds it to a chain of snapshots of the
1065  same filesystem tree, archival mode takes a filesystem tree and
1066  inserts it into a search tree anchored on the specified tag,
1067  indexing it on a list of key+value properties supplied at archival
1068  time. An archive tag is represented in the virtual filesystem as a
1069  directory full of archive objects, each identified by their full
1070  hash; each archive object references the filesystem root as well as
1071  the key+value properties, and optionally a parent link like a
1072  snapshot, as an archive can be made that explicitly replaces an
1073  earlier one and should replace it in the index; there is also a
1074  virtual directory for each indexed property which contains a
1075  directory for each value of the property, full of symlinks to the
1076  archive objects, and subdirectories that allow multi-property
1077  searches on other properties. The index itself is stored as a B-Tree
1078  with a reasonably small block size; when it's updated, the modified
1079  index blocks are replaced, thereby gaining new hashes, so their
1080  parents need replacing, all the way up the tree until a new root
1081  block is created. The existing block unlink mechanism in the
1082  backends will reclaim storage for blocks that are superceded, if the
1083  backend supports it. When this is done, ugarit will offer the option
1084  of snapshotting to a snapshot tag, or archiving to an archive tag,
1085  or archiving to an archive tag while replacing a specified archive
1086  object (nominated by path within the tag), which causes it to be
1087  removed from the index (except from the directory listing all
1088  archives by hash), and the new archive object is inserted,
1089  referencing the old one as a parent.
1090
1091* Dump/restore format. On a dump, walk an arbitrary subtree of an
1092  archive, serialising objects. Do not put any hashes in the dump
1093  format - dump out entire files, and just identify objects with
1094  sequential numbers when forming the directory / snapshot trees. On a
1095  restore, read the same format and slide it into an archive (creating
1096  any required top-level snapshot objects if the dump doesn't start
1097  from a snapshot) and putting it onto a specified tag. The
1098  intention is that this format can be used to migrate your stuff
1099  between archives, perhaps to change to a better backend.
1100
1101* Optional progress reporting callback from within store-file! and
1102  store-directory!, called on each block within a file or on each
1103  filesystem object, respectively.
1104
1105* Add a procedure to resolve a path within the archive node tree from
1106  any root node. Pass in the path as a list of strings, with the
1107  symbols `.` and `..` being usable as meta-characters to do nothing
1108  or to go up a level. Write a utility procedure to parse a string
1109  into such a form. Make it recognise and follow symlinks.
1110
1111* When symlinks are traversed by the path resolver and by the explore
1112  CLI, make `<tag>/current` be a symlink to the timestamp of the
1113  current snapshot rather than a clone of it, for neatness.
1114
1115## Front-end
1116
1117* Install progress reporting callbacks to report progress to user;
1118  option for quiet (no reporting), normal (reporting if >60s have
1119  passed since last time), or verbose (report every file), or very
1120  verbose (report every file and block).
1121
1122* Make the explore CLI let you cd into symlinks
1123
1124* Add a command to force removing a tag lock.
1125
1126* Add a command to list all the tags (with a * next to locked tags)
1127
1128* Add a command to list the contents of any directory in the archive
1129  node tree
1130
1131* API mode: Works something like the backend API, except at the
1132  archive level. Supports all the important archive operations, plus
1133  access to sexpr stream writers and key stream writers,
1134  archive-node-fold, etc. Requested by andyjpb, perhaps I can write
1135  the framework for this and then let him add API functions as he desires.
1136
1137* Command-line support to extract the contents of a given path in the
1138  archive, rather than needing to use explore mode. Also the option to
1139  extract given just a block key (useful when reading from keys logged
1140  manually at snapshot time).
1141
1142* FUSE/9p support. Mount it as a read-only filesystem :-D Then
1143  consider adding Fossil-style writing to the `current` of a snapshot,
1144  with copy-on-write of blocks to a buffer area on the local disk,
1145  then the option to make a snapshot of `current`. Put these into
1146  separate "ugarit-frontend-9p" and "ugarit-frontend-fuse" eggs, to
1147  control the dependencies.
1148
1149* Filesystem watching. Even with the hash-caching trick, a snapshot
1150  will still involve walking the entire directory tree and looking up
1151  every file in the hash cache. We can do better than that - some
1152  platforms provide an interface for receiving real-time notifications
1153  of changed or added files. Using this, we could allow ugarit to run
1154  in continuous mode, keeping a log of file notifications from the OS
1155  while it does an initial full snapshot. It can then wait for a
1156  specified period (one hour, perhaps?), accumulating names of files
1157  changed since it started, before then creating a new snapshot by
1158  uploading just the files it knows to have changed, while subsequent
1159  file change notifications go to a new list.
1160
1161## Testing
1162
1163* An option to verify a snapshot, walking every block in it checking
1164  there's no dangling references, and that everything matches its
1165  hash, without needing to put it into a filesystem, and applying any
1166  other sanity checks we can think of en route. Optionally compare it
1167  to an on-disk filesystem, while we're at it.
1168
1169* A unit test script around the `ugarit` command-line tool; the corpus
1170  should contain a mix of tiny and huge files and directories, awkward
1171  cases for sharing of blocks (many identical files in the same dir,
1172  etc), complex forms of file metadata, and so on. It should archive
1173  and restore the corpus several times over with each hash,
1174  compression, and encryption option.
1175
1176* Testing crashes. See about writing a test backend binary that either
1177  raises an error or just kills the process directly after N
1178  operations, and sit in a loop running it with increasing N. Take N
1179  from an environment variable to make it easier to automate this.
1180
1181* Extract the debugging backend from backend-devtools into a proper
1182  backend binary that takes a path to a log file and a backend command
1183  line to wrap.
1184
1185* Invoke the archive unit tests with every compression and encryption
1186  option, and different hashing algorithms with and without keys
1187
1188# Acknowledgements
1189
1190The original idea came from Venti, a content-addressed storage system
1191from Plan 9. Venti is usable directly by user applications, and is
1192also integrated with the Fossil filesystem to support snapshotting the
1193status of a Fossil filesystem. Fossil allows references to either be
1194to a block number on the Fossil partition or to a Venti key; so when a
1195filesystem has been snapshotted, all it now contains is a "root
1196directory" pointer into the Venti archive, and any files modified
1197therafter are copied-on-write into Fossil where they may be modified
1198until the next snapshot.
1199
1200We're nowhere near that exciting yet, but using FUSE, we might be able
1201to do something similar, which might be fun. However, Venti inspired
1202me when I read about it years ago; it showed me how elegant
1203content-addressed storage is. Finding out that the Git version control
1204system used the same basic tricks really just confirmed this for me.
1205
1206Also, I'd like to tip my hat to Duplicity. With the changing economics
1207of storage presented by services like Amazon S3 and rsync.net, I
1208looked to Duplicity as it provided both SFTP and S3 backends. However,
1209it worked in terms of full and incremental backups, a model that I
1210think made sense for magnetic tapes, but loses out to
1211content-addressed snapshots when you have random-access
1212media. Duplicity inspired me by its adoption of multiple backends, the
1213very backends I want to use, but I still hungered for a
1214content-addressed snapshot store.
1215
1216I'd also like to tip my hat to Box Backup. I've only used it a little,
1217because it requires a special server to manage the storage (and I want
1218to get my backups *off* of my servers), but it also inspires me with
1219directions I'd like to take Ugarit. It's much more aware of real-time
1220access to random-access storage than Duplicity, and has a very
1221interesting continuous background incremental backup mode, moving away
1222from the tape-based paradigm of backups as something you do on a
1223special day of the week, like some kind of religious observance. I
1224hope the author Ben, who is a good friend of mine, won't mind me
1225plundering his source code for details on how to request real-time
1226notification of changes from the filesystem, and how to read and write
1227extended attributes!
1228
1229Moving on from the world of backup, I'd like to thank the Chicken Team
1230for producing Chicken Scheme. Felix and the community at #chicken on
1231Freenode have particularly inspired me with their can-do attitudes to
1232combining programming-language elegance and pragmatic engineering -
1233two things many would think un-unitable enemies. Of course, they
1234didn't do it all themselves - R5RS Scheme and the SRFIs provided a
1235solid foundation to build on, and there's a cast of many more in the
1236Chicken community, working on other bits of Chicken or just egging
1237everyone on. And I can't not thank Henry Baker for writing the seminal
1238paper on the technique Chicken uses to implement full tail-calling
1239Scheme with cheap continuations on top of C; Henry already had my
1240admiration for his work on combining elegance and pragmatism in linear
1241logic. Why doesn't he return my calls? I even sent flowers.
1242
1243A special thanks should go to Christian Kellermann for porting Ugarit
1244to use Chicken 4 modules, too, which was otherwise a big bottleneck to
1245development, as I was stuck on Chicken 3 for some time! And to Andy
1246Bennett for many insightful conversations about future directions.
1247
1248Thanks to the early adopters who brought me useful feedback, too!
1249
1250And I'd like to thank my wife for putting up with me spending several
1251evenings and weekends and holiday days working on this thing...
1252
1253# Version history
1254
1255* 1.0.2: Made the file cache also commit periodically, rather than on
1256  every write, in order to improve performance. Counting blocks and
1257  bytes uploaded / reused, and file cache bytes as well as hits;
1258  reporting same in snapshot UI and logging same to snapshot
1259  metadata. Switched to the `posix-extras` egg and ditched our own
1260  `posixextras.scm` wrappers. Used the `parley` egg in the `ugarit
1261  explore` CLI for line editing. Added logging infrastructure,
1262  recording of snapshot logs in the snapshot. Added recovery from
1263  extraction errors. Listed lock state of tags in explore
1264  mode. Backend protocol v2 introduced (retaining v1 for
1265  compatability) allowing for an error on backend startup, and logging
1266  nonfatal errors, warnings, and info on startup and all protocol
1267  calls. Added `ugarit-archive-admin` command line interface to
1268  backend-specific administrative interfaces. Configuration of the
1269  splitlog backend (write protection, adjusting block size and logfile
1270  size limit and commit interval) is now possible via the admin
1271  interface. The admin interface also permits rebuilding the metadata
1272  index of a splitlog archive with the `reindex!` admin command.
1273
1274  * BUGFIX: Made file cache check the file hashes it finds in the
1275    cache actually exist in the archive, to protect against the case
1276    where a crash of some kind has caused unflushed changes to be
1277    lost; the file cache may well have committed changes that the
1278    backend hasn't, leading to references to nonexistant blocks. Note
1279    that we assume that archives are sequentially safe, eg if the
1280    final indirect block of a large file made it, all the partial
1281    blocks must have made it too.
1282
1283  * BUGFIX: Added an explicit `flush!` command to the backend
1284    protocol, and put explicit flushes at critical points in higher
1285    layers (`backend-cache`, the archive abstraction in the Ugarit
1286    core, and when tagging a snapshot) so that we ensure the blocks we
1287    point at are flushed before committing references to them in the
1288    `backend-cache` or file caches, or into tags, to ensure crash
1289    safety.
1290
1291  * BUGFIX: Made the splitlog backend never exceed the file size limit
1292    (except when passed blocks that, plus a header, are larger than
1293    it), rather than letting a partial block hang over the 'end'.
1294
1295  * BUGFIX: Fixed tag locking, which was broken all over the
1296    place. Concurrent snapshots to the same tag should now block for
1297    one another, although why you'd want to *do* that is questionable.
1298
1299  * BUGFIX: Fixed generation of non-keyed hashes, which was
1300    incorrectly appending the type to the hash without an outer
1301    hash. This breaks backwards compatability, but nobody was using
1302    the old algorithm, right? I'll introduce it as an option if
1303    required.
1304
1305* 1.0.1: Consistency check on read blocks by default. Removed warning
1306  about deletions from backend-cache; we need a new mechanism to
1307  report warnings from backends to the user. Made backend-cache and
1308  backend-fs/splitlog commit periodically rather than after every
1309  insert, which should speed up snapshotting a lot, and reused the
1310  prepared statements rather than re-preparing them all the
1311  time. BUGFIX: splitlog backend now creates log files with
1312  "rw-------" rather than "rwx------" permissions; and all sqlite
1313  databases (splitlog metadata, cache file, and file-cache file) are
1314  created with "rw-------" rather then "rw-r--r--".
1315
1316* 1.0: Migrated from gdbm to sqlite for metadata storage, removing the
1317  GPL taint. Unit test suite. backend-cache made into a separate
1318  backend binary. Removed backend-log. BUGFIX: file caching uses mtime *and*
1319  size now, rather than just mtime. Error handling so we skip objects
1320  that we cannot do something with, and proceed to try the rest of the
1321  operation.
1322
1323* 0.8: decoupling backends from the core and into separate binaries,
1324  accessed via standard input and output, so they can be run over SSH
1325  tunnels and other such magic.
1326
1327* 0.7: file cache support, sorting of directories so they're archived
1328  in canonical order, autoloading of hash/encryption/compression
1329  modules so they're not required dependencies any more.
1330
1331* 0.6: .ugarit support.
1332
1333* 0.5: Keyed hashing so attackers can't tell what blocks you have,
1334  markers in logs so the index can be reconstructed, sha2 support, and
1335  passphrase support.
1336
1337* 0.4: AES encryption.
1338
1339* 0.3: Added splitlog backend, and fixed a .meta file typo.
1340
1341* 0.2: Initial public release.
1342
1343* 0.1: Internal development release.
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