source: project/release/4/ugarit/trunk/README.txt @ 25565

Last change on this file since 25565 was 25565, checked in by Alaric Snell-Pym, 9 years ago

ugarit: tag locking, and strict enforcement of maximum file size in splitlog archives

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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). Whichever hash function you
359use, you will need to install the required Chicken egg with one of the
360following commands:
361
362    chicken-install -s tiger-hash  # for tiger
363    chicken-install -s sha2        # for the SHA hashes
364
365`double-check`, if present, causes Ugarit to perform extra internal
366consistency checks during backups, which will detect bugs but may slow
367things down.
368
369`lzma` is the recommended compression option for low-bandwidth
370backends or when space is tight, but it's very slow to compress;
371deflate or no compression at all are better for fast local
372archives. To have no compression at all, just remove the `(compression
373...)` line entirely. Likewise, to use compression, you need to install
374a Chicken egg:
375
376       chicken-install -s z3       # for deflate
377       chicken-install -s lzma     # for lzma
378
379Likewise, the `(encryption ...)` line may be omitted to have no
380encryption; the only currently supported algorithm is aes (in CBC
381mode) with a key given in hex, as a passphrase (hashed to get a key),
382or a passphrase read from the terminal on every run. The key may be
38316, 24, or 32 bytes for 128-bit, 192-bit or 256-bit AES. To specify a
384hex key, just supply it as a string, like so:
385
386      (encryption aes "00112233445566778899AABBCCDDEEFF")
387
388...for 128-bit AES,
389
390      (encryption aes "00112233445566778899AABBCCDDEEFF0011223344556677")
391
392...for 192-bit AES, or
393
394      (encryption aes "00112233445566778899AABBCCDDEEFF00112233445566778899AABBCCDDEEFF")
395
396...for 256-bit AES.
397
398Alternatively, you can provide a passphrase, and specify how large a
399key you want it turned into, like so:
400
401      (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"))
402
403Finally, the extra-paranoid can request that Ugarit prompt for a
404passphrase on every run and hash it into a key of the specified
405length, like so:
406
407      (encryption aes ([16|24|32] prompt))
408
409(note the lack of quotes around `prompt`, distinguishing it from a passphrase)
410
411Please read the "Security model" section below for details on the
412implications of different encryption setups.
413
414Again, as it is an optional feature, to use encryption, you must
415install the appropriate Chicken egg:
416
417       chicken-install -s aes
418
419A file cache, if enabled, significantly speeds up subsequent snapshots
420of a filesystem tree. The file cache is a file (which Ugarit will
421create if it doesn't already exist) mapping filenames to
422(mtime,size,hash) tuples; as it scans the filesystem, if it finds a
423file in the cache and the mtime and size have not changed, it will
424assume it is already archived under the specified hash. This saves it
425from having to read the entire file to hash it and then check if the
426hash is present in the archive. In other words, if only a few files
427have changed since the last snapshot, then snapshotting a directory
428tree becomes an O(N) operation, where N is the number of files, rather
429than an O(M) operation, where M is the total size of files involved.
430
431For example:
432
433      (storage "ssh ugarit@spiderman 'backend-fs splitlog /mnt/ugarit-data /mnt/ugarit-metadata/metadata 900000000'")
434      (hash tiger "Giung0ahKahsh9ahphu5EiGhAhth4eeyDahs2aiWAlohr6raYeequ8uiUr3Oojoh")
435      (encryption aes (32 "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"))
436      (compression lzma)
437      (file-cache "/var/ugarit/cache")
438
439Be careful to put a set of parentheses around each configuration
440entry. White space isn't significant, so feel free to indent things
441and wrap them over lines if you want.
442
443Keep copies of this file safe - you'll need it to do extractions!
444Print a copy out and lock it in your fire safe! Ok, currently, you
445might be able to recreate it if you remember where you put the
446storage, but encryption keys are harder to remember.
447
448## Your first backup
449
450Think of a tag to identify the filesystem you're backing up. If it's
451`/home` on the server `gandalf`, you might call it `gandalf-home`. If
452it's the entire filesystem of the server `bilbo`, you might just call
453it `bilbo`.
454
455Then from your shell, run (as root):
456
457      # ugarit snapshot <ugarit.conf> [-c] [-a] <tag> <path to root of filesystem>
458
459For example, if we have a `ugarit.conf` in the current directory:
460
461      # ugarit snapshot ugarit.conf -c localhost-etc /etc
462
463Specify the `-c` flag if you want to store ctimes in the archive;
464since it's impossible to restore ctimes when extracting from an
465archive, doing this is useful only for informational purposes, so it's
466not done by default. Similarly, atimes aren't stored in the archive
467unless you specify `-a`, because otherwise, there will be a lot of
468directory blocks uploaded on every snapshot, as the atime of every
469file will have been changed by the previous snapshot - so with `-a`
470specified, on every snapshot, every directory in your filesystem will
471be uploaded! Ugarit will happily restore atimes if they are found in
472an archive; their storage is made optional simply because uploading
473them is costly and rarely useful.
474
475## Exploring the archive
476
477Now you have a backup, you can explore the contents of the
478archive. This need not be done as root, as long as you can read
479`ugarit.conf`; however, if you want to extract files, run it as root
480so the uids and gids can be set.
481
482      $ ugarit explore <ugarit.conf>
483
484This will put you into an interactive shell exploring a virtual
485filesystem. The root directory contains an entry for every tag; if you
486type `ls` you should see your tag listed, and within that tag, you'll
487find a list of snapshots, in descending date order, with a special
488entry `current` for the most recent snapshot. Within a snapshot,
489you'll find the root directory of your snapshot, and will be able to
490`cd` into subdirectories, and so on:
491
492      > ls
493      Test <tag>
494      > cd Test
495      /Test> ls
496      2009-01-24 10:28:16 <snapshot>
497      2009-01-24 10:28:16 <snapshot>
498      current <snapshot>
499      /Test> cd current
500      /Test/current> ls
501      README.txt <file>
502      LICENCE.txt <symlink>
503      subdir <dir>
504      .svn <dir>
505      FIFO <fifo>
506      chardev <character-device>
507      blockdev <block-device>
508      /Test/current> ls -ll LICENCE.txt
509      lrwxr-xr-x 1000 100 2009-01-15 03:02:49 LICENCE.txt -> subdir/LICENCE.txt
510      target: subdir/LICENCE.txt
511      ctime: 1231988569.0
512
513As well as exploring around, you can also extract files or directories
514(or entire snapshots) by using the `get` command. Ugarit will do its
515best to restore the metadata of files, subject to the rights of the
516user you run it as.
517
518Type `help` to get help in the interactive shell.
519
520## Duplicating tags
521
522As mentioned above, you can duplicate a tag, creating two tags that
523refer to the same snapshot and its history but that can then have
524their own subsequent history of snapshots applied to each
525independently, with the following command:
526
527      $ ugarit fork <ugarit.conf> <existing tag> <new tag>
528
529## `.ugarit` files
530
531By default, Ugarit will archive everything it finds in the filesystem
532tree you tell it to snapshot. However, this might not always be
533desired; so we provide the facility to override this with `.ugarit`
534files, or global rules in your `.conf` file.
535
536Note: The syntax of these files is provisional, as I want to
537experiment with usability, as the current syntax is ugly. So please
538don't be surprised if the format changes in incompatible ways in
539subsequent versions!
540
541In quick summary, if you want to ignore all files or directories
542matching a glob in the current directory and below, put the following
543in a `.ugarit` file in that directory:
544
545      (* (glob "*~") exclude)
546
547You can write quite complex expressions as well as just globs. The
548full set of rules is:
549
550* `(glob "`*pattern*`")` matches files and directories whose names
551  match the glob pattern
552
553* `(name "`*name*`")` matches files and directories with exactly that
554  name (useful for files called `*`...)
555
556* `(modified-within ` *number* ` seconds)` matches files and
557  directories modified within the given number of seconds
558
559* `(modified-within ` *number* ` minutes)` matches files and
560  directories modified within the given number of minutes
561
562* `(modified-within ` *number* ` hours)` matches files and directories
563  modified within the given number of hours
564
565* `(modified-within ` *number* ` days)` matches files and directories
566  modified within the given number of days
567
568* `(not ` *rule*`)` matches files and directories that do not match
569  the given rule
570
571* `(and ` *rule* *rule...*`)` matches files and directories that match
572  all the given rules
573
574* `(or ` *rule* *rule...*`)` matches files and directories that match
575  any of the given rules
576
577Also, you can override a previous exclusion with an explicit include
578in a lower-level directory:
579
580    (* (glob "*~") include)
581
582You can bind rules to specific directories, rather than to "this
583directory and all beneath it", by specifying an absolute or relative
584path instead of the `*`:
585
586    ("/etc" (name "passwd") exclude)
587
588If you use a relative path, it's taken relative to the directory of
589the `.ugarit` file.
590
591You can also put some rules in your `.conf` file, although relative
592paths are illegal there, by adding lines of this form to the file:
593
594    (rule * (glob "*~") exclude)
595
596# Questions and Answers
597
598## What happens if a snapshot is interrupted?
599
600Nothing! Whatever blocks have been uploaded will be uploaded, but the
601snapshot is only added to the tag once the entire filesystem has been
602snapshotted. So just start the snapshot again. Any files that have
603already be uploaded will then not need to be uploaded again, so the
604second snapshot should proceed quickly to the point where it failed
605before, and continue from there.
606
607Unless the archive ends up with a partially-uploaded corrupted block
608due to being interrupted during upload, you'll be fine. The filesystem
609backend has been written to avoid this by writing the block to a file
610with the wrong name, then renaming it to the correct name when it's
611entirely uploaded.
612
613Actually, there is *one* caveat: blocks that were uploaded, but never
614make it into a finished snapshot, will be marked as "referenced" but
615there's no snapshot to delete to un-reference them, so they'll never
616be removed when you delete snapshots. (Not that snapshot deletion is
617implemented yet, mind). If this becomes a problem for people, we could
618write a "garbage collect" tool that regenerates the reference counts
619in an archive, leading to unused blocks (with a zero refcount) being
620unlinked.
621
622## Should I share a single large archive between all my filesystems?
623
624I think so. Using a single large archive means that blocks shared
625between servers - eg, software installed from packages and that sort
626of thing - will only ever need to be uploaded once, saving storage
627space and upload bandwidth. However, do not share an archive between
628servers that do not mutually trust each other, as they can all update
629the same tags, so can meddle with each other's snapshots - and read
630each other's snapshots.
631
632# Security model
633
634I have designed and implemented Ugarit to be able to handle cases
635where the actual archive storage is not entirely trusted.
636
637However, security involves tradeoffs, and Ugarit is configurable in
638ways that affect its resistance to different kinds of attacks. Here I
639will list different kinds of attack and explain how Ugarit can deal
640with them, and how you need to configure it to gain that
641protection.
642
643## Archive snoopers
644
645This might be somebody who can intercept Ugarit's communication with
646the archive at any point, or who can read the archive itself at their
647leisure.
648
649Ugarit's splitlog backend creates files with "rw-------" permissions
650out of the box to try and prevent this. This is a pain for people who
651want to share archives between UIDs, but we can add a configuration
652option to override this if that becomes a problem.
653
654### Reading your data
655
656If you enable encryption, then all the blocks sent to the archive are
657encrypted using a secret key stored in your Ugarit configuration
658file. As long as that configuration file is kept safe, and the AES
659algorithm is secure, then attackers who can snoop the archive cannot
660decode your data blocks. Enabling compression will also help, as the
661blocks are compressed before encrypting, which is thought to make
662cryptographic analysis harder.
663
664Recommendations: Use compression and encryption when there is a risk
665of archive snooping. Keep your Ugarit configuration file safe using
666UNIX file permissions (make it readable only by root), and maybe store
667it on a removable device that's only plugged in when
668required. Alternatively, use the "prompt" passphrase option, and be
669prompted for a passphrase every time you run Ugarit, so it isn't
670stored on disk anywhere.
671
672### Looking for known hashes
673
674A block is identified by the hash of its content (before compression
675and encryption). If an attacker was trying to find people who own a
676particular file (perhaps a piece of subversive literature), they could
677search Ugarit archives for its hash.
678
679However, Ugarit has the option to "key" the hash with a "salt" stored
680in the Ugarit configuration file. This means that the hashes used are
681actually a hash of the block's contents *and* the salt you supply. If
682you do this with a random salt that you keep secret, then attackers
683can't check your archive for known content just by comparing the hashes.
684
685Recommendations: Provide a secret string to your hash function in your
686Ugarit configuration file. Keep the Ugarit configuration file safe, as
687per the advice in the previous point.
688
689## Archive modifiers
690
691These folks can modify Ugarit's writes into the archive, its reads
692back from the archive, or can modify the archive itself at their leisure.
693
694Modifying an encrypted block without knowing the encryption key can at
695worst be a denial of service, corrupting the block in an unknown
696way. An attacker who knows the encryption key could replace a block
697with valid-seeming but incorrect content. In the worst case, this
698could exploit a bug in the decompression engine, causing a crash or
699even an exploit of the Ugarit process itself (thereby gaining the
700powers of a process inspector, as documented below). We can but hope
701that the decompression engine is robust. Exploits of the decryption
702engine, or other parts of Ugarit, are less likely due to the nature of
703the operations performed upon them.
704
705However, if a block is modified, then when Ugarit reads it back, the
706hash will no longer match the hash Ugarit requested, which will be
707detected and an error reported. The hash is checked after
708decryption and decompression, so this check does not protect us
709against exploits of the decompression engine.
710
711This protection is only afforded when the hash Ugarit asks for is not
712tampered with. Most hashes are obtained from within other blocks,
713which are therefore safe unless that block has been tampered with; the
714nature of the hash tree conveys the trust in the hashes up to the
715root. The root hashes are stored in the archive as "tags", which an
716archive modifier could alter at will. Therefore, the tags cannot be
717trusted if somebody might modify the archive. This is why Ugarit
718prints out the snapshot hash and the root directory hash after
719performing a snapshot, so you can record them securely outside of the
720archive.
721
722The most likely threat posed by archive modifiers is that they could
723simply corrupt or delete all of your archive, without needing to know
724any encryption keys.
725
726Recommendations: Secure your archives against modifiers, by whatever
727means possible. If archive modifiers are still a potential threat,
728write down a log of your root directory hashes from each snapshot, and keep
729it safe. When extracting your backups, use the `ls -ll` command in the
730interface to check the "contents" hash of your snapshots, and check
731they match the root directory hash you expect.
732
733## Process inspectors
734
735These folks can attach debuggers or similar tools to running
736processes, such as Ugarit itself.
737
738Ugarit backend processes only see encrypted data, so people who can
739attach to that process gain the powers of archive snoopers and
740modifiers, and the same conditions apply.
741
742People who can attach to the Ugarit process itself, however, will see
743the original unencrypted content of your filesystem, and will have
744full access to the encryption keys and hashing keys stored in your
745Ugarit configuration. When Ugarit is running with sufficient
746permissions to restore backups, they will be able to intercept and
747modify the data as it comes out, and probably gain total write access
748to your entire filesystem in the process.
749
750Recommendations: Ensure that Ugarit does not run under the same user
751ID as untrusted software. In many cases it will need to run as root in
752order to gain unfettered access to read the filesystems it is backing
753up, or to restore the ownership of files. However, when all the files
754it backs up are world-readable, it could run as an untrusted user for
755backups, and where file ownership is trivially reconstructible, it can
756do restores as a limited user, too.
757
758## Attackers in the source filesystem
759
760These folks create files that Ugarit will back up one day. By having
761write access to your filesystem, they already have some level of
762power, and standard Unix security practices such as storage quotas
763should be used to control them. They may be people with logins on your
764box, or more subtly, people who can cause servers to writes files;
765somebody who sends an email to your mailserver will probably cause
766that message to be written to queue files, as will people who can
767upload files via any means.
768
769Such attackers might use up your available storage by creating large
770files. This creates a problem in the actual filesystem, but that
771problem can be fixed by deleting the files. If those files get
772archived into Ugarit, then they are a part of that snapshot. If you
773are using a backend that supports deletion, then (when I implement
774snapshot deletion in the user interface) you could delete that entire
775snapshot to recover the wasted space, but that is a rather serious
776operation.
777
778More insidiously, such attackers might attempt to abuse a hash
779collision in order to fool the archive. If they have a way of creating
780a file that, for instance, has the same hash as your shadow password
781file, then Ugarit will think that it already has that file when it
782attempts to snapshot it, and store a reference to the existing
783file. If that snapshot is restored, then they will receive a copy of
784your shadow password file. Similarly, if they can predict a future
785hash of your shadow password file, and create a shadow password file
786of their own (perhaps one giving them a root account with a known
787password) with that hash, they can then wait for the real shadow
788password file to have that hash. If the system is later restored from
789that snapshot, then their chosen content will appear in the shadow
790password file. However, doing this requires a very fundamental break
791of the hash function being used.
792
793Recommendations: Think carefully about who has write access to your
794filesystems, directly or indirectly via a network service that stores
795received data to disk. Enforce quotas where appropriate, and consider
796not backing up "queue directories" where untrusted content might
797appear; migrate incoming content that passes acceptance tests to an
798area that is backed up. If necessary, the queue might be backed up to
799a non-snapshotting system, such as rsyncing to another server, so that
800any excessive files that appear in there are removed from the backup
801in due course, while still affording protection.
802
803# Future Directions
804
805Here's a list of planned developments, in approximate priority order:
806
807## General
808
809* More checks with `double-check` mode activated. Perhaps read blocks
810  back from the archive to check it matches the blocks sent, to detect
811  hash collisions. Maybe have levels of double-check-ness.
812
813* Migrate the source repo to Fossil (when there's a
814  kitten-technologies.co.uk migration to Fossil), and update the egg
815  locations thingy.
816
817* Profile the system. As of 1.0.1, having done the periodic SQLite
818  commits improvement, Ugarit is doing around 250KiB/sec on my home
819  fileserver, but using 87% CPU in the ugarit procesa and 25% in the
820  backend-fs process, when dealing with large files (so full 1MiB
821  blocks are being processed). This suggests that the main
822  block-handling loop in `store-file!` is less than efficient; reading
823  via `current-input-port` rather than using the POSIX egg `file-read`
824  functions may be a mistake, and there is probably more copying afoot
825  than we need.
826
827## Backends
828
829* Create ugarit-backend-protocol-2, and extend import-backend to
830  support it. The differences are:
831
832  * Extend the backend API to have all API calls return a possibly
833    empty list of log messages before the actual result. When
834    importing a backend, provide a logging callback which is passed
835    these lists and feeds them into a logging mechanism which prints
836    them and stores them in the archive object for later logging into
837    the snapshot. The same logging interface can then be used for
838    warnings from within ugarit-core itself as well.
839
840  * Extend the backend API to have an initial list of log messages and
841    a possible error or success for initialisation, inside the
842    header. Make the command-line wrappers for backends use this to
843    indicate startup failure.
844
845* Carefully document backend API for other backend authors: in
846  particular note behaviour in crash situations - we assume that after
847  a succesful flush! all previous blocks are safe, but after a flush,
848  if some blocks make it, then all previous blocks must have. Eg,
849  writes are done in order and periodically auto-flushed, in
850  effect. This invariant is required for the file-cache to be safe
851  (see v1.0.2).
852
853* Lock the archive for writing in backend-splitlog, so that two
854  snapshots to the same archive don't collide. Do we lock per `put!`
855  to allow interleaving, or is that too inefficient? In which case, we
856  need to hold a lock that persists for a while, and release it
857  periodically to allow other writers to the same archive to have a
858  chance.
859
860* Make backend-splitlog write the current log file offset as well as
861  number into the metadata on each flush, and on startup, either
862  truncate the file to that position (to remove anything written but
863  not flushed to the metadata) or scan the log onwards from that point
864  to find (complete) blocks that did not get flushed to the metadata.
865
866* Make `lock-tag!` fail if the tag is already locked. Make the archive
867  block and retry a few times in that case.
868
869* Extend the backend protocol with a special "admin" command that
870  allows for arbitrary backend-specific operations, and write an
871  ugarit-backend-admin CLI tool to administer backends with it. The
872  input should be a single s-expression as a list, and the result
873  should be an alist which is displayed to the user in a friendly
874  manner, as "Key: Value\n" lines.
875
876* Implement "info" admin commands for all backends, that list any
877  available stats, and at least the backend type and parameters.
878
879* Support for recreating the index and tags on a backend-splitlog if
880  they get corrupted, from the headers left in the log, as a "reindex"
881  admin command.
882
883* Support for flushing the cache on a backend-cache, via an admin
884  command, rather than having to delete the cache file.
885
886* Support for unlinking in backend-splitlog, by marking byte ranges as
887  unused in the metadata (and by touching the headers in the log so we
888  maintain the invariant that the metadata is a reconstructible cache)
889  and removing the entries for the unlinked blocks, perhaps provide an
890  option to attempt to re-use existing holes to put blocks in for
891  online reuse, and provide an offline compaction operation. Keep
892  stats in the index of how many byte ranges are unused, and how many
893  bytes unused, in each file, and report them in the info admin
894  interface, along with the option to compact any or all files. We'll
895  need to store refcounts in the backend metadata (should we log
896  reuses, then, so the metadata can always be reconstructed, or just
897  set them to NULL on a reconstruct); when this is enabled on an
898  existing archive with no refcounts, default them to NULL, and treat
899  a NULL refcount as "infinity".
900
901* Have read-only and unlinkable and block size config flags in the
902  backend-split metadata file, settable via admin commands.
903
904* For people doing remote backups who want to not hog resources, write
905  a proxy backend that throttles bandwidth usage. Make it record the
906  time it last sent a request to the backend, and the number of bytes
907  read and written; then when a new request comes in, delay it until
908  at least the largest of (write bandwidth quota * bytes written) and
909  (read bandwidth quota * bytes read) seconds has passed since the
910  last request was sent. NOTE: Start the clock when SENDING, so the
911  time spent handling the request is already counting towards
912  bandwidth quotas, or it won't be fair.
913
914* Support for SFTP as a storage backend. Store one file per block, as
915  per `backend-fs`, but remotely. See
916  http://tools.ietf.org/html/draft-ietf-secsh-filexfer-13 for sftp
917  protocol specs; popen an `ssh -p sftp` connection to the server then
918  talk that simple binary protocol. Tada! Ideally make an sftp egg,
919  then a "ugarit-backend-sftp" egg to keep the dependencies optional.
920
921* Support for S3 as a storage backend. There is now an S3 egg! Make an
922  "ugarit-backend-s3" egg to keep the dependencies optional.
923
924* Support for replicated archives. This will involve a special storage
925  backend that can wrap any number of other archives, each tagged with
926  a trust percentage and read and write load weightings. Each block
927  will be uploaded to enough archives to make the total trust be at
928  least 100%, by randomly picking the archives weighted by their write
929  load weighting. A read-only archive automatically gets its write
930  load weighting set to zero, and a warning issued if it was
931  configured otherwise. A local cache will be kept of which backends
932  carry which blocks, and reads will be serviced by picking the
933  archive that carries it and has the highest read load weighting. If
934  that archive is unavailable or has lost the block, then they will be
935  tried in read load order; and if none of them have it, an exhaustive
936  search of all available archives will be performed before giving up,
937  and the cache updated with the results if the block is found. In
938  order to correctly handle archives that were unavailable during
939  this, we might need to log an "unknown" for that block key / archive
940  pair, rather than assuming the block is not there, and check it
941  later. Users will be given an admin command to notify the backend of
942  an archive going missing forever, which will cause it to be removed
943  from the cache. Affected blocks should be examined and re-replicated
944  if their replication count is now too low. Another command should be
945  available to warn of impending deliberate removal, which will again
946  remove the archive from the cluster and re-replicate, the difference
947  being that the disappearing archive is usable for re-replicating
948  FROM, so this is a safe operation for blocks that are only on that
949  one archive. The individual physical archives that we put
950  replication on top of won't be "valid" archives unless they are 100%
951  replicated, as they'll contain references to blocks that are on
952  other archives. It might be a good idea to mark them as such with a
953  special tag to avoid people trying to restore directly from them;
954  the frontend should complain if you attempt to directly use an
955  archive with the special tag in place. A copy of the replication
956  configuration could be stored under a special tag to mark this fact,
957  and to enable easy finding of the proper replicated archive to work
958  from. There should be a configurable option to snapshot the cache to
959  the archives whenever the replicated archive is closed, too. The
960  command line to the backend, "backend-replicated", should point to
961  an sqlite file for the configuration and cache, and users should use
962  admin commands to add/remove/modify archives in the cluster.
963
964## Core
965
966* Make `fold-archive-node`'s listing of tags at the top level report
967  the lock status of the tags.
968
969* More stats. Log bytes written AFTER compression and encryption in
970  `archive-put!`. Log snapshot start and end times in the snapshot
971  object.
972
973* SIGINFO support. Add a SIGINFO handler that sets a flag, and make
974  the `store-file!` and `store-directory!` main loops look for the
975  flag and, if set, display what path we're working on, and perhaps a
976  quick summary of the bytes/blocks stored/skipped stats.
977
978* Clarify what characters are legal in tag names sent to backends, and
979  what are legal in human-supplied tag names, and check that
980  human-supplied tag names match a regular expression. Leave space for
981  system-only tag names for storing archive metadata; suggest making a
982  hash sign illegal in tag names.
983
984* Clarify what characters are legal in block keys. Ugarit will only
985  issue [a-zA-Z0-9] for normal blocks, but may use other characters
986  (hash?) for special metadata blocks; establish a contract of what
987  backends must support (a-z, A-Z, 0-9, hash?)
988
989* API documentation for the modules we export
990
991* Encrypt tags, with a hash inside to check it's decrypted
992  correctly. Add a special "#ugarit-archive-format" tag that records a
993  format version number, to note that this change has been
994  applied. Provide an upgrade tool. Don't do auto-upgrades, or
995  attackers will be able to drop in plaintext tags.
996
997* Store a test block in the archive that is used to check the same
998  encryption and hash settings are used for an archive, consistently
999  (changing compression setting is supported, but changing encryption
1000  or hash will lead to confusion). Encrypt the hash of the passphrase
1001  and store it in the test block, which should have a name that cannot
1002  clash with any actual hash (eg, use non-hex characters in its
1003  name). When the block does not exist, create it; when it does exist,
1004  check it against the current encryption and hashing settings to see
1005  if it matches. When creating a new block, if the "prompt" passphrase
1006  specification mechanism is in use, prompt again to confirm the
1007  passphrase. If no encryption is in use, check the hash algorithm
1008  doesn't change by storing the hash of a constant string,
1009  unencrypted. To make brute-forcing the passphrase or hash-salt
1010  harder, consider applying the hash a large number of times, to
1011  increase the compute cost of checking it. Thanks to Andy Bennett for
1012  this idea.
1013
1014* More `.ugarit` actions. Right now we just have exclude and include;
1015  we might specify less-safe operations such as commands to run before
1016  and after snapshotting certain subtrees, or filters (don't send this
1017  SVN repository; instead send the output of `svnadmin dump`),
1018  etc. Running arbitrary commands is a security risk if random users
1019  write their own `.ugarit` files - so we'd need some trust-based
1020  mechanism; they'd need to be explicitly enabled in `ugarit.conf`,
1021  then a `.ugarit` option could disable all unsafe operations in a
1022  subtree.
1023
1024* `.ugarit` rules for file sizes. In particular, a rule to exclude
1025  files above a certain size. Thanks to Andy Bennett for this idea.
1026
1027* Support for FFS flags, Mac OS X extended filesystem attributes, NTFS
1028  ACLs/streams, FAT attributes, etc... Ben says to look at Box Backup
1029  for some code to do that sort of thing.
1030
1031* Deletion support - letting you remove snapshots. Perhaps you might
1032  want to remove all snapshots older than a given number of days on a
1033  given tag. Or just remove X out of Y snapshots older than a given
1034  number of days on a given tag. We have the core support for this;
1035  just find a snapshot and `unlink-directory!` its contents, leaving a
1036  dangling pointer from the snapshot, and write the snapshot handling
1037  code to expect this. Again, check Box Backup for that.
1038
1039* Option, when backing up, to not cross mountpoints
1040
1041* Option, when backing up, to store inode number and mountpoint path
1042  in directory entries, and then when extracting, keeping a dictionary
1043  of this unique identifier to pathname, so that if a file to be
1044  extracted is already in the dictionary and the hash is the same, a
1045  hardlink can be created.
1046
1047* Archival mode as well as snapshot mode. Whereas a snapshot record
1048  takes a filesystem tree and adds it to a chain of snapshots of the
1049  same filesystem tree, archival mode takes a filesystem tree and
1050  inserts it into a search tree anchored on the specified tag,
1051  indexing it on a list of key+value properties supplied at archival
1052  time. An archive tag is represented in the virtual filesystem as a
1053  directory full of archive objects, each identified by their full
1054  hash; each archive object references the filesystem root as well as
1055  the key+value properties, and optionally a parent link like a
1056  snapshot, as an archive can be made that explicitly replaces an
1057  earlier one and should replace it in the index; there is also a
1058  virtual directory for each indexed property which contains a
1059  directory for each value of the property, full of symlinks to the
1060  archive objects, and subdirectories that allow multi-property
1061  searches on other properties. The index itself is stored as a B-Tree
1062  with a reasonably small block size; when it's updated, the modified
1063  index blocks are replaced, thereby gaining new hashes, so their
1064  parents need replacing, all the way up the tree until a new root
1065  block is created. The existing block unlink mechanism in the
1066  backends will reclaim storage for blocks that are superceded, if the
1067  backend supports it. When this is done, ugarit will offer the option
1068  of snapshotting to a snapshot tag, or archiving to an archive tag,
1069  or archiving to an archive tag while replacing a specified archive
1070  object (nominated by path within the tag), which causes it to be
1071  removed from the index (except from the directory listing all
1072  archives by hash), and the new archive object is inserted,
1073  referencing the old one as a parent.
1074
1075* Dump/restore format. On a dump, walk an arbitrary subtree of an
1076  archive, serialising objects. Do not put any hashes in the dump
1077  format - dump out entire files, and just identify objects with
1078  sequential numbers when forming the directory / snapshot trees. On a
1079  restore, read the same format and slide it into an archive (creating
1080  any required top-level snapshot objects if the dump doesn't start
1081  from a snapshot) and putting it onto a specified tag. The
1082  intention is that this format can be used to migrate your stuff
1083  between archives, perhaps to change to a better backend.
1084
1085* Optional progress reporting callback from within store-file! and
1086  store-directory!, called on each block within a file or on each
1087  filesystem object, respectively.
1088
1089* Add a procedure to resolve a path within the archive node tree from
1090  any root node. Pass in the path as a list of strings, with the
1091  symbols `.` and `..` being usable as meta-characters to do nothing
1092  or to go up a level. Write a utility procedure to parse a string
1093  into such a form.
1094
1095## Front-end
1096
1097* Install progress reporting callbacks to report progress to user;
1098  option for quiet (no reporting), normal (reporting if >60s have
1099  passed since last time), or verbose (report every file), or very
1100  verbose (report every file and block).
1101
1102* Add a command to force removing a tag lock.
1103
1104* Add a command to list all the tags (with a * next to locked tags)
1105
1106* Add a command to list the contents of any directory in the archive
1107  node tree
1108
1109* Better error messages
1110
1111* API mode: Works something like the backend API, except at the
1112  archive level. Supports all the important archive operations, plus
1113  access to sexpr stream writers and key stream writers,
1114  archive-node-fold, etc. Requested by andyjpb, perhaps I can write
1115  the framework for this and then let him add API functions as he desires.
1116
1117* Command-line support to extract the contents of a given path in the
1118  archive, rather than needing to use explore mode. Also the option to
1119  extract given just a block key (useful when reading from keys logged
1120  manually at snapshot time).
1121
1122* FUSE/9p support. Mount it as a read-only filesystem :-D Then
1123  consider adding Fossil-style writing to the `current` of a snapshot,
1124  with copy-on-write of blocks to a buffer area on the local disk,
1125  then the option to make a snapshot of `current`. Put these into
1126  separate "ugarit-frontend-9p" and "ugarit-frontend-fuse" eggs, to
1127  control the dependencies.
1128
1129* Filesystem watching. Even with the hash-caching trick, a snapshot
1130  will still involve walking the entire directory tree and looking up
1131  every file in the hash cache. We can do better than that - some
1132  platforms provide an interface for receiving real-time notifications
1133  of changed or added files. Using this, we could allow ugarit to run
1134  in continuous mode, keeping a log of file notifications from the OS
1135  while it does an initial full snapshot. It can then wait for a
1136  specified period (one hour, perhaps?), accumulating names of files
1137  changed since it started, before then creating a new snapshot by
1138  uploading just the files it knows to have changed, while subsequent
1139  file change notifications go to a new list.
1140
1141## Testing
1142
1143* An option to verify a snapshot, walking every block in it checking
1144  there's no dangling references, and that everything matches its
1145  hash, without needing to put it into a filesystem, and applying any
1146  other sanity checks we can think of en route. Optionally compare it
1147  to an on-disk filesystem, while we're at it.
1148
1149* A unit test script around the `ugarit` command-line tool; the corpus
1150  should contain a mix of tiny and huge files and directories, awkward
1151  cases for sharing of blocks (many identical files in the same dir,
1152  etc), complex forms of file metadata, and so on. It should archive
1153  and restore the corpus several times over with each hash,
1154  compression, and encryption option.
1155
1156* Testing crashes. See about writing a test backend binary that either
1157  raises an error or just kills the process directly after N
1158  operations, and sit in a loop running it with increasing N. Take N
1159  from an environment variable to make it easier to automate this.
1160
1161* Extract the debugging backend from backend-devtools into a proper
1162  backend binary that takes a path to a log file and a backend command
1163  line to wrap.
1164
1165# Acknowledgements
1166
1167The original idea came from Venti, a content-addressed storage system
1168from Plan 9. Venti is usable directly by user applications, and is
1169also integrated with the Fossil filesystem to support snapshotting the
1170status of a Fossil filesystem. Fossil allows references to either be
1171to a block number on the Fossil partition or to a Venti key; so when a
1172filesystem has been snapshotted, all it now contains is a "root
1173directory" pointer into the Venti archive, and any files modified
1174therafter are copied-on-write into Fossil where they may be modified
1175until the next snapshot.
1176
1177We're nowhere near that exciting yet, but using FUSE, we might be able
1178to do something similar, which might be fun. However, Venti inspired
1179me when I read about it years ago; it showed me how elegant
1180content-addressed storage is. Finding out that the Git version control
1181system used the same basic tricks really just confirmed this for me.
1182
1183Also, I'd like to tip my hat to Duplicity. With the changing economics
1184of storage presented by services like Amazon S3 and rsync.net, I
1185looked to Duplicity as it provided both SFTP and S3 backends. However,
1186it worked in terms of full and incremental backups, a model that I
1187think made sense for magnetic tapes, but loses out to
1188content-addressed snapshots when you have random-access
1189media. Duplicity inspired me by its adoption of multiple backends, the
1190very backends I want to use, but I still hungered for a
1191content-addressed snapshot store.
1192
1193I'd also like to tip my hat to Box Backup. I've only used it a little,
1194because it requires a special server to manage the storage (and I want
1195to get my backups *off* of my servers), but it also inspires me with
1196directions I'd like to take Ugarit. It's much more aware of real-time
1197access to random-access storage than Duplicity, and has a very
1198interesting continuous background incremental backup mode, moving away
1199from the tape-based paradigm of backups as something you do on a
1200special day of the week, like some kind of religious observance. I
1201hope the author Ben, who is a good friend of mine, won't mind me
1202plundering his source code for details on how to request real-time
1203notification of changes from the filesystem, and how to read and write
1204extended attributes!
1205
1206Moving on from the world of backup, I'd like to thank the Chicken Team
1207for producing Chicken Scheme. Felix and the community at #chicken on
1208Freenode have particularly inspired me with their can-do attitudes to
1209combining programming-language elegance and pragmatic engineering -
1210two things many would think un-unitable enemies. Of course, they
1211didn't do it all themselves - R5RS Scheme and the SRFIs provided a
1212solid foundation to build on, and there's a cast of many more in the
1213Chicken community, working on other bits of Chicken or just egging
1214everyone on. And I can't not thank Henry Baker for writing the seminal
1215paper on the technique Chicken uses to implement full tail-calling
1216Scheme with cheap continuations on top of C; Henry already had my
1217admiration for his work on combining elegance and pragmatism in linear
1218logic. Why doesn't he return my calls? I even sent flowers.
1219
1220A special thanks should go to Christian Kellermann for porting Ugarit
1221to use Chicken 4 modules, too, which was otherwise a big bottleneck to
1222development, as I was stuck on Chicken 3 for some time! And to Andy
1223Bennett for many insightful conversations about future directions.
1224
1225Thanks to the early adopters who brought me useful feedback, too!
1226
1227And I'd like to thank my wife for putting up with me spending several
1228evenings and weekends and holiday days working on this thing...
1229
1230# Version history
1231
1232* 1.0.2: Made the file cache also commit periodically, rather than on
1233  every write, in order to improve performance. Counting blocks and
1234  bytes uploaded / reused, and file cache bytes as well as hits;
1235  reporting same in snapshot UI and logging same to snapshot
1236  metadata. Switched to the `posix-extras` egg and ditched our own
1237  `posixextras.scm` wrappers. Used the `parley` egg in the `ugarit
1238  explore` CLI for line editing. BUGFIX: Made file cache check the
1239  file hashes it finds in the cache actually exist in the archive, to
1240  protect against the case where a crash of some kind has caused
1241  unflushed changes to be lost; the file cache may well have committed
1242  changes that the backend hasn't, leading to references to
1243  nonexistant blocks. Note that we assume that archives are
1244  sequentially safe, eg if the final indirect block of a large file
1245  made it, all the partial blocks must have made it too. BUGFIX: Added
1246  an explicit `flush!` command to the backend protocol, and put
1247  explicit flushes at critical points in higher layers
1248  (`backend-cache`, the archive abstraction in the Ugarit core, and
1249  when tagging a snapshot) so that we ensure the blocks we point at
1250  are flushed before committing references to them in the
1251  `backend-cache` or file caches, or into tags, to ensure crash
1252  safety. BUGFIX: Made the splitlog backend never exceed the file size
1253  limit (except when passed blocks that, plus a header, are larger
1254  than it), rather than letting a partial block hang over the
1255  'end'. BUGFIX: Fixed tag locking, which was broken all over the
1256  place. Concurrent snapshots to the same tag should now block for one
1257  another, although why you'd want to *do* that is questionable.
1258
1259* 1.0.1: Consistency check on read blocks by default. Removed warning
1260  about deletions from backend-cache; we need a new mechanism to
1261  report warnings from backends to the user. Made backend-cache and
1262  backend-fs/splitlog commit periodically rather than after every
1263  insert, which should speed up snapshotting a lot, and reused the
1264  prepared statements rather than re-preparing them all the
1265  time. BUGFIX: splitlog backend now creates log files with
1266  "rw-------" rather than "rwx------" permissions; and all sqlite
1267  databases (splitlog metadata, cache file, and file-cache file) are
1268  created with "rw-------" rather then "rw-r--r--".
1269
1270* 1.0: Migrated from gdbm to sqlite for metadata storage, removing the
1271  GPL taint. Unit test suite. backend-cache made into a separate
1272  backend binary. Removed backend-log. BUGFIX: file caching uses mtime *and*
1273  size now, rather than just mtime. Error handling so we skip objects
1274  that we cannot do something with, and proceed to try the rest of the
1275  operation.
1276
1277* 0.8: decoupling backends from the core and into separate binaries,
1278  accessed via standard input and output, so they can be run over SSH
1279  tunnels and other such magic.
1280
1281* 0.7: file cache support, sorting of directories so they're archived
1282  in canonical order, autoloading of hash/encryption/compression
1283  modules so they're not required dependencies any more.
1284
1285* 0.6: .ugarit support.
1286
1287* 0.5: Keyed hashing so attackers can't tell what blocks you have,
1288  markers in logs so the index can be reconstructed, sha2 support, and
1289  passphrase support.
1290
1291* 0.4: AES encryption.
1292
1293* 0.3: Added splitlog backend, and fixed a .meta file typo.
1294
1295* 0.2: Initial public release.
1296
1297* 0.1: Internal development release.
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