Ugarit
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Introduction

Ugarit is a backup/archival system based around content-addressible storage.

News

Current development priority: Performance, better error handling, and fixing bugs! After I've cleaned house a little, I'll be focussing on replicated backend storage (ticket [f1f2ce8cdc]), as I now have a cluster of storage devices at home.

About Ugarit

What's content-addressible storage?

Traditional backup systems work by storing copies of your files somewhere. Perhaps they go onto tapes, or perhaps they're in archive files written to disk. They will either be full dumps, containing a complete copy of your files, or incrementals or differentials, which only contain files that have been modified since some point. This saves making repeated copies of unchanging files, but it means that to do a full restore, you need to start by extracting the last full dump then applying one or more incrementials, or the latest differential, to get the latest state.

Not only do differentials and incrementals let you save space, they also give you a history - you can restore up to a previous point in time, which is invaluable if the file you want to restore was deleted a few backup cycles ago!

This technology was developed when the best storage technology for backups was magnetic tape, because each dump is written sequentially (and restores are largely sequential, unless you're skipping bits to pull out specific files).

However, these days, random-access media such as magnetic disks and SSDs are cheap enough to compete with magnetic tape for long-term bulk storage (especially when one considers the cost of a tape drive or two). And having fast random access means we can take advantage of different storage techniques.

A content-addressible store is a key-value store, except that the keys are always computed from the values. When a given object is stored, it is hashed, and the hash used as the key. This means you can never store the same object twice; the second time you'll get the same hash, see the object is already present, and re-use the existing copy. Therefore, you get deduplication of your data for free.

But how do you find things again, if you can't choose the keys?

When an object is stored, you need to record the key so you can find it again later. In Ugarit, we are storing a tree-like directory structure. Files are uploaded and their hashes obtained, and then a directory object is constructed containing a list of the files in the directory, and listing the key of the Ugarit objects storing the contents of each file. This directory object itself has a hash, which is stored inside the directory entry in the parent directory, and so on up to the root. The root of a tree stored in a Ugarit vault has no parent directory to contain it, so at that point, we store the key of the root in a named "tag" that we can look up by name when we want it.

Therefore, everything in a Ugarit vault can be found by starting with a named tag and retrieving the object whose key it contains, then finding keys inside that object and looking up the objects they refer to, until we find the object we want.

When you use Ugarit to backup your filesystem, it uploads a complete snapshot of every file in the filesystem, like a full dump. But because the vault is content-addressed, it automatically avoids uploading anything it already has a copy of, so all we upload is an incremental dump - but in the vault, it looks like a full dump, and so can be restored on its own without having to restore a chain of incrementals.

Also, the same storage can be shared between multiple systems that all back up to it - and the incremental upload algorithm will mean that any files shared between the servers will only need to be uploaded once. If you back up a complete server, than go and back up another that is running the same distribution, then all the files in /bin and so on that are already in the storage will not need to be backed up again; the system will automatically spot that they're already there, and not upload them again.

So what's that mean in practice?

You can run Ugarit to back up any number of filesystems to a shared storage area (known as a vault, and on every backup, Ugarit will only upload files or parts of files that aren't already in the vault - be they from the previous snapshot, earlier snapshots, snapshot of entirely unrelated filesystems, etc. Every time you do a snapshot, Ugarit builds an entire complete directory tree of the snapshot in the vault - but reusing any parts of files, files, or entire directories that already exist anywhere in the vault, and only uploading what doesn't already exist.

The support for parts of files means that, in many cases, gigantic files like database tables and virtual disks for virtual machines will not need to be uploaded entirely every time they change, as the changed sections will be identified and uploaded.

Because a complete directory tree exists in the vault for any snapshot, the extraction algorithm is incredibly simple - and, therefore, incredibly reliable and fast. Simple, reliable, and fast are just what you need when you're trying to reconstruct the filesystem of a live server.

Also, it means that you can do lots of small snapshots. If you run a snapshot every hour, then only a megabyte or two might have changed in your filesystem, so you only upload a megabyte or two - yet you end up with a complete history of your filesystem at hourly intervals in the vault.

Conventional backup systems usually either store a full backup then incrementals to their archives, meaning that doing a restore involves reading the full backup then reading every incremental since and applying them - so to do a restore, you have to download *every version* of the filesystem you've ever uploaded, or you have to do periodic full backups (even though most of your filesystem won't have changed since the last full backup) to reduce the number of incrementals required for a restore. Better results are had from systems that use a special backup server to look after the archive storage, which accept incremental backups and apply them to the snapshot they keep in order to maintain a most-recent snapshot that can be downloaded in a single run; but they then restrict you to using dedicated servers as your archive stores, ruling out cheaply scalable solutions like Amazon S3, or just backing up to a removable USB or eSATA disk you attach to your system whenever you do a backup. And dedicated backup servers are complex pieces of software; can you rely on something complex for the fundamental foundation of your data security system?

System Requirements

Ugarit should run on any POSIX-compliant system that can run Chicken Scheme. It stores and restores all the file attributes reported by the stat system call - POSIX mode permissions, UID, GID, mtime, and optionally atime and ctime (although the ctime cannot be restored due to POSIX restrictions). Ugarit will store files, directories, device and character special files, symlinks, and FIFOs.

Support for extended filesystem attributes - ACLs, alternative streams, forks and other metadata - is possible, due to the extensible directory entry format; support for such metadata will be added as required.

Currently, only local filesystem-based vault storage backends are complete: these are suitable for backing up to a removable hard disk or a filesystem shared via NFS or other protocols. However, the backend can be accessed via an SSH tunnel, so a remote server you are able to install Ugarit on to run the backends can be used as a remote vault.

However, the next backend to be implemented will be one for Amazon S3, and an SFTP backend for storing vaults anywhere you can ssh to. Other backends will be implemented on demand; a vault can, in principle, be stored on anything that can store files by name, report on whether a file already exists, and efficiently download a file by name. This rules out magnetic tapes due to their requirement for sequential access.

Although we need to trust that a backend won't lose data (for now), we don't need to trust the backend not to snoop on us, as Ugarit optionally encrypts everything sent to the vault.

Terminology

A Ugarit backend is the software module that handles backend storage. An actual storage area - an instance of a backend is called a storage, and is used to implement a vault; currently, every storage is a valid vault, but the planned future introduction of a distributed storage backend will enable multiple storages (which are not, themselves, valid vaults as they only contain some subset of the information required) to be combined into an aggregrate storage, which then holds the actual vault. Note that the contents of a storage is purely a set of blocks, and a series of named tags containing references to them; the storage does not know the details of encryption and hashing, so cannot make any sense of its contents.

For example, if you use the recommended "splitlog" filesystem backend, your vault might be /mnt/bigdisk on the server prometheus. The backend (which is compiled along with the other filesystem backends in the backend-fs binary) must be installed on prometheus, and Ugarit clients all over the place may then use it via ssh to prometheus. However, even with the filesystem backends, the actual storage might not be on prometheus where the backend runs - /mnt/bigdisk might be an NFS mount, or a mount from a storage-area network. This ability to delegate via SSH is particularly useful with the "cache" backend, which reduces latency by storing a cache of what blocks exist in a backend, thereby making it quicker to identify already-stored files; a cluster of servers all sharing the same vault might all use SSH tunnels to access an instance of the "cache" backend on one of them (using some local disk to store the cache), which proxies the actual vault storage to a vault on the other end of a high-latency Internet link, again via an SSH tunnel.

A vault is where Ugarit stores backups (as chains of snapshots) and, in future, archives (as chains of archive deltas). Backups and archives are identified by tags, which are the top-level named entry points into a vault. A vault is based on top of a storage, along with a choice of hash function, compression algorithm, and encryption that are used to map the logical world of snapshots and archive deltas into the physical world of blocks stored in the storage.

A snapshot is a copy of a filesystem tree in the vault, with a header block that gives some metadata about it. A backup consists of a number of snapshots of a given filesystem.

An archive delta is a set of filesystem trees, each along with metadata about it. Whereas a backup is organised around a series of timed snapshots, an archive is organised around the metadata; the filesystem trees in the archive are identified by their properties.

What's in a vault?

A Ugarit vault contains a load of blocks, each up to a maximum size (usually 1MiB, although other backends might impose smaller limits). Each block is identified by the hash of its contents; this is how Ugarit avoids ever uploading the same data twice, by checking to see if the data to be uploaded already exists in the vault by looking up the hash. The contents of the blocks are compressed and then encrypted before upload.

Every file uploaded is, unless it's small enough to fit in a single block, chopped into blocks, and each block uploaded. This way, the entire contents of your filesystem can be uploaded - or, at least, only the parts of it that aren't already there! The blocks are then tied together to create a snapshot by uploading blocks full of the hashes of the data blocks, and directory blocks are uploaded listing the names and attributes of files in directories, along with the hashes of the blocks that contain the files' contents. Even the blocks that contain lists of hashes of other blocks are subject to checking for pre-existence in the vault; if only a few MiB of your hundred-GiB filesystem has changed, then even the index blocks and directory blocks are re-used from previous snapshots.

Once uploaded, a block in the vault is never again changed. After all, if its contents changed, its hash would change, so it would no longer be the same block! However, every block has a reference count, tracking the number of index blocks that refer to it. This means that the vault knows which blocks are shared between multiple snapshots (or shared *within* a snapshot - if a filesystem has more than one copy of the same file, still only one copy is uploaded), so that if a given snapshot is deleted, then the blocks that only that snapshot is using can be deleted to free up space, without corrupting other snapshots by deleting blocks they share. Keep in mind, however, that not all storage backends may support this - there are certain advantages to being an append-only vault. For a start, you can't delete something by accident! The supplied fs backend supports deletion, while the splitlog backend does not yet. However, the actual snapshot deletion command hasn't been implemented yet either, so it's a moot point for now...

Finally, the vault contains objects called tags. Unlike the blocks, the tags contents can change, and they have meaningful names rather than being identified by hash. Tags identify the top-level blocks of snapshots within the system, from which (by following the chain of hashes down through the index blocks) the entire contents of a snapshot may be found. Unless you happen to have recorded the hash of a snapshot somewhere, the tags are where you find snapshots from when you want to do a restore!

Whenever a snapshot is taken, as soon as Ugarit has uploaded all the files, directories, and index blocks required, it looks up the tag you have identified as the target of the snapshot. If the tag already exists, then the snapshot it currently points to is recorded in the new snapshot as the "previous snapshot"; then the snapshot header containing the previous snapshot hash, along with the date and time and any comments you provide for the snapshot, and is uploaded (as another block, identified by its hash). The tag is then updated to point to the new snapshot.

This way, each tag actually identifies a chronological chain of snapshots. Normally, you would use a tag to identify a filesystem being backed up; you'd keep snapshotting the filesystem to the same tag, resulting in all the snapshots of that filesystem hanging from the tag. But if you wanted to remember any particular snapshot (perhaps if it's the snapshot you take before a big upgrade or other risky operation), you can duplicate the tag, in effect 'forking' the chain of snapshots much like a branch in a version control system.

Using Ugarit

Installation

Install Chicken Scheme using their installation instructions.

Ugarit can then be installed by typing (as root):

chicken-install ugarit

See the chicken-install manual for details if you have any trouble, or wish to install into your home directory.

Setting up a vault

Firstly, you need to know the vault identifier for the place you'll be storing your vaults. This depends on your backend. The vault identifier is actually the command line used to invoke the backend for a particular vault; communication with the vault is via standard input and output, which is how it's easy to tunnel via ssh.

Local filesystem backends

These backends use the local filesystem to store the vaults. Of course, the "local filesystem" on a given server might be an NFS mount or mounted from a storage-area network.

Logfile backend

The logfile backend works much like the original Venti system. It's append-only - you won't be able to delete old snapshots from a logfile vault, even when I implement deletion. It stores the vault in two sets of files; one is a log of data blocks, split at a specified maximum size, and the other is the metadata: an sqlite database used to track the location of blocks in the log files, the contents of tags, and a count of the logs so a filename can be chosen for a new one.

To set up a new logfile vault, just choose where to put the two parts. It would be nice to put the metadata file on a different physical disk to the logs directory, to reduce seeking. If you only have one disk, you can put the metadata file in the log directory ("metadata" is a good name).

You can then refer to it using the following vault identifier:

"backend-fs splitlog ...log directory... ...metadata file... max-logfile-size"

For most platforms, a max-logfile-size of 900000000 (900 MB) should suffice. For now, don't go much bigger than that on 32-bit systems until Chicken's file-position function is fixed to work with files more than 1GB in size.

Filesystem backend

The filesystem backend creates vaults by storing each block or tag in its own file, in a directory. To keep the objects-per-directory count down, it'll split the files into subdirectories. Because of this, it uses a stupendous number of inodes (more than the filesystem being backed up). Only use it if you don't mind that; splitlog is much more efficient.

To set up a new filesystem-backend vault, just create an empty directory that Ugarit will have write access to when it runs. It will probably run as root in order to be able to access the contents of files that aren't world-readable (although that's up to you), so be careful of NFS mounts that have maproot=nobody set!

You can then refer to it using the following vault identifier:

"backend-fs fs ...path to directory..."

Proxying backends

These backends wrap another vault identifier which the actual storage task is delegated to, but add some value along the way.

SSH tunnelling

It's easy to access a vault stored on a remote server. The caveat is that the backend then needs to be installed on the remote server! Since vaults are accessed by running the supplied command, and then talking to them via stdin and stdout, the vault identified needs only be:

"ssh ...hostname... '...remote vault identifier...'"

Cache backend

The cache backend is used to cache a list of what blocks exist in the proxied backend, so that it can answer queries as to the existance of a block rapidly, even when the proxied backend is on the end of a high-latency link (eg, the Internet). This should speed up snapshots, as existing files are identified by asking the backend if the vault already has them.

The cache backend works by storing the cache in a local sqlite file. Given a place for it to store that file, usage is simple:

"backend-cache ...path to cachefile... '...proxied vault identifier...'"

The cache file will be automatically created if it doesn't already exist, so make sure there's write access to the containing directory.

- WARNING - WARNING - WARNING - WARNING - WARNING - WARNING -

If you use a cache on a vault shared between servers, make sure that you either:

or

If a block is deleted from a vault, and a cache on that vault is not aware of the deletion (as it did not go "through" the caching proxy), then the cache will record that the block exists in the vault when it does not. This will mean that if a snapshot is made through the cache that would use that block, then it will be assumed that the block already exists in the vault when it does not. Therefore, the block will not be uploaded, and a dangling reference will result!

Some setups which *are* safe:

Writing a ugarit.conf

ugarit.conf should look something like this:

(storage <vault identifier>)
(hash tiger "<salt>")
[double-check]
[(compression [deflate|lzma])]
[(encryption aes <key>)]
[(file-cache "<path>")]
[(rule ...)]

The hash line chooses a hash algorithm. Currently Tiger-192 (tiger), SHA-256 (sha256), SHA-384 (sha384) and SHA-512 (sha512) are supported; if you omit the line then Tiger will still be used, but it will be a simple hash of the block with the block type appended, which reveals to attackers what blocks you have (as the hash is of the unencrypted block, and the hash is not encrypted). This is useful for development and testing or for use with trusted vaults, but not advised for use with vaults that attackers may snoop at. Providing a salt string produces a hash function that hashes the block, the type of block, and the salt string, producing hashes that attackers who can snoop the vault cannot use to find known blocks (see the "Security model" section below for more details).

I would recommend that you create a salt string from a secure entropy source, such as:

dd if=/dev/random bs=1 count=64 | base64 -w 0

Whichever hash function you use, you will need to install the required Chicken egg with one of the following commands:

chicken-install -s tiger-hash # for tiger chicken-install -s sha2 # for the SHA hashes

double-check, if present, causes Ugarit to perform extra internal consistency checks during backups, which will detect bugs but may slow things down.

lzma is the recommended compression option for low-bandwidth backends or when space is tight, but it's very slow to compress; deflate or no compression at all are better for fast local vaults. To have no compression at all, just remove the (compression ...) line entirely. Likewise, to use compression, you need to install a Chicken egg:

chicken-install -s z3 # for deflate chicken-install -s lzma # for lzma

WARNING: The lzma egg is currently rather difficult to install, and needs rewriting to fix this problem.

Likewise, the (encryption ...) line may be omitted to have no encryption; the only currently supported algorithm is aes (in CBC mode) with a key given in hex, as a passphrase (hashed to get a key), or a passphrase read from the terminal on every run. The key may be 16, 24, or 32 bytes for 128-bit, 192-bit or 256-bit AES. To specify a hex key, just supply it as a string, like so:

(encryption aes "00112233445566778899AABBCCDDEEFF")

...for 128-bit AES,

(encryption aes "00112233445566778899AABBCCDDEEFF0011223344556677")

...for 192-bit AES, or

(encryption aes "00112233445566778899AABBCCDDEEFF00112233445566778899AABBCCDDEEFF")

...for 256-bit AES.

Alternatively, you can provide a passphrase, and specify how large a key you want it turned into, like so:

(encryption aes (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"))

I would recommend that you generate a long passphrase from a secure entropy source, such as:

dd if=/dev/random bs=1 count=64 | base64 -w 0

Finally, the extra-paranoid can request that Ugarit prompt for a passphrase on every run and hash it into a key of the specified length, like so:

(encryption aes (24|32 prompt))

(note the lack of quotes around prompt, distinguishing it from a passphrase)

Please read the "Security model" section below for details on the implications of different encryption setups.

Again, as it is an optional feature, to use encryption, you must install the appropriate Chicken egg:

chicken-install -s aes

A file cache, if enabled, significantly speeds up subsequent snapshots of a filesystem tree. The file cache is a file (which Ugarit will create if it doesn't already exist) mapping filenames to (mtime,size,hash) tuples; as it scans the filesystem, if it finds a file in the cache and the mtime and size have not changed, it will assume it is already stored under the specified hash. This saves it from having to read the entire file to hash it and then check if the hash is present in the vault. In other words, if only a few files have changed since the last snapshot, then snapshotting a directory tree becomes an O(N) operation, where N is the number of files, rather than an O(M) operation, where M is the total size of files involved.

For example:

(storage "ssh ugarit@spiderman 'backend-fs splitlog /mnt/ugarit-data /mnt/ugarit-metadata/metadata 900000000'") (hash tiger "i3HO7JeLCSa6Wa55uqTRqp4jppUYbXoxme7YpcHPnuoA+11ez9iOIA6B6eBIhZ0MbdLvvFZZWnRgJAzY8K2JBQ") (encryption aes (32 "FN9m34J4bbD3vhPqh6+4BjjXDSPYpuyskJX73T1t60PP0rPdC3AxlrjVn4YDyaFSbx5WRAn4JBr7SBn2PLyxJw")) (compression lzma) (file-cache "/var/ugarit/cache")

Be careful to put a set of parentheses around each configuration entry. White space isn't significant, so feel free to indent things and wrap them over lines if you want.

Keep copies of this file safe - you'll need it to do extractions! Print a copy out and lock it in your fire safe! Ok, currently, you might be able to recreate it if you remember where you put the storage, but encryption keys and hash salts are harder to remember...

Your first backup

Think of a tag to identify the filesystem you're backing up. If it's /home on the server gandalf, you might call it gandalf-home. If it's the entire filesystem of the server bilbo, you might just call it bilbo.

Then from your shell, run (as root):

# ugarit snapshot <ugarit.conf> -c -a <tag> <path to root of filesystem>

For example, if we have a ugarit.conf in the current directory:

# ugarit snapshot ugarit.conf -c localhost-etc /etc

Specify the -c flag if you want to store ctimes in the vault; since it's impossible to restore ctimes when extracting from an vault, doing this is useful only for informational purposes, so it's not done by default. Similarly, atimes aren't stored in the vault unless you specify -a, because otherwise, there will be a lot of directory blocks uploaded on every snapshot, as the atime of every file will have been changed by the previous snapshot - so with -a specified, on every snapshot, every directory in your filesystem will be uploaded! Ugarit will happily restore atimes if they are found in a vault; their storage is made optional simply because uploading them is costly and rarely useful.

Exploring the vault

Now you have a backup, you can explore the contents of the vault. This need not be done as root, as long as you can read ugarit.conf; however, if you want to extract files, run it as root so the uids and gids can be set.

$ ugarit explore <ugarit.conf>

This will put you into an interactive shell exploring a virtual filesystem. The root directory contains an entry for every tag; if you type ls you should see your tag listed, and within that tag, you'll find a list of snapshots, in descending date order, with a special entry current for the most recent snapshot. Within a snapshot, you'll find the root directory of your snapshot under "contents", and will be able to cd into subdirectories, and so on:

> ls Test <tag> > cd Test /Test> ls 2009-01-24 10:28:16 <snapshot> 2009-01-24 10:28:16 <snapshot> current <snapshot> /Test> cd current /Test/current> ls contents <dir> /Test/current> cd contents /Test/current/contents> ls README.txt <file> LICENCE.txt <symlink> subdir <dir> .svn <dir> FIFO <fifo> chardev <character-device> blockdev <block-device> /Test/current/contents> ls -ll LICENCE.txt lrwxr-xr-x 1000 100 2009-01-15 03:02:49 LICENCE.txt -> subdir/LICENCE.txt target: subdir/LICENCE.txt ctime: 1231988569.0

As well as exploring around, you can also extract files or directories (or entire snapshots) by using the get command. Ugarit will do its best to restore the metadata of files, subject to the rights of the user you run it as.

Type help to get help in the interactive shell.

The interactive shell supports command-line editing, history and tab completion for your convenience.

Extracting things directly

As well as using the interactive explore mode, it is also possible to directly extract something from the vault, given a path.

Given the sample vault from the previous example, it would be possible to extract the README.txt file with the following command:

ugarit extract ugarit.conf /Test/current/contents/README.txt

Duplicating tags

As mentioned above, you can duplicate a tag, creating two tags that refer to the same snapshot and its history but that can then have their own subsequent history of snapshots applied to each independently, with the following command:

$ ugarit fork <ugarit.conf> <existing tag> <new tag>

Storage administration

Each backend offers a number of administrative commands for administering the storage underlying vaults. These are accessible via the ugarit-storage-admin command line interface.

To use it, run it with the following command:

$ ugarit-storage-admin '<vault identifier>'

The available commands differ between backends, but all backends support the info and help commands, which give basic information about the vault, and list all available commands, respectively. Some offer a stats command that examines the vault state to give interesting statistics, but which may be a time-consuming operation.

Administering splitlog vaults

The splitlog backend offers a wide selection of administrative commands. See the help command on a splitlog vault for details. The following facilities are available:

* Configuring the frequency of automatic synching of the vault state to disk. Lowering this harms performance when writing to the vault, but decreases the number of in-progress block writes that can fail in a crash.
* Enable or disable write protection of the vault
* Reindex the vault, rebuilding the block and tag state from the contents of the log. If the metadata file is damaged or lost, reindexing can rebuild it (although any configuration changes made via other admin commands will need manually repeating as they are not logged).

.ugarit files

By default, Ugarit will vault everything it finds in the filesystem tree you tell it to snapshot. However, this might not always be desired; so we provide the facility to override this with .ugarit files, or global rules in your .conf file.

Note: The syntax of these files is provisional, as I want to experiment with usability, as the current syntax is ugly. So please don't be surprised if the format changes in incompatible ways in subsequent versions!

In quick summary, if you want to ignore all files or directories matching a glob in the current directory and below, put the following in a .ugarit file in that directory:

(* (glob "*~") exclude)

You can write quite complex expressions as well as just globs. The full set of rules is:

Also, you can override a previous exclusion with an explicit include in a lower-level directory:

(* (glob "*~") include)

You can bind rules to specific directories, rather than to "this directory and all beneath it", by specifying an absolute or relative path instead of the `*`:

("/etc" (name "passwd") exclude)

If you use a relative path, it's taken relative to the directory of the .ugarit file.

You can also put some rules in your .conf file, although relative paths are illegal there, by adding lines of this form to the file:

(rule * (glob "*~") exclude)

Questions and Answers

What happens if a snapshot is interrupted?

Nothing! Whatever blocks have been uploaded will be uploaded, but the snapshot is only added to the tag once the entire filesystem has been snapshotted. So just start the snapshot again. Any files that have already be uploaded will then not need to be uploaded again, so the second snapshot should proceed quickly to the point where it failed before, and continue from there.

Unless the vault ends up with a partially-uploaded corrupted block due to being interrupted during upload, you'll be fine. The filesystem backend has been written to avoid this by writing the block to a file with the wrong name, then renaming it to the correct name when it's entirely uploaded.

Actually, there is *one* caveat: blocks that were uploaded, but never make it into a finished snapshot, will be marked as "referenced" but there's no snapshot to delete to un-reference them, so they'll never be removed when you delete snapshots. (Not that snapshot deletion is implemented yet, mind). If this becomes a problem for people, we could write a "garbage collect" tool that regenerates the reference counts in a vault, leading to unused blocks (with a zero refcount) being unlinked.

Should I share a single large vault between all my filesystems?

I think so. Using a single large vault means that blocks shared between servers - eg, software installed from packages and that sort of thing - will only ever need to be uploaded once, saving storage space and upload bandwidth. However, do not share a vault between servers that do not mutually trust each other, as they can all update the same tags, so can meddle with each other's snapshots - and read each other's snapshots.

CAVEAT

It's not currently safe to have multiple concurrent snapshots to the same split log backend; this will soon be fixed, however.

Security model

I have designed and implemented Ugarit to be able to handle cases where the actual vault storage is not entirely trusted.

However, security involves tradeoffs, and Ugarit is configurable in ways that affect its resistance to different kinds of attacks. Here I will list different kinds of attack and explain how Ugarit can deal with them, and how you need to configure it to gain that protection.

Vault snoopers

This might be somebody who can intercept Ugarit's communication with the vault at any point, or who can read the vault itself at their leisure.

Ugarit's splitlog backend creates files with "rw-------" permissions out of the box to try and prevent this. This is a pain for people who want to share vaults between UIDs, but we can add a configuration option to override this if that becomes a problem.

Reading your data

If you enable encryption, then all the blocks sent to the vault are encrypted using a secret key stored in your Ugarit configuration file. As long as that configuration file is kept safe, and the AES algorithm is secure, then attackers who can snoop the vault cannot decode your data blocks. Enabling compression will also help, as the blocks are compressed before encrypting, which is thought to make cryptographic analysis harder.

Recommendations: Use compression and encryption when there is a risk of vault snooping. Keep your Ugarit configuration file safe using UNIX file permissions (make it readable only by root), and maybe store it on a removable device that's only plugged in when required. Alternatively, use the "prompt" passphrase option, and be prompted for a passphrase every time you run Ugarit, so it isn't stored on disk anywhere.

Looking for known hashes

A block is identified by the hash of its content (before compression and encryption). If an attacker was trying to find people who own a particular file (perhaps a piece of subversive literature), they could search Ugarit vaults for its hash.

However, Ugarit has the option to "key" the hash with a "salt" stored in the Ugarit configuration file. This means that the hashes used are actually a hash of the block's contents *and* the salt you supply. If you do this with a random salt that you keep secret, then attackers can't check your vault for known content just by comparing the hashes.

Recommendations: Provide a secret string to your hash function in your Ugarit configuration file. Keep the Ugarit configuration file safe, as per the advice in the previous point.

Vault modifiers

These folks can modify Ugarit's writes into the vault, its reads back from the vault, or can modify the vault itself at their leisure.

Modifying an encrypted block without knowing the encryption key can at worst be a denial of service, corrupting the block in an unknown way. An attacker who knows the encryption key could replace a block with valid-seeming but incorrect content. In the worst case, this could exploit a bug in the decompression engine, causing a crash or even an exploit of the Ugarit process itself (thereby gaining the powers of a process inspector, as documented below). We can but hope that the decompression engine is robust. Exploits of the decryption engine, or other parts of Ugarit, are less likely due to the nature of the operations performed upon them.

However, if a block is modified, then when Ugarit reads it back, the hash will no longer match the hash Ugarit requested, which will be detected and an error reported. The hash is checked after decryption and decompression, so this check does not protect us against exploits of the decompression engine.

This protection is only afforded when the hash Ugarit asks for is not tampered with. Most hashes are obtained from within other blocks, which are therefore safe unless that block has been tampered with; the nature of the hash tree conveys the trust in the hashes up to the root. The root hashes are stored in the vault as "tags", which an vault modifier could alter at will. Therefore, the tags cannot be trusted if somebody might modify the vault. This is why Ugarit prints out the snapshot hash and the root directory hash after performing a snapshot, so you can record them securely outside of the vault.

The most likely threat posed by vault modifiers is that they could simply corrupt or delete all of your vault, without needing to know any encryption keys.

Recommendations: Secure your vaults against modifiers, by whatever means possible. If vault modifiers are still a potential threat, write down a log of your root directory hashes from each snapshot, and keep it safe. When extracting your backups, use the ls -ll command in the interface to check the "contents" hash of your snapshots, and check they match the root directory hash you expect.

Process inspectors

These folks can attach debuggers or similar tools to running processes, such as Ugarit itself.

Ugarit backend processes only see encrypted data, so people who can attach to that process gain the powers of vault snoopers and modifiers, and the same conditions apply.

People who can attach to the Ugarit process itself, however, will see the original unencrypted content of your filesystem, and will have full access to the encryption keys and hashing keys stored in your Ugarit configuration. When Ugarit is running with sufficient permissions to restore backups, they will be able to intercept and modify the data as it comes out, and probably gain total write access to your entire filesystem in the process.

Recommendations: Ensure that Ugarit does not run under the same user ID as untrusted software. In many cases it will need to run as root in order to gain unfettered access to read the filesystems it is backing up, or to restore the ownership of files. However, when all the files it backs up are world-readable, it could run as an untrusted user for backups, and where file ownership is trivially reconstructible, it can do restores as a limited user, too.

Attackers in the source filesystem

These folks create files that Ugarit will back up one day. By having write access to your filesystem, they already have some level of power, and standard Unix security practices such as storage quotas should be used to control them. They may be people with logins on your box, or more subtly, people who can cause servers to writes files; somebody who sends an email to your mailserver will probably cause that message to be written to queue files, as will people who can upload files via any means.

Such attackers might use up your available storage by creating large files. This creates a problem in the actual filesystem, but that problem can be fixed by deleting the files. If those files get stored into Ugarit, then they are a part of that snapshot. If you are using a backend that supports deletion, then (when I implement snapshot deletion in the user interface) you could delete that entire snapshot to recover the wasted space, but that is a rather serious operation.

More insidiously, such attackers might attempt to abuse a hash collision in order to fool the vault. If they have a way of creating a file that, for instance, has the same hash as your shadow password file, then Ugarit will think that it already has that file when it attempts to snapshot it, and store a reference to the existing file. If that snapshot is restored, then they will receive a copy of your shadow password file. Similarly, if they can predict a future hash of your shadow password file, and create a shadow password file of their own (perhaps one giving them a root account with a known password) with that hash, they can then wait for the real shadow password file to have that hash. If the system is later restored from that snapshot, then their chosen content will appear in the shadow password file. However, doing this requires a very fundamental break of the hash function being used.

Recommendations: Think carefully about who has write access to your filesystems, directly or indirectly via a network service that stores received data to disk. Enforce quotas where appropriate, and consider not backing up "queue directories" where untrusted content might appear; migrate incoming content that passes acceptance tests to an area that is backed up. If necessary, the queue might be backed up to a non-snapshotting system, such as rsyncing to another server, so that any excessive files that appear in there are removed from the backup in due course, while still affording protection.

Acknowledgements

The Ugarit implementation contained herein is the work of Alaric Snell-Pym and Christian Kellermann, with advice, ideas, encouragement and guidance from many.

The original idea came from Venti, a content-addressed storage system from Plan 9. Venti is usable directly by user applications, and is also integrated with the Fossil filesystem to support snapshotting the status of a Fossil filesystem. Fossil allows references to either be to a block number on the Fossil partition or to a Venti key; so when a filesystem has been snapshotted, all it now contains is a "root directory" pointer into the Venti archive, and any files modified therafter are copied-on-write into Fossil where they may be modified until the next snapshot.

We're nowhere near that exciting yet, but using FUSE, we might be able to do something similar, which might be fun. However, Venti inspired me when I read about it years ago; it showed me how elegant content-addressed storage is. Finding out that the Git version control system used the same basic tricks really just confirmed this for me.

Also, I'd like to tip my hat to Duplicity. With the changing economics of storage presented by services like Amazon S3 and rsync.net, I looked to Duplicity as it provided both SFTP and S3 backends. However, it worked in terms of full and incremental backups, a model that I think made sense for magnetic tapes, but loses out to content-addressed snapshots when you have random-access media. Duplicity inspired me by its adoption of multiple backends, the very backends I want to use, but I still hungered for a content-addressed snapshot store.

I'd also like to tip my hat to Box Backup. I've only used it a little, because it requires a special server to manage the storage (and I want to get my backups *off* of my servers), but it also inspires me with directions I'd like to take Ugarit. It's much more aware of real-time access to random-access storage than Duplicity, and has a very interesting continuous background incremental backup mode, moving away from the tape-based paradigm of backups as something you do on a special day of the week, like some kind of religious observance. I hope the author Ben, who is a good friend of mine, won't mind me plundering his source code for details on how to request real-time notification of changes from the filesystem, and how to read and write extended attributes!

Moving on from the world of backup, I'd like to thank the Chicken Team for producing Chicken Scheme. Felix and the community at #chicken on Freenode have particularly inspired me with their can-do attitudes to combining programming-language elegance and pragmatic engineering - two things many would think un-unitable enemies. Of course, they didn't do it all themselves - R5RS Scheme and the SRFIs provided a solid foundation to build on, and there's a cast of many more in the Chicken community, working on other bits of Chicken or just egging everyone on. And I can't not thank Henry Baker for writing the seminal paper on the technique Chicken uses to implement full tail-calling Scheme with cheap continuations on top of C; Henry already had my admiration for his work on combining elegance and pragmatism in linear logic. Why doesn't he return my calls? I even sent flowers.

A special thanks should go to Christian Kellermann for porting Ugarit to use Chicken 4 modules, too, which was otherwise a big bottleneck to development, as I was stuck on Chicken 3 for some time! And to Andy Bennett for many insightful conversations about future directions.

Thanks to the early adopters who brought me useful feedback, too!

And I'd like to thank my wife for putting up with me spending several evenings and weekends and holiday days working on this thing...

Version history

BUGFIX: Logging of messages from storage backends wasn't happening correctly in the Ugarit core, leading to errors when the cache backend (which logs an info message at close time) was closed and the log message had nowhere to go.
BUGFIX: Made file cache check the file hashes it finds in the cache actually exist in the vault, to protect against the case where a crash of some kind has caused unflushed changes to be lost; the file cache may well have committed changes that the backend hasn't, leading to references to nonexistant blocks. Note that we assume that vaults are sequentially safe, eg if the final indirect block of a large file made it, all the partial blocks must have made it too.
BUGFIX: Added an explicit flush! command to the backend protocol, and put explicit flushes at critical points in higher layers (backend-cache, the vault abstraction in the Ugarit core, and when tagging a snapshot) so that we ensure the blocks we point at are flushed before committing references to them in the backend-cache or file caches, or into tags, to ensure crash safety.
BUGFIX: Made the splitlog backend never exceed the file size limit (except when passed blocks that, plus a header, are larger than it), rather than letting a partial block hang over the 'end'.
BUGFIX: Fixed tag locking, which was broken all over the place. Concurrent snapshots to the same tag should now block for one another, although why you'd want to *do* that is questionable.
BUGFIX: Fixed generation of non-keyed hashes, which was incorrectly appending the type to the hash without an outer hash. This breaks backwards compatability, but nobody was using the old algorithm, right? I'll introduce it as an option if required.
BUGFIX: splitlog backend now creates log files with "rw-------" rather than "rwx------" permissions; and all sqlite databases (splitlog metadata, cache file, and file-cache file) are created with "rw-------" rather then "rw-r--r--".
BUGFIX: file caching uses mtime *and* size now, rather than just mtime. Error handling so we skip objects that we cannot do something with, and proceed to try the rest of the operation.