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bup: It backs things up

bup is a program that backs things up. It's short for "backup." Can you believe that nobody else has named an open source program "bup" after all this time? Me neither.

Despite its unassuming name, bup is pretty cool. To give you an idea of just how cool it is, I wrote you this poem:

                         Bup is teh awesome
                      What rhymes with awesome?
                        I guess maybe possum
                       But that's irrelevant.

Hmm. Did that help? Maybe prose is more useful after all.

Reasons bup is awesome

bup has a few advantages over other backup software:

  • It uses a rolling checksum algorithm (similar to rsync) to split large files into chunks. The most useful result of this is you can backup huge virtual machine (VM) disk images, databases, and XML files incrementally, even though they're typically all in one huge file, and not use tons of disk space for multiple versions.

  • It uses the packfile format from git (the open source version control system), so you can access the stored data even if you don't like bup's user interface.

  • Unlike git, it writes packfiles directly (instead of having a separate garbage collection / repacking stage) so it's fast even with gratuitously huge amounts of data. bup's improved index formats also allow you to track far more filenames than git (millions) and keep track of far more objects (hundreds or thousands of gigabytes).

  • Data is "automagically" shared between incremental backups without having to know which backup is based on which other one - even if the backups are made from two different computers that don't even know about each other. You just tell bup to back stuff up, and it saves only the minimum amount of data needed.

  • You can back up directly to a remote bup server, without needing tons of temporary disk space on the computer being backed up. And if your backup is interrupted halfway through, the next run will pick up where you left off. And it's easy to set up a bup server: just install bup on any machine where you have ssh access.

  • Bup can use "par2" redundancy to recover corrupted backups even if your disk has undetected bad sectors.

  • Even when a backup is incremental, you don't have to worry about restoring the full backup, then each of the incrementals in turn; an incremental backup acts as if it's a full backup, it just takes less disk space.

  • You can mount your bup repository as a FUSE filesystem and access the content that way, and even export it over Samba.

  • It's written in python (with some C parts to make it faster) so it's easy for you to extend and maintain.

Reasons you might want to avoid bup

  • This is a very early version. Therefore it will most probably not work for you, but we don't know why. It is also missing some probably-critical features.

  • It requires python >= 2.5, a C compiler, and an installed git version >= 1.5.3.1. It also requires par2 if you want fsck to be able to generate the information needed to recover from some types of corruption.

  • It currently only works on Linux, MacOS X >= 10.4, NetBSD, Solaris, or Windows (with Cygwin). Patches to support other platforms are welcome.

  • Any items in "Things that are stupid" below.

Getting started

From source

  • Check out the bup source code using git:

     git clone git://github.com/bup/bup

  • Install the required python libraries (including the development libraries).

    On very recent Debian/Ubuntu versions, this may be sufficient (run as root):

         apt-get build-dep bup

    Otherwise try this (substitute python2.5-dev or python2.6-dev if you have an older system):

         apt-get install python2.7-dev python-fuse
     apt-get install python-pyxattr python-pylibacl
     apt-get install linux-libc-dev
     apt-get install acl attr
     apt-get install python-tornado # optional

    On CentOS (for CentOS 6, at least), this should be sufficient (run as root):

         yum groupinstall "Development Tools"
     yum install python python-devel
     yum install fuse-python pyxattr pylibacl
     yum install perl-Time-HiRes

    In addition to the default CentOS repositories, you may need to add RPMForge (for fuse-python) and EPEL (for pyxattr and pylibacl).

    On Cygwin, install python, make, rsync, and gcc4.

    If you would like to use the optional bup web server on systems without a tornado package, you may want to try this:

         pip install tornado

  • Build the python module and symlinks:

     make

  • Run the tests:

     make test

    (The tests should pass. If they don't pass for you, stop here and send an email to bup-list@googlegroups.com.)

  • You can install bup via "make install", and override the default destination with DESTDIR and PREFIX.

    Files are normally installed to "$DESTDIR/$PREFIX" where DESTDIR is empty by default, and PREFIX is set to /usr. So if you wanted to install bup to /opt/bup, you might do something like this:

     make install DESTDIR=/opt/bup PREFIX=''

From binary packages

Binary packages of bup are known to be built for the following OSes:

Using bup

  • Get help for any bup command:

     bup help
 bup help init
 bup help index
 bup help save
 bup help restore
 ...

  • Initialize the default BUP_DIR (~/.bup):

     bup init

  • Make a local backup (-v or -vv will increase the verbosity):

     bup index /etc
 bup save -n local-etc /etc

  • Restore a local backup to ./dest:

     bup restore -C ./dest local-etc/latest/etc
 ls -l dest/etc

  • Look at how much disk space your backup took:

     du -s ~/.bup

  • Make another backup (which should be mostly identical to the last one; notice that you don't have to specify that this backup is incremental, it just saves space automatically):

     bup index /etc
 bup save -n local-etc /etc

  • Look how little extra space your second backup used (on top of the first):

     du -s ~/.bup

  • Get a list of your previous backups:

     bup ls local-etc

  • Restore your first backup again:

     bup restore -C ./dest-2 local-etc/2013-11-23-11195/etc

  • Make a backup on a remote server (which must already have the 'bup' command somewhere in the server's PATH (see /etc/profile, etc/environment, ~/.profile, or ~/.bashrc), and be accessible via ssh. Make sure to replace SERVERNAME with the actual hostname of your server):

     ssh SERVERNAME bup init
 bup index /etc
 bup save -r SERVERNAME: -n local-etc /etc

  • Restore a backup from a remote server. (FAIL: unfortunately, unlike "bup join", "bup restore" does not yet support remote restores. See both "bup join" and "Things that are stupid" below.)

  • Defend your backups from death rays (OK fine, more likely from the occasional bad disk block). This writes parity information (currently via par2) for all of the existing data so that bup may be able to recover from some amount of repository corruption:

     bup fsck -g

  • Use split/join instead of index/save/restore. Try making a local backup using tar:

     tar -cvf - /etc | bup split -n local-etc -vv

  • Try restoring the tarball:

     bup join local-etc | tar -tf -

  • Look at how much disk space your backup took:

     du -s ~/.bup

  • Make another tar backup:

     tar -cvf - /etc | bup split -n local-etc -vv

  • Look at how little extra space your second backup used on top of the first:

    du -s ~/.bup

  • Restore the first tar backup again (the ~1 is git notation for "one older than the most recent"):

     bup join local-etc~1 | tar -tf -

  • Get a list of your previous split-based backups:

     GIT_DIR=~/.bup git log local-etc

  • Make a backup on a remote server:

     tar -cvf - /etc | bup split -r SERVERNAME: -n local-etc -vv

  • Try restoring the remote backup tarball:

     bup join -r SERVERNAME: local-etc | tar -tf -

That's all there is to it!

Notes on FreeBSD

  • FreeBSD's default 'make' command doesn't like bup's Makefile. In order to compile the code, run tests and install bup, you need to install GNU Make from the port named 'gmake' and use its executable instead in the commands seen above. (i.e. 'gmake test' runs bup's test suite)

  • Python's development headers are automatically installed with the 'python' port so there's no need to install them separately.

  • To use the 'bup fuse' command, you need to install the fuse kernel module from the 'fusefs-kmod' port in the 'sysutils' section and the libraries from the port named 'py-fusefs' in the 'devel' section.

  • The 'par2' command can be found in the port named 'par2cmdline'.

  • In order to compile the documentation, you need pandoc which can be found in the port named 'hs-pandoc' in the 'textproc' section.

Notes on NetBSD/pkgsrc

  • See pkgsrc/sysutils/bup, which should be the most recent stable release and includes man pages. It also has a reasonable set of dependencies (git, par2, py-fuse-bindings).

  • The "fuse-python" package referred to is hard to locate, and is a separate tarball for the python language binding distributed by the fuse project on sourceforge. It is available as pkgsrc/filesystems/py-fuse-bindings and on NetBSD 5, "bup fuse" works with it.

  • "bup fuse" presents every directory/file as inode 0. The directory traversal code ("fts") in NetBSD's libc will interpret this as a cycle and error out, so "ls -R" and "find" will not work.

  • There is no support for ACLs. If/when some entrprising person fixes this, adjust t/compare-trees.

Notes on Cygwin

  • There is no support for ACLs. If/when some enterprising person fixes this, adjust t/compare-trees.

  • In t/test.sh, two tests have been disabled. These tests check to see that repeated saves produce identical trees and that an intervening index doesn't change the SHA1. Apparently Cygwin has some unusual behaviors with respect to access times (that probably warrant further investigation). Possibly related: http://cygwin.com/ml/cygwin/2007-06/msg00436.html

Notes on OS X

  • There is no support for ACLs. If/when some enterprising person fixes this, adjust t/compare-trees.

How it works

Basic storage:

bup stores its data in a git-formatted repository. Unfortunately, git itself doesn't actually behave very well for bup's use case (huge numbers of files, files with huge sizes, retaining file permissions/ownership are important), so we mostly don't use git's code except for a few helper programs. For example, bup has its own git packfile writer written in python.

Basically, 'bup split' reads the data on stdin (or from files specified on the command line), breaks it into chunks using a rolling checksum (similar to rsync), and saves those chunks into a new git packfile. There is one git packfile per backup.

When deciding whether to write a particular chunk into the new packfile, bup first checks all the other packfiles that exist to see if they already have that chunk. If they do, the chunk is skipped.

git packs come in two parts: the pack itself (.pack) and the index (.idx). The index is pretty small, and contains a list of all the objects in the pack. Thus, when generating a remote backup, we don't have to have a copy of the packfiles from the remote server: the local end just downloads a copy of the server's index files, and compares objects against those when generating the new pack, which it sends directly to the server.

The "-n" option to 'bup split' and 'bup save' is the name of the backup you want to create, but it's actually implemented as a git branch. So you can do cute things like checkout a particular branch using git, and receive a bunch of chunk files corresponding to the file you split.

If you use '-b' or '-t' or '-c' instead of '-n', bup split will output a list of blobs, a tree containing that list of blobs, or a commit containing that tree, respectively, to stdout. You can use this to construct your own scripts that do something with those values.

The bup index:

'bup index' walks through your filesystem and updates a file (whose name is, by default, ~/.bup/bupindex) to contain the name, attributes, and an optional git SHA1 (blob id) of each file and directory.

'bup save' basically just runs the equivalent of 'bup split' a whole bunch of times, once per file in the index, and assembles a git tree that contains all the resulting objects. Among other things, that makes 'git diff' much more useful (compared to splitting a tarball, which is essentially a big binary blob). However, since bup splits large files into smaller chunks, the resulting tree structure doesn't exactly correspond to what git itself would have stored. Also, the tree format used by 'bup save' will probably change in the future to support storing file ownership, more complex file permissions, and so on.

If a file has previously been written by 'bup save', then its git blob/tree id is stored in the index. This lets 'bup save' avoid reading that file to produce future incremental backups, which means it can go very fast unless a lot of files have changed.

Things that are stupid for now but which we'll fix later

Help with any of these problems, or others, is very welcome. Join the mailing list (see below) if you'd like to help.

  • 'bup restore' can't pull directly from a remote server.

    So in one sense "save -r" is a dead-end right now. Obviously you can use "ssh SERVER bup restore -C ./dest..." to create a tree you can transfer elsewhere via rsync/tar/whatever, but that's lame.

    Until we fix it, you may be able to mount the remote BUP_DIR via sshfs and then restore "normally", though that hasn't been officially tested.

  • 'bup save' and 'bup restore' have immature metadata support.

    On the plus side, they actually do have support now, but it's new, and not remotely as well tested as tar/rsync/whatever's. However, you have to start somewhere, and as of 0.25, we think it's ready for more general use. Please let us know if you have any trouble.

    Also, if any strip or graft-style options are specified to 'bup save', then no metadata will be written for the root directory. That's obviously less than ideal.

  • bup is overly optimistic about mmap. Right now bup just assumes that it can mmap as large a block as it likes, and that mmap will never fail. Yeah, right... If nothing else, this has failed on 32-bit architectures (and 31-bit is even worse -- looking at you, s390).

    To fix this, we might just implement a FakeMmap[1] class that uses normal file IO and handles all of the mmap methods[2] that bup actually calls. Then we'd swap in one of those whenever mmap fails.

    This would also require implementing some of the methods needed to support "[]" array access, probably at a minimum getitem, setitem, and setslice [3].

    [1] http://comments.gmane.org/gmane.comp.sysutils.backup.bup/613 [2] http://docs.python.org/2/library/mmap.html [3] http://docs.python.org/2/reference/datamodel.html#emulating-container-types

  • 'bup index' is slower than it should be.

    It's still rather fast: it can iterate through all the filenames on my 600,000 file filesystem in a few seconds. But it still needs to rewrite the entire index file just to add a single filename, which is pretty nasty; it should just leave the new files in a second "extra index" file or something.

  • bup could use inotify for really efficient incremental backups.

    You could even have your system doing "continuous" backups: whenever a file changes, we immediately send an image of it to the server. We could give the continuous-backup process a really low CPU and I/O priority so you wouldn't even know it was running.

  • bup currently has no way to prune old backups.

    Because of the way the packfile system works, backups become "entangled" in weird ways and it's not actually possible to delete one pack (corresponding approximately to one backup) without risking screwing up other backups.

    git itself has lots of ways of optimizing this sort of thing, but its methods aren't really applicable here; bup packfiles are just too huge. We'll have to do it in a totally different way. There are lots of options. For now: make sure you've got lots of disk space :)

    Until we fix this, one possible workaround is to just start a new BUP_DIR occasionally, i.e. bup-2013-10, bup-2013-11...

  • bup has never been tested on anything but Linux, MacOS, and Windows+Cygwin.

    There's nothing that makes it inherently non-portable, though, so that's mostly a matter of someone putting in some effort. (For a "native" Windows port, the most annoying thing is the absence of ssh in a default Windows installation.)

  • bup needs better documentation.

    According to a recent article about bup in Linux Weekly News (https://lwn.net/Articles/380983/), "it's a bit short on examples and a user guide would be nice." Documentation is the sort of thing that will never be great unless someone from outside contributes it (since the developers can never remember which parts are hard to understand).

  • bup is "relatively speedy" and has "pretty good" compression.

    ...according to the same LWN article. Clearly neither of those is good enough. We should have awe-inspiring speed and crazy-good compression. Must work on that. Writing more parts in C might help with the speed.

  • bup has no GUI.

    Actually, that's not stupid, but you might consider it a limitation. There are a bunch of Linux GUI backup programs; someday I expect someone will adapt one of them to use bup.

More Documentation

bup has an extensive set of man pages. Try using 'bup help' to get started, or use 'bup help SUBCOMMAND' for any bup subcommand (like split, join, index, save, etc.) to get details on that command.

For further technical details, please see ./DESIGN.

How you can help

bup is a work in progress and there are many ways it can still be improved. If you'd like to contribute patches, ideas, or bug reports, please join the bup mailing list.

You can find the mailing list archives here:

http://groups.google.com/group/bup-list

and you can subscribe by sending a message to:

bup-list+subscribe@googlegroups.com

Please see ./HACKING for additional information, i.e. how to submit patches (hint - no pull requests), how we handle branches, etc.

Have fun,

Avery