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bup 0.01: 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.
- 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.
- 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.
- 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 version 0.01. What that means is this is the very first version.
Therefore it will most probably not work for you, but we don't know why.
- It requires python 2.5, a C compiler, and an installed git version >= 1.5.2.
- It only works on Linux (for now).
- It has almost no documentation. Not even a man page! This file is all
you get for now.
Getting started
---------------
- check out the bup source code using git:
git clone git://github.com/apenwarr/bup
- install the python 2.5 development libraries. On Debian or Ubuntu, this
is:
apt-get install python2.5-dev
- 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
me an email.)
- Try making a local backup:
tar -cvf - /etc | bup split -n local-etc -vv
- Try restoring your backup:
bup join local-etc | tar -tf -
- 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):
tar -cvf - /etc | bup split -n local-etc -vv
- Look how little extra space your second backup used on top of the first:
du -s ~/.bup
- Restore your old 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 backups:
GIT_DIR=~/.bup git log local-etc
- make a backup on a remote server (which must already have the 'bup' command
somewhere in the PATH, and be accessible via ssh; make sure to replace
SERVERNAME with the actual hostname of your server):
tar -cvf - /etc | bup split -r SERVERNAME: -n local-etc -vv
- try restoring the remote backup:
bup join -r SERVERNAME: local-etc | tar -tf -
That's all there is to it!
How it works
------------
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.
'bup save' basically just runs 'bup split' a whole bunch of times, once per
file in a directory hierarchy, 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.
Things that are stupid for now but which we'll fix later
--------------------------------------------------------
Help with any of these problems, or others, is very, very welcome. Let me
know if you'd like to help. Maybe we can start a mailing list.
- bup's incremental backup algorithm is braindead.
Bup reads the contents of every single file you want to back up, *then*
it checks if it has that content already, and if not, it backs up the
file. Now, it happens to do that very fast (using mmap'ed git packfile
indexes), all things considered, but it's not nearly as fast as simply
noticing that the file inode+ctime is the same as before and just
skipping it. There's nothing preventing us from adding this
optimization, though. (Perhaps we could use the git indexfile format for
tracking this?)
- 'bup save' is incomplete and there's no 'bup restore' yet.
'bup save' is supposed to recursively go through a given directory and
store all the files efficiently, and then you could use 'bup restore' to
restore all or some of them. However, these features don't really work
yet.
Instead, for now the best way to use bup is to feed 'bup split' a big tar
file of your backup, then restore that tar file later with 'bup join'.
This is cute, but inefficient; for example, tar files don't have an
index, so to restore a single file would require linearly reading through
the entire tarball. (This is exactly like what always happens when you
make a backup using tar, but if we use git's native trees/blobs the way
they're meant to be used, it will be ridiculously faster.)
- 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 features that prune away *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 :)
- bup doesn't ever validate existing backups/packs to ensure they're
correct.
This would be easy to implement (given that git uses hashes and CRCs all
over the place), but nobody has implemented it. For now, you could try
doing a test restore of your tarball; doing so should trigger git's error
handling if any of the objects are corrupted.
- bup has never been tested on anything but Linux.
There's nothing that makes it *inherently* non-portable, though, so
that's mostly a matter of someone putting in some effort.
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, please email me at <apenwarr@gmail.com>. If
enough people are interested, perhaps we should start a mailing list for
it!
Have fun,
Avery
January 2010
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