2 bup: It backs things up
3 =======================
5 bup is a program that backs things up. It's short for "backup." Can you
6 believe that nobody else has named an open source program "bup" after all
9 Despite its unassuming name, bup is pretty cool. To give you an idea of
10 just how cool it is, I wrote you this poem:
13 What rhymes with awesome?
15 But that's irrelevant.
17 Hmm. Did that help? Maybe prose is more useful after all.
20 Reasons bup is awesome
21 ----------------------
23 bup has a few advantages over other backup software:
25 - It uses a rolling checksum algorithm (similar to rsync) to split large
26 files into chunks. The most useful result of this is you can backup huge
27 virtual machine (VM) disk images, databases, and XML files incrementally,
28 even though they're typically all in one huge file, and not use tons of
29 disk space for multiple versions.
31 - It uses the packfile format from git (the open source version control
32 system), so you can access the stored data even if you don't like bup's
35 - Unlike git, it writes packfiles *directly* (instead of having a separate
36 garbage collection / repacking stage) so it's fast even with gratuitously
37 huge amounts of data. bup's improved index formats also allow you to
38 track far more filenames than git (millions) and keep track of far more
39 objects (hundreds or thousands of gigabytes).
41 - Data is "automagically" shared between incremental backups without having
42 to know which backup is based on which other one - even if the backups
43 are made from two different computers that don't even know about each
44 other. You just tell bup to back stuff up, and it saves only the minimum
45 amount of data needed.
47 - You can back up directly to a remote bup server, without needing tons of
48 temporary disk space on the computer being backed up. And if your backup
49 is interrupted halfway through, the next run will pick up where you left
50 off. And it's easy to set up a bup server: just install bup on any
51 machine where you have ssh access.
53 - Bup can use "par2" redundancy to recover corrupted backups even if your
54 disk has undetected bad sectors.
56 - Even when a backup is incremental, you don't have to worry about
57 restoring the full backup, then each of the incrementals in turn; an
58 incremental backup *acts* as if it's a full backup, it just takes less
61 - You can mount your bup repository as a FUSE filesystem and access the
62 content that way, and even export it over Samba.
64 - It's written in python (with some C parts to make it faster) so it's easy
65 for you to extend and maintain.
68 Reasons you might want to avoid bup
69 -----------------------------------
71 - This is a very early version. Therefore it will most probably not work
72 for you, but we don't know why. It is also missing some
73 probably-critical features.
75 - It requires python >= 2.4, a C compiler, and an installed git version >=
78 - It currently only works on Linux, MacOS X >= 10.4, or Windows (with
79 Cygwin). Patches to support other platforms are welcome.
85 - check out the bup source code using git:
87 git clone git://github.com/apenwarr/bup
89 - install the python 2.5 development libraries. On Debian or Ubuntu, this
91 apt-get install python2.5-dev
93 - build the python module and symlinks:
101 (The tests should pass. If they don't pass for you, stop here and send
104 - Try making a local backup as a tar file:
106 tar -cvf - /etc | bup split -n local-etc -vv
108 - Try restoring your backup tarball:
110 bup join local-etc | tar -tf -
112 - Look at how much disk space your backup took:
116 - Make another backup (which should be mostly identical to the last one;
117 notice that you don't have to *specify* that this backup is incremental,
118 it just saves space automatically):
120 tar -cvf - /etc | bup split -n local-etc -vv
122 - Look how little extra space your second backup used on top of the first:
126 - Restore your old backup again (the ~1 is git notation for "one older than
129 bup join local-etc~1 | tar -tf -
131 - get a list of your previous backups:
133 GIT_DIR=~/.bup git log local-etc
135 - make a backup on a remote server (which must already have the 'bup' command
136 somewhere in the PATH, and be accessible via ssh; make sure to replace
137 SERVERNAME with the actual hostname of your server):
139 tar -cvf - /etc | bup split -r SERVERNAME: -n local-etc -vv
141 - try restoring the remote backup tarball:
143 bup join -r SERVERNAME: local-etc | tar -tf -
145 - try using the new (slightly experimental) 'bup index' and 'bup save'
146 style backups, which bypass 'tar' but have some missing features (see
147 "Things that are stupid" below):
150 bup save -n local-etc /etc
152 - do it again and see how fast an incremental backup can be:
155 bup save -n local-etc /etc
157 (You can also use the "-r SERVERNAME:" option to 'bup save', just like
158 with 'bup split' and 'bup join'. The index itself is always local,
159 so you don't need -r there.)
161 That's all there is to it!
169 bup stores its data in a git-formatted repository. Unfortunately, git
170 itself doesn't actually behave very well for bup's use case (huge numbers of
171 files, files with huge sizes, retaining file permissions/ownership are
172 important), so we mostly don't use git's *code* except for a few helper
173 programs. For example, bup has its own git packfile writer written in
176 Basically, 'bup split' reads the data on stdin (or from files specified on
177 the command line), breaks it into chunks using a rolling checksum (similar to
178 rsync), and saves those chunks into a new git packfile. There is one git
181 When deciding whether to write a particular chunk into the new packfile, bup
182 first checks all the other packfiles that exist to see if they already have that
183 chunk. If they do, the chunk is skipped.
185 git packs come in two parts: the pack itself (*.pack) and the index (*.idx).
186 The index is pretty small, and contains a list of all the objects in the
187 pack. Thus, when generating a remote backup, we don't have to have a copy
188 of the packfiles from the remote server: the local end just downloads a copy
189 of the server's *index* files, and compares objects against those when
190 generating the new pack, which it sends directly to the server.
192 The "-n" option to 'bup split' and 'bup save' is the name of the backup you
193 want to create, but it's actually implemented as a git branch. So you can
194 do cute things like checkout a particular branch using git, and receive a
195 bunch of chunk files corresponding to the file you split.
197 If you use '-b' or '-t' or '-c' instead of '-n', bup split will output a
198 list of blobs, a tree containing that list of blobs, or a commit containing
199 that tree, respectively, to stdout. You can use this to construct your own
200 scripts that do something with those values.
204 'bup index' walks through your filesystem and updates a file (whose name is,
205 by default, ~/.bup/bupindex) to contain the name, attributes, and an
206 optional git SHA1 (blob id) of each file and directory.
208 'bup save' basically just runs the equivalent of 'bup split' a whole bunch
209 of times, once per file in the index, and assembles a git tree
210 that contains all the resulting objects. Among other things, that makes
211 'git diff' much more useful (compared to splitting a tarball, which is
212 essentially a big binary blob). However, since bup splits large files into
213 smaller chunks, the resulting tree structure doesn't *exactly* correspond to
214 what git itself would have stored. Also, the tree format used by 'bup save'
215 will probably change in the future to support storing file ownership, more
216 complex file permissions, and so on.
218 If a file has previously been written by 'bup save', then its git blob/tree
219 id is stored in the index. This lets 'bup save' avoid reading that file to
220 produce future incremental backups, which means it can go *very* fast unless
221 a lot of files have changed.
224 Things that are stupid for now but which we'll fix later
225 --------------------------------------------------------
227 Help with any of these problems, or others, is very welcome. Join the
228 mailing list (see below) if you'd like to help.
230 - 'bup save' doesn't know about file metadata.
232 That means we aren't saving file attributes, mtimes, ownership, hard
233 links, MacOS resource forks, etc. Clearly this needs to be improved.
235 - There's no 'bup restore' yet.
237 'bup save' saves files in the standard git 'tree of blobs' format, so you
238 could then "restore" the files using something like 'git checkout'. But
239 that's a git command, not a bup command, so it's hard to explain and
240 doesn't support retrieving objects from a remote bup server without first
241 fetching and packing an entire (possibly huge) pack, which could be very
242 slow. Also, like 'bup save', you would need extra features in order to
243 properly restore file metadata. And files that bup has split into
244 chunks will need to be recombined. Although there's no restore tool,
245 'bup fuse' does accomplish some of this already.
247 - 'bup index' is slower than it should be.
249 It's still rather fast: it can iterate through all the filenames on my
250 600,000 file filesystem in a few seconds. But it still needs to rewrite
251 the entire index file just to add a single filename, which is pretty
252 nasty; it should just leave the new files in a second "extra index" file
255 - bup could use inotify for *really* efficient incremental backups.
257 You could even have your system doing "continuous" backups: whenever a
258 file changes, we immediately send an image of it to the server. We could
259 give the continuous-backup process a really low CPU and I/O priority so
260 you wouldn't even know it was running.
262 - bup currently has no features that prune away *old* backups.
264 Because of the way the packfile system works, backups become "entangled"
265 in weird ways and it's not actually possible to delete one pack
266 (corresponding approximately to one backup) without risking screwing up
269 git itself has lots of ways of optimizing this sort of thing, but its
270 methods aren't really applicable here; bup packfiles are just too huge.
271 We'll have to do it in a totally different way. There are lots of
272 options. For now: make sure you've got lots of disk space :)
274 - bup has never been tested on anything but Linux, MacOS, and Linux+Cygwin.
276 There's nothing that makes it *inherently* non-portable, though, so
277 that's mostly a matter of someone putting in some effort. (For a
278 "native" Windows port, the most annoying thing is the absence of ssh in
279 a default Windows installation.)
281 - bup has no GUI. Actually, that's not stupid, but you might consider it
282 a limitation. There are a bunch of Linux GUI backup programs; someday
283 I expect someone will adapt one of them to use bup.
289 bup is a work in progress and there are many ways it can still be improved.
290 If you'd like to contribute patches, ideas, or bug reports, please join the
293 You can find the mailing list archives here:
295 http://groups.google.com/group/bup-list
297 and you can subscribe by sending a message to:
299 bup-list+subscribe@googlegroups.com