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bup 0.04: 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 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 >= 1.5.2.
 
 - It currently only works on Linux, MacOS X 10.5, or Windows (with Cygwin).
   Patches to support other platforms are welcome.
 
 - 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 as a tar file:
 
        tar -cvf - /etc | bup split -n local-etc -vv
        
 - Try restoring your backup tarball:
 
        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 tarball:
 
        bup join -r SERVERNAME: local-etc | tar -tf -
        
 - try using the new (slightly experimental) 'bup index' and 'bup save'
   style backups, which bypass 'tar' but have some missing features (see
   "Things that are stupid" below):
        
        bup index -uv /etc
        bup save -n local-etc /etc
        
 - do it again and see how fast an incremental backup can be:
 
        bup index -uv /etc
        bup save -n local-etc /etc
        
   (You can also use the "-r SERVERNAME:" option to 'bup save', just like
    with 'bup split' and 'bup join'.  The index itself is always local,
    so you don't need -r there.)
        
That's all there is to it!


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, very welcome.  Let me
know if you'd like to help.  Maybe we can start a mailing list.

 - 'bup save' doesn't know about file metadata.
 
   That means we aren't saving file attributes, mtimes, ownership, hard
   links, MacOS resource forks, etc.  Clearly this needs to be improved.

 - There's no 'bup restore' yet.
 
   'bup save' saves files in the standard git 'tree of blobs' format, so you
   could then "restore" the files using something like 'git checkout'.  But
   that's a git command, not a bup command, so it's hard to explain and
   doesn't support retrieving objects from a remote bup server without first
   fetching and packing an entire (possibly huge) pack, which could be very
   slow.  Also, like 'bup save', you would need extra features in order to
   properly restore file metadata.  And files that bup has split into
   chunks would need to be recombined somehow.
   
 - '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 sometimes you just want to
   change a filename or two, so this is needlessly slow.  There should be
   a way to binary search through the file list rather than always going
   through it sequentially.  And if you only add a couple of filenames,
   there's no need to rewrite the entire index; 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 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.  'git fsck' would
   theoreticaly work too, but it's too slow for huge backups.

 - bup has never been tested on anything but Linux, MacOS, and Linux+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 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.


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

Have fun,

Avery
January 2010