Feb 14, 2010

My "simple" (ok, not quite) backup system - implementation (backup host)

According to the general plan, with backed-up side scripts in place, some backup-grab mechanism is needed on the backup host.

So far, sshd provides secure channel and authentication, launching control script as a shell, backed-up side script provides hostname:port for one-shot ssh link on the commandline, with private key to this link and backup-exclusion paths list piped in.

All that's left to do on this side is to read the data from a pipe and start rsync over this link, with a few preceding checks, like a free space check, so backup process won't be strangled by its abscence and as many as possible backups will be preserved for as long as possible, removing them right before receiving new ones.

Historically, this script also works with any specified host, interactively logging into it as root for rsync operation, so there's bit of interactive voodoo involved, which isn't relevant for the remotely-initiated backup case.

Ssh parameters for rsync transport are passed to rsync itself, since it starts ssh process, via "--rsh" option. Inside the script,these are accumulated in bak_src_ext variable

Note that in case then this script is started as a shell, user is not a root, yet it needs to store filesystem metadata like uids, gids, acls, etc.
To that end, rsync can employ user_xattr's, although it looks extremely unportable and inproper to me, since nothing but rsync will translate them back to original metadata, so rsync need to be able to change fs metadata directly, and to that end there's posix capabilities.

I use custom module for capability manipulation, as well as other convenience modules here and there, their purpose is quite obvious and replacing these with stdlib functions should be pretty straightforward, if necessary.

Activating the inherited capabilities:

bak_user = os.getuid()
if bak_user:
    from fgc.caps import Caps
    import pwd
    os.putenv('HOME', pwd.getpwuid(bak_user).pw_dir)

But first things first - there's data waiting on commandline and stdin. Getting the hostname and port...

bak_src = argz[0]
try: bak_src, bak_src_ext = bak_src.split(':')
except: bak_src_ext = tuple()
else: bak_src_ext = '-p', bak_src_ext

...and the key / exclusions:

bak_key = bak_sub('.key_{0}'.format(bak_host))
password, reply = it.imap(
    op.methodcaller('strip', spaces), sys.stdin.read().split('\n\n\n', 1) )
open(bak_key, 'w').write(password)
sh.chmod(bak_key, 0400)
bak_src_ext += '-i', os.path.realpath(bak_key)

Then, basic rsync invocation options can be constructed:

sync_optz = [ '-HaAXz',
    '--rsync-path=ionice -c3 rsync',
    '--rsh=ssh {0}'.format(' '.join(bak_src_ext)) ]

Excluded paths list here is written to a local file, to keep track which paths were excluded in each backup. "--super" option is actually necessary if local user is not root, rsync drops all the metadata otherwise. "HaAX" is like "preserve all" flags - Hardlinks, ownership/modes/times ("a" flag), Acl's, eXtended attrs. "--rsh" here is the ssh command, with parameters, determined above.

Aside from that, there's also need to specify hardlink destination path, which should be a previous backup, and that traditionnaly is the domain of ugly perlisms - regexps.

bakz_re = re.compile(r'^([^.].*)\.\d+-\d+-\d+.\d+$') # host.YYYY-mm-dd.unix_time
bakz = list( bak for bak in os.listdir(bak_root)
 if bakz_re.match(bak) ) # all backups
bakz_host = sorted(it.ifilter(op.methodcaller(
    'startswith', bak_host ), bakz), reverse=True)

So, the final sync options come to these:

src = '{0}:/'.format(src)
sync_optz = list(dta.chain( sync_optz, '--link-dest={0}'\
        .format(os.path.realpath(bakz_host[0])), src, bak_path ))\
    if bakz_host else list(dta.chain(sync_optz, src, bak_path))

The only interlude is to cleanup backup partition if it gets too crowded:

## Free disk space check / cleanup
ds, df = sh.df(bak_root)
min_free = ( max(min_free_avg( (ds-df) / len(bakz)), min_free_abs*G)
    if min_free_avg and bakz else min_free_abs*G )

def bakz_rmq():
    '''Iterator that returns bakz in order of removal'''
    bakz_queue = list( list(bakz) for host,bakz in it.groupby(sorted(bakz),
        key=lambda bak: bakz_re.match(bak).group(1)) )
    while bakz_queue:
        if len(bakz_queue[-1]) <= min_keep: break
        yield bakz_queue[-1].pop()

if df < min_free:
    for bak in bakz_rmq():
        log.info('Removing backup: {0}'.format(bak))
        sh.rr(bak, onerror=False)
        ds, df = sh.df(bak_root)
        if df >= min_free: break
        log.fatal( 'Not enough space on media:'
                ' {0:.1f}G, need {1:.1f}G, {2} backups min)'\
            .format( op.truediv(df, G),
                op.truediv(min_free, G), min_keep ), crash=2 )

And from here it's just to start rsync and wait 'till the job's done.

This thing works for months now, and saved my day on many occasions, but the most important thing here I think is the knowledge that the backup is there should you need one, so you never have to worry about breaking your system or losing anything important there, whatever you do.

Here's the full script.

Actually, there's more to the story, since just keeping backups on single local harddisk (raid1 of two disks, actually) isn't enough for me.
Call this paranoia, but setting up system from scratch and restoring all the data I have is a horrible nightmare, and there are possibility of fire, robbery, lighting, voltage surge or some other disaster that can easily take this disk(s) out of the picture, and few gigabytes of space in the web come almost for free these days - there are p2p storages like wuala, dropbox, google apps/mail with their unlimited quotas...
So, why not upload all this stuff there and be absolutely sure it'd never go down, whatever happens? Sure thing.
Guess I'll write a note on the topic as much to document it for myself as for the case someone might find it useful as well, plus the ability to link it instead of explaining ;)
Member of The Internet Defense League