On Wed, Sep 02, 2009 at 10:14:05AM -0500, Les Mikesell wrote:
> Pieter Wuille wrote:
> > In our case, the BackupPC pool is stored on an XFS filesystem on an LVM
> > volume, allowing a xfsfreeze/sync/snapshot/xfsunfreeze, and using
> > devfiles.pl on the snapshot. Instead of xfsfreeze+unfreeze, a backuppc
> > stop/umount + mount/backuppc start is also possible. If no system for making
> > snapshots is available, you would need to suspend backuppc during the whole
> > synchronisation.
> > In fact, the BackupPC volume is already encrypted on our backup server
> > itself, allowing very cheap encrypted offsite backups (simply not sending
> > the keyfile to the remote side is enough...)
> >
> > The result: offsite backups of our 400GiB pool, containing 350GiB data, of
> > which about 2GiB changes daily, is synchronised 5 times a week with offsite
> > backup in 12-15 hours, requiring nearly no bandwidth. This seems mostly
> > limited by the slow disk I/O on the receiver side (25MiB/s).
> >
> > Hope you find this interesting/useful,
>
> The one thing that would bother me about this approach is that you would
> have a fairly long window of time while the remote filesystem chunks are
> being updated. While rsync normally creates a copy of an individual
> file and does not delete the original until the copy is complete, a
> mis-matched set of filesystem chunks would likely not be usable. Since
> disasters always happen at the worst possible time, I'd want to be sure
> you could recover from losing the primary filesystem (site?) in the
> middle of a remote copy. This might be done by keeping a 2nd copy of
> the files at the remote location, keeping them on an LVM with a snapshot
> taken before each update, or perhaps catting them together onto a
> removable device for fast access after the chunks update.
You're very right, and i thought about it too. Instead of using a RAID1 on
the offsite backup, there are two separate backups on the offsite machine,
and synchronisation switches between them. This also enables the use of
rsync's --inplace option.
Keeping an LVM snapshot is a possibility, but it becomes somewhat complex to
manage: you get a snapshot of a volume containing a filesystem whose files
correspond to parts of a snapshot of a volume containing an (encrypted)
filesystem containing a directory that corresponds to a pool of backups...
Catting the part files together to a device after transmission isn't a
complete solution: what if the machine crashes during the catting...?
--
Pieter
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