[Dovecot] SIS and restoring from backups

[Timo Sirainen]

On 24.3.2012, at 14.54, Charles Marcus wrote:

> On 2012-03-24 8:08 AM, Timo Sirainen <tss at iki.fi> wrote:
>> You can do full backups from a filesystem snapshot, which works
>> "well enough" (might leave some unused attachments lying around in
>> some rare cases, but that can also happen if Dovecot crashes/dies).
> 
> But the problem isn't with backups, but with restores, right?

Ah, right. Then it gets tricky.

>> The other possibility is to already use dsync (doveadm backup) to do
>> full backups. With the redesigned dsync you would be able to do
>> incremental backups also. In any case the solution involves
>> de-SISing mails for backup.
> 
> So, this would make the backup storage requirements larger - maybe dramatically larger for sites that have a lot of large attachments?

Some backup systems can do internal deduplication.

> I currently use rsnapshot to keep many multiple (daily, weekly, and monthly) hardlinked snapshots, each of which consumes only a tiny fraction of extra storage over and above the first/main snapshot.
> 
> Am I correct that enabling SIS as it is currently implemented would break this backup tool?

I'm not sure. Are you running rsnapshot on live filesystem or on a snapshot? On live filesystem there would be race conditions.

> I was also thinking of asking about how to provide read-only access to these backup snapshots to the users in some kind of special namespace, so that they could all essentially go 'back in time' to grab any emails that they may have inadvertently deleted...

This should be possible, just point the namespace to such snapshot. You may need to point CONTROL dir to some temporary directory and index dir as well to either temp or to memory.