[Discuss] Backing up LVM partitions using snapshots

[derek at ihtfp.com (Derek Atkins)]

On Wed, December 14, 2011 2:00 pm, Richard Pieri wrote:
> On 12/14/2011 12:34 PM, Bill Bogstad wrote:
>> I've been watching the (second?) incarnation of this thread for a
>> while now and I think that I see your point.  I wonder if the "TRIM"
>> functionality that is being added to filesystems in order to handle
>> SSDs could help with this.
>
> I don't think so.  The problem I describe is that once a dump goes
> missing then any differentials against it will have inconsistencies
> between the file data and the file metadata structures.  TRIMming freed
> blocks won't make this go away.  It might make things worse what with
> dangling inode lists pointing to de-allocated SSD blocks.
>
>
> As an aside, enterprise backup systems like Amanda and Bacula and TSM
> do, indeed, maintain databases of backed up files and what media they
> are on.

This is also the reasoning why you should perform regular full dumps,
because it resets the history necessary to perform a backup.  For example,
you could perform monthly full backups, then weekly incrementals, and then
daily incrementals off the weekly incrementals.  So worst case you need 11
restores to get to any point in time (full, 4xweekly incremental, 6 daily
incrementals off the weekly).

There are many other strategies that you can use, but yes, if you lose an
intermediary incremental then yes, you've effectively lost everything that
happened after that, until your next higher-level (or lower-level,
depending which way you look at it) dump.  E.g., if in the above scenario
you lose a daily incremental then you lose all data for the rest of the
week, whereas if you lose a weekly incremental then you lose all data for
the rest of the month.

-derek

-- 
       Derek Atkins                 617-623-3745
       derek at ihtfp.com             www.ihtfp.com
       Computer and Internet Security Consultant