[Discuss] Backing up LVM partitions using snapshots

[richard.pieri at gmail.com (Richard Pieri)]

On Sun, Dec 11, 2011 at 9:15 PM, <markw at mohawksoft.com> wrote:

> Now, you make a HUGE (5%) change to your disk, you back-up 3 million
> changed blocks! You overlay those new blocks on to a list of old blocks
> and create a new list for the backup. Even though you've done an
> incremental backup, you still have a "whole" representation of the volume.
>

I make that 5% change and do an incremental backup.  Then I make a
different 5% change and do another incremental backup.  Here I have two
reasonable choices: I can calculate delta against the most recent
incremental backup or I can calculate against the most recent full backup.

The first option is the more efficient as far as capacity goes but is
vulnerable to loss.  If I lose the first incremental and have to do a
restore then I have everything from the full and everything from the second
incremental but there is a 5% "hole" of changes that I cannot restore.
 That's 3 million blocks of data that my second 3 million blocks may depend
on.  One specific example is the file system superblocks which are pointing
to inode data that isn't there.  My file system is in an inconsistent
state.  You have not addressed this point.

The second option is resistent to loss of incremental backups but requires
successively more capacity for each day's incremental backups with
cumulative deltas potentially exceeding the size of the base volume.

Sorry, Mark.  All that I see here is a complex, dodgy technical solution
looking for a problem that doesn't exist.