[Discuss] Backing up LVM partitions using snapshots

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

On Dec 11, 2011, at 2:54 PM, markw at mohawksoft.com wrote:
> 
> How?

Let's posit a 7 day backup cycle.  On day 1 you do your full backup (all volume blocks).  On day's 2 through 7 you do incremental backups (changed volume blocks).

Let's say that the volume contains your company's code repository.  Users are working on this volume almost constantly.  Regardless of what they do, the file system blocks are going to be changing just as frequently, and what you see on day of your cycle 5 may not look at all like what you saw on day 1.

As a specific example, I create a file on day 1 that spans 3 volume blocks.  On day 2 I change the first block but the remaining two blocks remain unchanged (I'm doing random I/O for performance reasons, just as one would with a database).  On day 3 I make changes to the second block.  Your block-level backups for this file would contain 3 blocks on day 1, 1 block on day 1 and 1 block on day 3.

On day 6 I accidentally delete the file and I contact you to restore it.  You go to your day 1 backup and do the full restore which gets the original version of the file.  Then you go to restore the day 2 incremental and find that it is unusable.  You can certainly restore the day 3 incremental but I'm still missing the block of data backed up on day 2, a block of data that can not be recovered because the backup of it is gone.  Much of my work is lost and I have to do it all over again.

Now, consider the effect of missing blocks on directory data and inode tables.

--Rich P.