[DRBD-user] Snapshot with DRBD

[drbd at bobich.net]

On Thu, 14 Feb 2008, H.D. wrote:

>> I disagree. It's not a solution because syncing the FS isn't sufficient. 
>> With LVM you still get the FS level consistency as it is at the time of the 
>> snapshot. The problem is that applications like DBs maintain their internal 
>> buffers and caches, and these will not necessarily be consistent with the 
>> data on disk, FS freeze or not.
>
> Syncing the FS _is_ sufficient. That is why DBMS implement write ahead logs. 
> Recovering from a snapshot is the same as recovering from a power failure. 
> And if a DBMS can't do that, it doesn't conform to ACID, and no one serious 
> would use it. There is no serious DBMS out there that is unhappy with FS 
> snapshots, even MySQL / innoDB is fine.

Syncing the FS is sufficient for data safety on the level of journalling 
that keeps the FS consistent after a power failure. It is NOT sufficient 
for data consistency I would expect from a proper backup.

> In case of XFS it's actually better than a power failure, both meta data and 
> data is flushed, as well as an umount record written.

Many file systems have data AND metadata journalling available, including 
ext3 (even if ext3 by default only journals metadata).

> Have you ever tried running a DBMS on copyfs? There is a reason why no one is 
> doing that. First there is no reason to do it, second performance is 
> unacceptable.

No, it never occured to me to run a DBMS on CopyFS. As you quite rightly 
point out, there is no point in doing it. As for DB backups, backup up a 
replicated server is a much safer and more sensible option than 
snapshotting. Even if snapshotting provided consistent images (which it 
doesn't), it still impairs the performance of a live server (it 
effectively does the same thing CopyFS does). Having a separate replicated 
DB server gives you both the guaranteed consistency and doesn't have a 
serious performance hit on the master server.

Hmm... I think we've drifted a tad far off the DRBD topic...

Gordan