[DRBD-user] drbd or rsync?
greg.freemyer at gmail.com
Wed Apr 8 19:34:43 CEST 2009
On Wed, Apr 8, 2009 at 1:13 PM, Peter Sabaini <peter at sabaini.at> wrote:
> On Wednesday 08 April 2009 18:38:54 you wrote:
>> > A crucial factor is wether you can tolerate stale data. Running rsync
>> > periodically will probably result in not-quite-uptodate replication when
>> > the storm troopers come rushing in and cut your power. If that is not a
>> > concern, eg. because your data doesn't change all that often, or you
>> > simply don't care about a few lost updates, then rsync is IMHO simpler to
>> > set up.
>> > In contrast, DRBD does real replication. Depending on your link/network
>> > quality and the chosen protocol you can guarantee that local writes are
>> > only considered complete if the remote side also has completed.
>> Maybe it is a re-incarnation of executive order 6102, to melt down the
>> servers for their gold content?
>> Anyhow, provided the bandwidth is sufficient, I prefer the DRBD
>> option. However, will the backups be successful, if the backup drive
>> is attached to the secondary? Most of what I've read suggests that
>> you shouldn't even mount the secondary in read-only mode (although
>> maybe an LVM snapshot can be mounted for taking a backup).
> DRBD won't allow you to mount a secondary. This means you either have to use a
> Dual Primary setup (requiring a cluster filesystem) or make a block-level
> backup (don't know if thats adivsable). Maybe you can temporarily disconnect
> DRBD for the backup, promote the secondary, mount + backup, demote again, and
> reconnect -- Im not really sure if this would work though, you'd have to try.
I also don't think a snapshot will work either. The first step in
creating a snapshot is to quiesce every thing, then create the
snapshot. Since the quiesce would have to happen on the primary, I
don't think you could properly coordinate a snapshot being created on
If tape is critical to your situation, you may need to stick to rsync.
FYI: I think this should be added to the drdb wishlist. I think some
commercial SAN devices that replicate have to ability to mount the
remote copy by using snapshot technology on the remote.
And I know some support what Peter describes which is effectively:
stop block transmissions from primary to secondary
mount secondary in read-only form
release drdb to continue updates
If drdb doesn't support that, I'd also like it to go on the wishlist.
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