[DRBD-user] DRBD Replication Modes

Florian Haas florian at hastexo.com
Sun Dec 9 18:34:17 CET 2012

Note: "permalinks" may not be as permanent as we would like,
direct links of old sources may well be a few messages off.

On Fri, Dec 7, 2012 at 7:09 PM, Nathan Joyes
<njoyes at interprisesoftware.com> wrote:
> We are investigating the possibility of using DRBD to perform synchronous
> replication of our PostgreSQL database server. After reading the
> Replication modes section of the DRBD features documentation we concluded
> that we would need to use protocol C, which is describes writes to be
> considered complete only after both the local and the remote disk writes
> have been confirmed.  However, during our testing we were able to
> continuously write to the primary device after disabling network traffic
> on the secondary.


>  We also noticed that during high write loads on the
> primary the number of kilobytes out of sync would grow, but eventually the
> secondary would catch up.

Only when in disconnected mode. And then after the connection is
restored, you resync:

> After reading the documentation, we were surprised at the testing results.

Um, sorry, you seem to not have reviewed the documentation in
sufficient detail. :)

> Is this expected and/or correct behavior, or have we incorrectly configured
> something for the DRBD device?

Perfectly expected.

Now, if you're looking for PostgreSQL HA, the alternative to DRBD is
obviously to use Postgres synchronous replication, available since
9.1. Also integrated with Pacemaker. Since that option forgoes DRBD,
though, a discussion of that option would be off-topic for this list,
and if you're interested in pursuing that option I'd encourage you to
take this discussion to the Pacemaker or linux-ha mailing list.


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