[Sheepdog] Databases, write barriers etc

MORITA Kazutaka morita.kazutaka at lab.ntt.co.jp
Mon Aug 8 18:18:57 CEST 2011

At Mon, 8 Aug 2011 16:44:58 +0100,
Brian Candler wrote:
> I am considering using a Sheepdog-based KVM cluster to run a Postgres
> database, where data consistency is important.
> Are there any problems or issues I should consider in this application? Does
> Sheepdog support write barriers?  Should I use ext4 with journal
> checksumming?

Yes, both would work with Sheepdog.

> I notice the design doc says "The write request is successful only when all
> replicas can be successfully updated", so I guess fsync shouldn't return
> until all data has been written.

Yes, exactly.

> But I'd be grateful for any real-world experiences with databases on
> KVM+Sheepdog.

I think the performance could be a problem.  The latency of a Sheepdog
is higher than other local disk formats because of data replication
and network latency, so Sheepdog would show a poorer performance with
something like databases.



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