[Sheepdog] Databases, write barriers etc

MORITA Kazutaka morita.kazutaka at lab.ntt.co.jp
Tue Aug 9 13:23:05 CEST 2011

At Tue, 9 Aug 2011 11:27:54 +0100,
Brian Candler wrote:
> On Tue, Aug 09, 2011 at 06:59:54PM +0900, MORITA Kazutaka wrote:
> > > If an app wants to update a single block, will this require Sheepdog to
> > > fetch an entire 4MB object, modify part of it, and write it back?
> > 
> > The app sends only the data to be written, and the server daemon
> > overwrites the existing object.  The app doesn't fetch the entire
> > object.
> Thank you. So it's just like a normal block write, and the issues are (a)
> network latency, and (b) software latency and scheduling delays in the
> gateway and object manager processes.
> network latency would exist in an iSCSI SAN environment anyway.
> I know very little about how Linux schedules KVM guests. Would it help to
> run the sheep process with a negative 'nice' value?

I'm not sure how it affect the performance results, sorry.  Perhaps it
would improve the performance, but we need tests to confirm that.



> Regards,
> Brian.
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