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Re: [Qemu-devel] [PATCH] sheepdog: implement SD_OP_FLUSH_VDI operation


From: Christoph Hellwig
Subject: Re: [Qemu-devel] [PATCH] sheepdog: implement SD_OP_FLUSH_VDI operation
Date: Tue, 24 Apr 2012 08:58:17 +0200
User-agent: Mutt/1.5.17 (2007-11-01)

On Tue, Apr 24, 2012 at 01:26:43AM +0900, MORITA Kazutaka wrote:
> SD_FLAG_CMD_CACHE is ignored in the older version of Sheepdog, so,
> even if we specify cache=writeback or cache=none, the data is written
> with O_DSYNC always and cannot be cached in the server's page cache or
> volatile disk cache either.  I think it is safe.
> 
> It seems that there is another version problem with this patch;
> bdrv_co_flush_to_disk() results in error with the older Sheepdog which
> doesn't support SD_OP_FLUSH_VDI.  The simple fix is to increment
> SD_PROTO_VER and prevent the newer qemu from connecting to the older
> Sheepdog cluster, I think.

Incrementin the version seems like a must.  But qemu can still connect
to older protocol versions fine if connecting to the current one fails,
t just must not send SD_OP_FLUSH_VDI (and preferably not set SD_FLAG_CMD_CACHE)
in that case.




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