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Re: [Qemu-devel] [PATCH] sheepdog: implement direct write semantics


From: Stefan Hajnoczi
Subject: Re: [Qemu-devel] [PATCH] sheepdog: implement direct write semantics
Date: Fri, 4 Jan 2013 17:38:30 +0100
User-agent: Mutt/1.5.21 (2010-09-15)

On Thu, Jan 03, 2013 at 09:43:53PM +0800, Liu Yuan wrote:
> On 12/25/2012 04:45 PM, Liu Yuan wrote:
> > Well, I found setting cache=directsync will contain 'BDRV_O_CACHE_WB'.
> > Is this a bug for current master? If no, my current scheme will be the
> > only way to bypass cache of sheepdog.
> 
> Ping. Can anyone confirm it is a bug that 'cache=directsync' will pass
> BDRV_O_CACHE_WB' in the flags?

Hi Yuan,
BDRV_O_NOCACHE means bypass host page cache (O_DIRECT).

BDRV_O_CACHE_WB specifies the cache semantics that the guest sees - that
means whether the disk cache is writethrough or writeback.

In other words, BDRV_O_NOCACHE is a host performance tweak while
BDRV_O_CACHE_WB changes the cache safety of the BlockDriverState.  A
protocol driver like sheepdog doesn't need to look at BDRV_O_CACHE_WB
because it is implemented in block.c (see bdrv_co_do_writev() where QEMU
will flush when after each write when !bs->enable_write_cache).

Stefan



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