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Date: Mon, 17 May 2010 14:40:18 +0200
From: Christoph Hellwig <hch@lst.de>
Subject: Re: [Qemu-devel] Re: [PATCH 2/2] Add flush=off parameter to -drive
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To: Paul Brook <paul@codesourcery.com>
Cc: Kevin Wolf <kwolf@redhat.com>, Alexander Graf <agraf@suse.de>, Christoph Hellwig <hch@lst.de>, qemu-devel@nongnu.org, armbru@redhat.com

On Tue, May 11, 2010 at 11:11:12PM +0100, Paul Brook wrote:
> .. though it may be a kernel bug rather that a qemu bug, depending on the 
> exact details.  Either way, I consider any mode that inhibits host filesystem 
> write cache but not volatile drive cache to be pretty worthless.  Either we 
> guaranteed data integrity on completion or we don't.

O_DIRECT just means bypassing the pagecache, it does not mean flushing
the disk cache on every access, which for certain workloads can be very
painful.  It also doesn't require a synchronous writeout of metadata
required to reach the data, e.g. in case when we have to allocate blocks
for a sparse image file.

To get the behaviour you want you need O_DIRECT|O_DSYNC, which is
something that is not exposed by qemu's current cache= suboption.

If we want to implement this properly we need to split the cache option,
as I already mentioned.  This would also have benefits in other areas,
but again refer to my previous mail for that.