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Date: Wed, 2 Sep 2009 21:49:12 +0200
From: Christoph Hellwig <hch@lst.de>
Subject: Re: [Qemu-devel] [PATCH 1/4] block: add enable_write_cache flag
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To: Anthony Liguori <anthony@codemonkey.ws>
Cc: Christoph Hellwig <hch@lst.de>, qemu-devel@nongnu.org

On Wed, Sep 02, 2009 at 08:13:52AM -0500, Anthony Liguori wrote:
> >performance should be kind of a worst case.  All these patches are
> >with Linux 2.6.31-rc8 + my various barrier fixes on guest and host,
> >using ext3 with barrier=1 on both.
> >  
> 
> Does barrier=0 make a performance difference?  IOW, would the typical 
> default ext3 deployment show worse behavior?

Note for this tyical ext3 deployment the barrier patches are kinda
useless, because we still don't have any data integrity guarantees at
all.  Anyway, here are the numbers with barrier=0 on host and guest:

data=writeback, no write cache advertised:

	9m37.890s, 9m38.303s, 9m38.423s, 9m38.861s, 9m39.599s

data=writeback, write cache advertized (and backed by fdatasync):

	9m39.649s, 9m39.772s, 9m40.149s, 9m41.737s, 9m41.996s