From mboxrd@z Thu Jan  1 00:00:00 1970
From: Christoph Hellwig <hch@lst.de>
Subject: Re: sync guest calls made async on host - SQLite performance
Date: Wed, 14 Oct 2009 19:02:15 +0200
Message-ID: <20091014170215.GA22621@lst.de>
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Cc: Christoph Hellwig <hch@lst.de>,
	Matthew Tippett <tippettm@gmail.com>,
	Dustin Kirkland <dustin.kirkland@gmail.com>,
	Anthony Liguori <anthony@codemonkey.ws>, RW <kvm@tauceti.net>,
	kvm@vger.kernel.org
To: Avi Kivity <avi@redhat.com>
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On Thu, Oct 15, 2009 at 01:56:40AM +0900, Avi Kivity wrote:
> Does virtio say it has a write cache or not (and how does one say it?)?

Historically it didn't and the only safe way to use virtio was in
cache=writethrough mode.  Since qemu git as of 4th Sempember and Linux
2.6.32-rc there is a virtio-blk feature to communicate the existance
of a volatile write cache, and the support for a cache flush command.
With the combination of these two data=writeback and data=none modes
are safe for the first time.