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Date: Wed, 29 Jun 2011 16:21:42 +0200
From: Kevin Wolf <kwolf@redhat.com>
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Subject: Re: [Qemu-devel] Default cache mode
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To: Christoph Hellwig <hch@lst.de>
Cc: Qemu-devel@nongnu.org, Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>, Frediano Ziglio <freddy77@gmail.com>, Avi Kivity <avi@redhat.com>

Am 29.06.2011 16:12, schrieb Christoph Hellwig:
> On Wed, Jun 29, 2011 at 04:11:53PM +0200, Kevin Wolf wrote:
>> If the guest is acting correctly, then this assumption is definitely
>> true with cache=none/writeback. What needs to happen is that the guest
>> issues a disk flush before sending your "transaction ok" message. If it
>> doesn't do that it can fail even on real hardware. cache=writethrough
>> makes things safe that aren't even safe on real hardware, that's why I
>> consider it extremely conservative.
> 
> Depends on your defintion of real hardware.  Traditionally disks always
> had WCE=0 semantics.  When ATA disks added volatile writecaches and people
> started seeing problems due to it the advertisement of cache modes and
> cache flush commands were added.  For non-ATA disks like SAS or FC
> WCE=1 still is the typical delivery setting.

Sure, I'm not saying that it definitely breaks on any hardware, just
that it can break.

Kevin