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Date: Tue, 14 Oct 2008 17:43:50 +0200
From: Avi Kivity <avi@redhat.com>
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Subject: Re: [Qemu-devel] [RFC] Disk integrity in QEMU
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To: Anthony Liguori <anthony@codemonkey.ws>
Cc: Chris Wright <chrisw@redhat.com>, Mark McLoughlin <markmc@redhat.com>, kvm-devel <kvm-devel@lists.sourceforge.net>, Laurent Vivier <Laurent.Vivier@bull.net>, qemu-devel@nongnu.org, Ryan Harper <ryanh@us.ibm.com>

Anthony Liguori wrote:
> Avi Kivity wrote:
>> I don't think O_DIRECT should be qemu's default, since anyone using qemu
>> directly is likely a "causal virtualization" user.  Management systems
>> like ovirt should definitely default to O_DIRECT (really, they shouldn't
>> even offer caching).
>>   
>
> ovirt isn't a good example because the default storage model is
> iSCSI.  Since you aren't preserving zero-copy, I doubt that you'll see
> any advantage to using O_DIRECT (I suspect the code paths aren't even
> different).

If you have a hardware iSCSI initiator then O_DIRECT pays off.  Even for
a software initiator, the write path could be made zero copy.  The read
path doesn't look good though.

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.