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Re: [Qemu-devel] [RFC] Disk integrity in QEMU

From: Avi Kivity
Subject: Re: [Qemu-devel] [RFC] Disk integrity in QEMU
Date: Tue, 14 Oct 2008 17:43:50 +0200
User-agent: Thunderbird (X11/20080723)

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 

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