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Date: Tue, 28 Oct 2008 12:45:58 -0500
From: Anthony Liguori <anthony@codemonkey.ws>
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Subject: Re: [Qemu-devel] [RFC] Disk integrity in QEMU
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To: qemu-devel@nongnu.org
Cc: Chris Wright <chrisw@redhat.com>, Mark McLoughlin <markmc@redhat.com>, Ryan Harper <ryanh@us.ibm.com>, kvm-devel <kvm-devel@lists.sourceforge.net>, Laurent Vivier <Laurent.Vivier@bull.net>

Ian Jackson wrote:
> Anthony Liguori writes ("[Qemu-devel] [RFC] Disk integrity in QEMU"):
>   
>> So to summarize, I think we should enable O_DSYNC by default to ensure 
>> that guest data integrity is not dependent on the host OS, and that 
>> practically speaking, cache=off is only useful for very specialized 
>> circumstances.  Part of the patch I'll follow up with includes changes 
>> to the man page to document all of this for users.
>>     
>
> I have a patch which does this and allows the host to control the
> buffering with the IDE cache control facility.
>   

Do you mean that the guest can control host disk cachability?  We've 
switched to always use O_DSYNC by default.  There was a very long thread 
about it including benchmarks.  With the right posix-aio tuning, we can 
use O_DSYNC without hurting performance*.

* Write performance drops but only because write performance was greater 
than native before.  It now is at native performance.

Regards,

Anthony Liguori

> I'll be resubmitting it shortly (if I manage to get round to it before
> going away for three weeks on Thursday lunchtime ...)
>
> Ian.
>
>
>