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Date: Sun, 12 Oct 2008 14:59:28 -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: Avi Kivity <avi@redhat.com>
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>

Avi Kivity wrote:
> But would increase latency, memory bus utilization, and cpu overhead.
>
> In the cases where the page cache buys us something (host page cache
> significantly larger than guest size), that's understandable.  But for
> the other cases, why bother?  Especially when many systems don't have
> this today.
>
> Let me phrase this another way: is there an argument against O_DIRECT? 
>   

It slows down any user who frequently restarts virtual machines.  It 
slows down total system throughput when there are multiple virtual 
machines sharing a single disk.  This later point is my primary concern 
because in the future, I expect disk sharing to be common in some form 
(either via common QCOW base images or via CAS).

I'd like to see a benchmark demonstrating that O_DIRECT improves overall 
system throughput in any scenario today.  I just don't buy the cost of 
the extra copy today is going to be significant since the CPU cache is 
already polluted.  I think the burden of proof is on O_DIRECT because 
it's quite simple to demonstrate where it hurts performance (just the 
time it takes to do two boots of the same image).

> In a significant fraction of deployments it will be both simpler and faster.
>   

I think this is speculative.  Is there any performance data to back this up?


Regards,

Anthony Liguori