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

From: Anthony Liguori
Subject: Re: [Qemu-devel] [RFC] Disk integrity in QEMU
Date: Sun, 12 Oct 2008 14:59:28 -0500
User-agent: Thunderbird (X11/20080925)

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?


Anthony Liguori

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