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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: Fri, 10 Oct 2008 08:55:12 -0500
User-agent: Thunderbird (X11/20080925)

Paul Brook wrote:
But that said, I'm concerned that this is far too confusing for users.
I don't think anyone is relying on disk write performance when in
write-back mode simply because the guest already has a page cache so
writes are already being completed instantaneously from the
application's perspective.

This isn't entirely true. With IDE devices you don't have command queueing, so it's easy for a large write to stall subsequent reads for a relatively long time. I'm not sure how much this effects qemu, but I've definitely seen it happening on real hardware.

I think that suggests we should have a cache=wb option and if people report slow downs with IDE, we can observe if cache=wb helps. My suspicion is that it's not going to have a practical impact because as long as the operations are asynchronous (via DMA), then you're getting native-like performance.

My bigger concern is synchronous IO operations because then a guest VCPU is getting far less time to run and that may have a cascading effect on performance.

Anyway, I'll work up a new patch with cache=wb and repost.


Anthony Liguori


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