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Re: [Xen-users] disk performance about half in domU? + question about Xe

To: xen-users@xxxxxxxxxxxxxxxxxxx
Subject: Re: [Xen-users] disk performance about half in domU? + question about XenSource
From: Tony Hoyle <tmh@xxxxxxxxxxxx>
Date: Mon, 13 Aug 2007 22:47:07 +0100
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Johnn Tan wrote:
Based on some tests we ran, it seems the biggest performance hit you get from running within domU is from disk I/O. We did some mysql read and write tests, and for both, our performance is about half that compared to native. Has that been others' experience?

Is there any way to make this better? We are using physical partitions. In contrast, cpu/memory tests appear to be near native.


Does buying commercial Xen (XenSource) help in anyway? Do they have optimized disk drivers?

There's also a fairly big hit on network I/O.

I don't believe you can get it any faster with the current architecture as it doesn't support any kind of DMA.. it's all software copying of data from one place to another.

One thing to do is give the dom0 its own processor - makes quite a big difference. If course because there's only one process in dom0 doing the whole I/O (qemu-dm) it'll slow down with each new domU you start up.

loopback partitions are faster because a lot gets cached in RAM, although because of that they're not so great if you get a sudden power loss. Never had a kernel that supported blktap so I don't know what kind of speed you can get out of that.

Tony

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