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

[Tony Hoyle <tmh@xxxxxxxxxxxx>]

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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