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Re: [Xen-devel] question about disk performance in domU

To: "Santos, Jose Renato G" <joserenato.santos@xxxxxx>
Subject: Re: [Xen-devel] question about disk performance in domU
From: xuehai zhang <hai@xxxxxxxxxxxxxxx>
Date: Mon, 21 Nov 2005 16:41:53 -0600
Cc: keahey@xxxxxxxxxxx, Tim Freeman <tfreeman@xxxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxx
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Renato and Tim,
Thank you for your replies.
Is there anyway to flush the dom0 file buffer before running hdparm on domU 
each time?
Xuehai

Santos, Jose Renato G wrote:

-----Original Message-----
From: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx [mailto:xen-devel-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of Tim Freeman
Sent: Monday, November 21, 2005 12:45 PM
To: xen-devel@xxxxxxxxxxxxxxxxxxx
Cc: keahey@xxxxxxxxxxx; xuehai zhang
Subject: Re: [Xen-devel] question about disk performance in domU


So the "Timing buffered disk reads" show much higher results. I see the DMA zone is larger in domU, but since this is mapped to a loopback file I'm guessing the physical disk performance should only be affected by what dom0's DMA zone is set to on the node running the domU if that is all that was going on. But dom0 performance seems comparable to native linux. Is this huge "timing buffered disk read" difference accurate? Does the domU benefit from some other cache of the loopback file in dom0?

Yes. It seems to me that this is the effect of the file buffer cache on dom0. hdparm flushes the file cache on domU to make sure that there is no data in the file buffer cache when measuring device access times (reported as buffered disk
  reads). However when  using VBDs mapped to files, the data is
  also cached on dom0 file buffer. Therefore data is not coming
directly from the device but from dom0 file cache. Note how the amount of data that is read in 3 sec increases at each step,
  since data read in previous steps come from dom0 file cache.

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