[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] question about disk performance in domU
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 FreemanSent: 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 domUSo 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 diskreads). However when using VBDs mapped to files, the data is also cached on dom0 file buffer. Therefore data is not comingdirectly 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. _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxxxxxxxx http://lists.xensource.com/xen-devel
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