[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] Re: poor domU VBD performance.
> My dd command was always the same: "dd if=/dev/hdb6 bs=64k count=1000" and > it took 1.6 seconds on hdb6 and 2.2 seconds on hda1 when running in Dom0 > and it took 4.6 seconds on hdb6 and 5.8 seconds on hda1 when running on > DomU. I did one experiment with count=10000 and it took ten times as long > in each of the four cases. > > I have done the following tests: > DomU : dd if=/dev/hdb6 of=/dev/null bs=1024k count=4000 ; duration 301 sec > DomU : dd if=/dev/hdb6 of=/dev/null bs=1024k count=4000 ; duration 370 sec > > Dom0 : dd if=/dev/hdb6 of=/dev/null bs=1024k count=4000 ; duration 115 sec > Dom0 : dd if=/dev/hda1 of=/dev/null bs=1024k count=4000 ; duration 140 sec OK, I have produced this with both dd and o-direct now. On o-direct, I needed what was the effective dd block request size (128k) and I got similar results. My results are much worse, due to that I am driving 14 disks: dom0: 153.5 MB/sec domU: 12.7 MB/sec It looks like there might be a problem were we are not getting a timely response back from dom0 VBD driver that the io request is complete, which limits the number of outstanding requests to a level which cannot keep the disk utilized well. If you drive enough IO outstanding requests (which can be done with either o-direct with large request or a much larger readahead setting with buffered IO), it's not an issue. In the domU, can you try setting the readahead size to a much larger value using hdparm? Something like hdparm -a 2028, then run dd? -Andrew _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxxxxxxxx http://lists.xensource.com/xen-devel
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