Hi, I'm having trouble benchmarking disk i/o in my vm's. The data I get seems bogus. I have two centos 6 guests which use a raw image as volume. Each volume is stored on its own physical disk and both disks are the same model. The host system is fedora 15 with the virt-preview repo enabled. The disks for the guests use virtio and caching is set to none. My problem is that I get very different results when I benchmark I/O in these guests even though I shouldn't. Doing a seekmark I get: guest 1: 120 seeks/s guest 2: 220 seeks/s "hdparm -t" shows: guest 1: 100 MB/s guest 1: 160 MB/s What's worse is that when I restart the guests the results change and suddenly guest 1 is a lot faster and guest 2 is a lot slower however as long as the guests are running repeating the benchmarks give consistent results. When I test the disks on the host directly I get: seekmark: /dev/sdb: 75 seeks/s /dev/sdc: 75 seeks/s hdparm -t: /dev/sdb: 140 MB/s /dev/sdc: 140 MB/s What bugs me is not so much the absolute numbers (for now) but the fact that these guests give so wildly inconsistent results. Even if the jump from 75 seeks/s to 120 seeks/s from host to guest is explainable by the way block i/o is handled in the virtualization layer I would still expect both guests to return similar results and I would also expect to see similar results across restarts of a single guest. I've attached the definition for both guests even though they are virtually identical. Does anyone have an idea what's happening here? Regards, Dennis -------------- next part -------------- A non-text attachment was scrubbed... Name: gw1.xml Type: text/xml Size: 1794 bytes Desc: not available URL: <http://lists.centos.org/pipermail/centos-virt/attachments/20111008/0e698e7b/attachment-0012.xml> -------------- next part -------------- A non-text attachment was scrubbed... Name: gw2.xml Type: text/xml Size: 1794 bytes Desc: not available URL: <http://lists.centos.org/pipermail/centos-virt/attachments/20111008/0e698e7b/attachment-0013.xml>
On Saturday, October 8, 2011 at 12:45 PM, Dennis Jacobfeuerborn wrote:> Hi, > I'm having trouble benchmarking disk i/o in my vm's. The data I get seems > bogus.have you tried perf top, you can see their other performance statistics there. there is more going on during a disk write than writing what your guests want. I think there is hardware passthrough for disks also. -- Nehemiah I. Dacres Saint Louis University: Advanced Technology Group Linux System Administrator Sent with Sparrow (http://www.sparrowmailapp.com/?sig) -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.centos.org/pipermail/centos-virt/attachments/20111009/efab2fb6/attachment-0006.html>
On 10/09/2011 04:32 PM, Nehemiah wrote:> On Saturday, October 8, 2011 at 12:45 PM, Dennis Jacobfeuerborn wrote: > >> Hi, >> I'm having trouble benchmarking disk i/o in my vm's. The data I get seems >> bogus. > > have you tried perf top, you can see their other performance statistics > there. there is more going on during a disk write than writing what your > guests want. I think there is hardware passthrough for disks also.At the moment I'm not interested in the benchmarks themselves but the reason why they show such an odd behavior. The fact that there might be "more going on" doesn't account for the results I'm seeing. Regards, Dennis