I think I know why this is happening, and I think it is correct. I ran a test again only looking at Dom0 and found that it was similar to Dom1. The 'read sectors' from /proc/diskstats were about 2x the pgpgin from /proc/vmstat. This is whay I see on my physical hardware as well. The reason that I am not seeing the same results when reading data from Dom1 is because I am only looking at the physical block device (/dev/sda in this case). When I look at the diskstats from physical device /dev/sda and device between used for blkback /dev/tda they show the 2 reads. Since they are both "block devices" I assume they need to page in to make the data available. So I think this is not really an issue, just a matter of perspective. Deron On Wed, Apr 9, 2014 at 4:15 AM, Stefano Stabellini < stefano.stabellini at eu.citrix.com> wrote: > On Sun, 6 Apr 2014, Deron wrote: > > Hello. > > > > I have been doing some performance tests with Xen and CentOS and have > found some strange statistics when comparing the > > virtual memory (/proc/vmstat) and I/O (/proc/diskstat) statistics > between the Domain 0 and guests. > > > > When I look at the "pgpgin" statistic it is exactly the same as the > "read sectors" in Dom0. However, when I look at > > the same statistics in the guest, the "read sectors" is exactly 2x > "pgpgin". > > > > I have tried this on 2 different versions, each on a different physical > server. Here is an example of the statistics > > over a 30 second period (while running a benchmark in Dom1) in the Dom0 > and Guest Domain. > > pgpgin read sectors > > ======= ========= > > Dom0 1698024 1698024 > > Dom1 848412 1696824 > > > > I have looked at some various settings to see if I could explain it. I > can't tell if the Dom1 guest kernel is > > reporting incorrectly, or there is some other explanation. If anyone > has any ideas about this I would appreciate any > > feedback. > > > > Dom 0 kernels > > 3.10.29-11.el6.centos.alt.x86_64 > > 3.4.54-8.el6.centos.alt.x86_64 > > > > Guest kernel > > 2.6.32-431.el6.x86_64 > > The numbers that you are seeing are probably wrong due to an accounting > issue in blkfront or blkback. It would be interesting to know whether > this happens even with the most recent kernels. > _______________________________________________ > CentOS-virt mailing list > CentOS-virt at centos.org > http://lists.centos.org/mailman/listinfo/centos-virt > > -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.centos.org/pipermail/centos-virt/attachments/20140429/2133eeed/attachment-0006.html>