<div dir="ltr">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. <div>
<br></div><div>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.</div>
<div><br></div><div>So I think this is not really an issue, just a matter of perspective. </div><div><br></div><div>Deron</div><div><br></div></div><div class="gmail_extra"><br><br><div class="gmail_quote">On Wed, Apr 9, 2014 at 4:15 AM, Stefano Stabellini <span dir="ltr"><<a href="mailto:stefano.stabellini@eu.citrix.com" target="_blank">stefano.stabellini@eu.citrix.com</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">On Sun, 6 Apr 2014, Deron wrote:<br>
> Hello.<br>
><br>
> I have been doing some performance tests with Xen and CentOS and have found some strange statistics when comparing the<br>
> virtual memory (/proc/vmstat) and I/O (/proc/diskstat) statistics between the Domain 0 and guests. <br>
><br>
> When I look at the "pgpgin" statistic it is exactly the same as the "read sectors" in Dom0. However, when I look at<br>
> the same statistics in the guest, the "read sectors" is exactly 2x "pgpgin".<br>
><br>
> I have tried this on 2 different versions, each on a different physical server. Here is an example of the statistics<br>
> over a 30 second period (while running a benchmark in Dom1) in the Dom0 and Guest Domain.<br>
> pgpgin read sectors<br>
> ======= =========<br>
> Dom0 1698024 1698024<br>
> Dom1 848412 1696824<br>
><br>
> I have looked at some various settings to see if I could explain it. I can't tell if the Dom1 guest kernel is<br>
> reporting incorrectly, or there is some other explanation. If anyone has any ideas about this I would appreciate any<br>
> feedback.<br>
><br>
> Dom 0 kernels<br>
> 3.10.29-11.el6.centos.alt.x86_64<br>
> 3.4.54-8.el6.centos.alt.x86_64<br>
><br>
> Guest kernel<br>
> 2.6.32-431.el6.x86_64<br>
<br>
The numbers that you are seeing are probably wrong due to an accounting<br>
issue in blkfront or blkback. It would be interesting to know whether<br>
this happens even with the most recent kernels.<br>_______________________________________________<br>
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<br></blockquote></div><br></div>