<div dir="ltr"><br><div class="gmail_extra"><br><div class="gmail_quote">On Wed, Nov 30, 2016 at 9:48 AM, Martin Polednik <span dir="ltr"><<a href="mailto:mpolednik@redhat.com" target="_blank">mpolednik@redhat.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><span class="">On 29/11/16 22:01 +0200, Yaniv Kaul wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
It appears that VDSM changes the following params:<br>
vm.dirty_ratio = 5<br>
vm.dirty_background_ratio = 2<br>
<br>
Any idea why? Because we use cache=none it's irrelevant anyway?<br>
</blockquote>
<br></span>
It's not really irrelevant, the host still uses disk cache. Anyway,<br>
there is BZ[1] with a presentation[2] that (imho reasonably) states:<br>
<br>
"Reduce dirty page limits in KVM host to allow<br>
direct I/O writer VMs to compete successfully<br>
with buffered writer processes for storage<br>
access"<br></blockquote><div><br></div><div>Thanks, but it really makes no sense to me. The direct IO by the VMs is going to a different storage than what the host is writing to, in most cases.</div><div>The host would write to the local disk, the VMs - to a shared storage, across NFS or block layer or so. Moreover, their IO is not buffered.</div><div>There is very little IO coming from the host itself, generally (I hope so!). </div><div><br></div><div>Partially unrelated - the trend today is actually to put NOOP on the VMs - the deadline is quite meaningless, as the host scheduler will reschedule anyway as it see fits.</div><div>Most likely it is also a deadline scheduler (but could be NOOP as well if it's an all flash array, for example). Therefore there is no reason for anything but simple NOOP on the VMs themselves.</div><div><br></div><div>In short, I think it's an outdated decision that perhaps should be revisited. Not urgent, though.</div><div>Y.</div><div><br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<br>
I wonder why virtual-host tuned profile doesn't contain these values:<br>
<br>
$ grep vm.dirty /usr/lib/tuned/virtual-host/tu<wbr>ned.conf<br>
vm.dirty_background_ratio = 5<br>
<br>
[1]<a href="https://bugzilla.redhat.com/show_bug.cgi?id=740887" rel="noreferrer" target="_blank">https://bugzilla.redhat.com<wbr>/show_bug.cgi?id=740887</a><br>
[2]<a href="http://perf1.lab.bos.redhat.com/bengland/laptop/rhev/rhev-vm-rsptime.pdf" rel="noreferrer" target="_blank">http://perf1.lab.bos.redhat<wbr>.com/bengland/laptop/rhev/<wbr>rhev-vm-rsptime.pdf</a><br>
<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
TIA,<br>
Y.<br>
</blockquote>
<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
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</blockquote>
<br>
</blockquote></div><br></div></div>