<html><body><div style="color:#000; background-color:#fff; font-family:HelveticaNeue, Helvetica Neue, Helvetica, Arial, Lucida Grande, sans-serif;font-size:10pt"><div><span>Hello Richard,</span></div><div style="color: rgb(0, 0, 0); font-size: 13.3333px; font-family: HelveticaNeue,Helvetica Neue,Helvetica,Arial,Lucida Grande,sans-serif; background-color: transparent; font-style: normal;"><span><br></span></div><div style="color: rgb(0, 0, 0); font-size: 13.3333px; font-family: HelveticaNeue,Helvetica Neue,Helvetica,Arial,Lucida Grande,sans-serif; background-color: transparent; font-style: normal;"><span>I tried two things with sparse=true:</span></div><div style="color: rgb(0, 0, 0); font-size: 13.3333px; font-family: HelveticaNeue,Helvetica Neue,Helvetica,Arial,Lucida Grande,sans-serif; background-color: transparent; font-style: normal;"><span>cache=None and my VM wouldnt</span></div><div style="color: rgb(0, 0, 0); font-size: 13.3333px;
font-family: HelveticaNeue,Helvetica Neue,Helvetica,Arial,Lucida Grande,sans-serif; background-color: transparent; font-style: normal;"><span>cache=writeback and my VM builds OK.</span></div><div style="color: rgb(0, 0, 0); font-size: 13.3333px; font-family: HelveticaNeue,Helvetica Neue,Helvetica,Arial,Lucida Grande,sans-serif; background-color: transparent; font-style: normal;"><br><span></span></div><div style="color: rgb(0, 0, 0); font-size: 13.3333px; font-family: HelveticaNeue,Helvetica Neue,Helvetica,Arial,Lucida Grande,sans-serif; background-color: transparent; font-style: normal;"><span>When I use cache=None I get the problem that I described in my 1st posting, that there is a problem mounting: /.</span></div><div style="color: rgb(0, 0, 0); font-size: 13.3333px; font-family: HelveticaNeue,Helvetica Neue,Helvetica,Arial,Lucida Grande,sans-serif; background-color: transparent; font-style: normal;"><br><span></span></div><div style="color: rgb(0,
0, 0); font-size: 13.3333px; font-family: HelveticaNeue,Helvetica Neue,Helvetica,Arial,Lucida Grande,sans-serif; background-color: transparent; font-style: normal;"><span>Is this normal?</span></div><div style="color: rgb(0, 0, 0); font-size: 13.3333px; font-family: HelveticaNeue,Helvetica Neue,Helvetica,Arial,Lucida Grande,sans-serif; background-color: transparent; font-style: normal;"><br></div><div style="color: rgb(0, 0, 0); font-size: 13.3333px; font-family: HelveticaNeue,Helvetica Neue,Helvetica,Arial,Lucida Grande,sans-serif; background-color: transparent; font-style: normal;">Regards,<br><span></span></div><div style="color: rgb(0, 0, 0); font-size: 13.3333px; font-family: HelveticaNeue,Helvetica Neue,Helvetica,Arial,Lucida Grande,sans-serif; background-color: transparent; font-style: normal;"><span>James</span></div><div style="display: block;" class="yahoo_quoted"> <div style="font-family: HelveticaNeue, Helvetica Neue, Helvetica, Arial,
Lucida Grande, sans-serif; font-size: 10pt;"> <div style="font-family: HelveticaNeue, Helvetica Neue, Helvetica, Arial, Lucida Grande, Sans-Serif; font-size: 12pt;"> <div dir="ltr"> <font face="Arial" size="2"> On Monday, 31 March 2014, 11:00, Richard W.M. Jones <rjones@redhat.com> wrote:<br> </font> </div> <div class="y_msg_container">On Mon, Mar 31, 2014 at 09:59:22AM +0100, James Harrison wrote:<br clear="none">[...]<br clear="none"><br clear="none">I wasn't aware that cache=none was necessary for live migration. It<br clear="none">seems really broken that this is necessary, see Stefan's explanation<br clear="none">here:<br clear="none"><br clear="none"><a shape="rect" href="https://groups.google.com/forum/#!topic/synnefo-devel/9MU5ujOcOt4" target="_blank">https://groups.google.com/forum/#!topic/synnefo-devel/9MU5ujOcOt4</a><br clear="none"><br clear="none">> Also Caching=None is set because there is a disk cache on the disk.<br
clear="none"><br clear="none">The qemu cache mode isn't really relevant to this. But I guess if<br clear="none">your disk has a huge cache (let's say it's a SAN LUN or a very<br clear="none">expensive RAID controller) then enabling the host cache<br clear="none">(ie. cache=writeback) might be overkill.<br clear="none"><br clear="none">> Would double caching would slow down I/O?<br clear="none"><br clear="none">Not in my testing. cache=writeback is a little faster.<br clear="none"><br clear="none">There are some caveats:<br clear="none"><br clear="none"> - cache=writeback is only safe if your guest sends write barriers,<br clear="none"> ie. recent Linux and recent Windows guests are OK, Linux and<br clear="none"> Windows prior to ~2008/2009 are NOT safe.<br clear="none"><br clear="none"> - Our performance team reckon that cache=none is better than<br clear="none"> cache=writeback, but their test configuration
fully provisions host<br clear="none"> memory with guests. You only benefit if your host actually has<br clear="none"> some free memory to give to the cache ...<br clear="none"><br clear="none"> - It may make migration impossible.<br clear="none"><br clear="none"> - Libguestfs is not representative of what real guests do.<div class="yqt8637523669" id="yqtfd74516"><br clear="none"><br clear="none">Rich.<br clear="none"><br clear="none">-- <br clear="none">Richard Jones, Virtualization Group, Red Hat <a shape="rect" href="http://people.redhat.com/~rjones" target="_blank">http://people.redhat.com/~rjones</a></div><br clear="none">virt-top is 'top' for virtual machines. Tiny program with many<br clear="none">powerful monitoring features, net stats, disk stats, logging, etc.<br clear="none"><a shape="rect" href="http://people.redhat.com/~rjones/virt-top" target="_blank">http://people.redhat.com/~rjones/virt-top</a><div
class="yqt8637523669" id="yqtfd70554"><br clear="none"></div><br><br></div> </div> </div> </div> </div></body></html>