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I also experienced really bad disk I/O performance with qcow2 images
(under CentOS 6.4 hosts.)<br>
When I converted the disk image to a raw logical volume (created
with lvm2) I get almost bare-metal disk I/O performance.<br>
<br>
Also note mentioning: check if your disk partitions are properly
aligned and begin at 4k block boundaries. I use parted for this. For
more info see
<a class="moz-txt-link-freetext" href="http://rainbow.chard.org/2013/01/30/how-to-align-partitions-for-best-performance-using-parted/">http://rainbow.chard.org/2013/01/30/how-to-align-partitions-for-best-performance-using-parted/</a>
or google it.<br>
<br>
There are more performance tuning options, e.g. you can set
vm.swappiness = 0 on the host's Linux kernel. You can also try
different kernel scheduling options, etc. These gave me only minor
performace gains. The most important part was getting away from
qcow2 and using properly aligned disk partitions.<br>
<br>
Zoltan<br>
<br>
<div class="moz-cite-prefix">On 5/4/2014 12:58 PM, Luca Gervasi
wrote:<br>
</div>
<blockquote
cite="mid:CAPJi5KPEmY12NN7XapzfpnPoB6zonfjB3yfRw-OPKfkgRT90nQ@mail.gmail.com"
type="cite">
<div dir="ltr">Hello,
<div><br>
</div>
<div>i'm trying to convert my physical web servers to a virtual
guest. What i'm experiencing is a poor disk i/o, compared to
the physical counterpart (having strace telling me that each
write takes approximately 100 times the time needed on
physical).</div>
<div><br>
</div>
<div>Tested hardware is pretty good (HP Proliant 360p Gen8 with
2xSAS 15k rpm 48 Gb Ram).</div>
<div><br>
</div>
<div>The hypervisor part is a minimal Centos 6.5 with libvirt.</div>
<div>The guest is configured using: VirtIO as disk bus, qcow2
storage format (thick allocation), cache mode: none (needed
for for live migration - this could be changed if is the
bottleneck), IO mode: default.</div>
<div><br>
</div>
<div>Is someone willing to give me some adivices? :)</div>
<div><br>
</div>
<div>Thanks</div>
<div>
<br>
</div>
<div>Luca</div>
</div>
<br>
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</pre>
</blockquote>
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