[CentOS-virt] CentOS 6 kvm disk write performance

Julian price centos.org at julianprice.org.uk
Sat Aug 11 14:34:10 EDT 2012

 >> I have 2 similar servers. Since upgrading one from CentOS 5.5 to 6, 
disk write performance in kvm guest VMs is much worse.

Philip Durbin wrote:
 > Nice post, Julian. It generated some feedback at 
http://irclog.perlgeek.de/crimsonfu/2012-08-10 and a link to 
 > Phil

Thanks Phil for linking to my post on #crimsonfu, and reporting the 
result back here.

In response to the points jimi_c raised there:

 > I don't see his test numbers for different caching options

All the test figures I gave are using cache=writethrough.
cache=writeback produced much better figures than even the host (about 
180MB/s), because it is really writing to memory, but I don't think it's 
a safe option to use.
cache=none produced worse figures.
I didn't include the figures because I did those tests before I started 
using bonnie++ (I was just timing copying files before that) and I'd 
already ruled cacheing out as a solution.
 > also, is he doing deadline on the guest, host, or both?

deadline on the host - didn't try it on the guest.

 > not sure if they implemented it yet, but they were talking about a vm 
host setting for tuned
 > and one for guests
 > yeah, Wagner mentioned it in his summit presentation: 
 > they should be available in rhel 6.3 according to his presentation

Well, tuned-adm is a gift for part-time sysadmins like myself.

Some of the guest disk write figures were close to the host's & better 
than CentOS 5 after doing...

yum install tuned
tuned-adm profile virtual-host

..in the host and...

yum install tuned
tuned-adm profile virtual-guest

...in the guest.

Here are the new bonnie++ guest block write figures in MB/s. all using 
tuned-host and virtio, with & without tuned-guest.  Not sure why there's 
so much variation, but at least they're all much better.

45    tuned-host
73    tuned-host
50    tuned-host + tuned-guest
37    tuned-host + tuned-guest

 > rhel/centos 6 probably went with a more conservative tuning option

Certainly looks that way.  It's be interesting to know what & why.

Before jimi_c provided the tuned-adm tip, I was hoping that running the 
VM off a block device might be the answer.
qemu-img convert /media/vm027/hda.raw -O /dev/vm/vm031

...but they are worse than running off a raw virtual disk file.

16    tuned-host virtio
20    tuned-host virtio
27    tuned-host tuned-guest virtio
24    tuned-host tuned-guest virtio

I'm not convinced, maybe there are other factors at work.  I'd 
investigate further if my plan A wasn't back on track.

The bonnie++ figures I gave before measuring host disk write performance 
were for the host *root* partition, not the LVM volumes that the guest 
VMs use.  What if the problem is LVM, not KVM?  So I did some timings 
comparing the root drive with LVM volumes, some with & some without 
tuned-host.  'archive' and 'vmxxx' are the lvm volume names: (Note: 
these timings are done in the host, not in a guest)

69    /
69    /
70    /
65    / + tuned-host
64    / + tuned-host

55    archive
56    archive
53    vm027
48    vm027
33    vm027 + tuned-host
38    vm027 + tuned-host
53    vm022 + tuned-host
85    vm022 + tuned-host

This indicates that there is a wide variation in performance between 
different LVM volumes, and all the LVM volumes are performing worse than 
the root.  (It's interesting that with tuned-host, the times seem to be 
mostly worse, but with greater deviation.)  I repeated the above test on 
the CentOS 5 server (without tuned-host of course) and found the same 
thing - LVMs perform worse than root and vary widely:

54    /
50    /
39    archive
45    archive
39    archive
49    vm022
34    vm027
33    vm027

A slight performance hit might be expected for LVM, but I though it was 
meant to be negligible?  If the figures fell into 2 bands - good and bad 
- then I'd be looking for a specific problem like a sector alignment, 
but they don't, and isn't sector alignment meant to be fixed on CentOS 
6?  The variation in performance indicates a problem of variable 
severity like fragmentation or the position on the physical disk - but I 
don't think either of those are likely causes, because there's only one 
file in each volume, and physical disk position shouldn't have such a 
marked effect should it?  Any other suggestions?


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