Good afternoon list.
I have a CentOS 6 server running six KVM VMs, and am struggling with
poor disk write performance in the VMs.
Setup summary:
Physical server has 2 x 500GB SATA drives in a RAID1 mirror, and 3 x
2TB SATA drives in a RAID5 stripeset. The 2 x 500GB drives host /,
/boot, and the VM .img files.
The .img files are mounted as per the following example in virsh xml:
<disk type='file' device='disk'>
<driver name='qemu' type='raw' cache='none' io='native'/>
<source file='/vm/pool/server05.img'/>
<target dev='vda' bus='virtio'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x04'
function='0x0'/>
</disk>
Each virtual server also has a slice of the RAID5 array for data,
including a CentOS 5.7 Samba server that has a 3TB piece.
The Samba drive is mounted in virsh xml as follows:
<disk type='block' device='disk'>
<driver name='qemu' type='raw' cache='none' io='native'/>
<source dev='/dev/md2'/>
<target dev='vdb' bus='virtio'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x06'
function='0x0'/>
</disk>
Mount parameters in the VM are:
/dev/vdb1 /mnt/data ext3 suid,dev,exec,noatime 0 0
When I first built the server performance in the VMs was terribly bad
with constant CPU IOWAIT, but after a lot of searching the addition of
io=native significantly improved it.
It is copying file data to the Samba share that is causing me issues.
Some files I use are many GBs in size. Copies start at 110MBps across
the network, but once the Samba server's network copy cache fills up
(the server has 4GB RAM allocated, and the cache seems to fill after
about 2.5 to 3GB copied from the network) they hit a wall and crawl to
35MBps.
Allocating the Samba server more RAM tricks Windows into thinking the
copy is finished sooner (although it still hits a brick wall once the
larger cache is full), but of course the underlying data write
continues for quite some time after Windows is done.
I have done a fair bit of analysis with dstat and using dd copies, and
file writes onto the VM's RAID5 slice sit between 35 and 40MBps
consistently.
Using Samba on the physical server (not my normal practice, but for
troubleshooting) using a different partition on the same RAID5 array,
I get a consistent 75MBps overall write speed.
Reads from the partitions are comparable speed from both physical and
virtual servers (~100MBps).
Are there any hints from anyone please about how I can improve the
write performance in the VM? I realise it will likely never achieve
the bare metal speed, but closer than 50% would be good...
Thanks
Simon.
--
Simon Wilson
M: 0400 12 11 16
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