Hi,
I'm having trouble benchmarking disk i/o in my vm's. The data I get seems
bogus.
I have two centos 6 guests which use a raw image as volume. Each volume is
stored on its own physical disk and both disks are the same model. The host
system is fedora 15 with the virt-preview repo enabled. The disks for the
guests use virtio and caching is set to none.
My problem is that I get very different results when I benchmark I/O in
these guests even though I shouldn't. Doing a seekmark I get:
guest 1: 120 seeks/s
guest 2: 220 seeks/s
"hdparm -t" shows:
guest 1: 100 MB/s
guest 1: 160 MB/s
What's worse is that when I restart the guests the results change and
suddenly guest 1 is a lot faster and guest 2 is a lot slower however as
long as the guests are running repeating the benchmarks give consistent
results.
When I test the disks on the host directly I get:
seekmark:
/dev/sdb: 75 seeks/s
/dev/sdc: 75 seeks/s
hdparm -t:
/dev/sdb: 140 MB/s
/dev/sdc: 140 MB/s
What bugs me is not so much the absolute numbers (for now) but the fact
that these guests give so wildly inconsistent results. Even if the jump
from 75 seeks/s to 120 seeks/s from host to guest is explainable by the way
block i/o is handled in the virtualization layer I would still expect both
guests to return similar results and I would also expect to see similar
results across restarts of a single guest.
I've attached the definition for both guests even though they are virtually
identical.
Does anyone have an idea what's happening here?
Regards,
Dennis
<domain type='kvm'>
<name>gw1</name>
<uuid>ac24997b-a63c-e136-2ad4-061e7e9a72f8</uuid>
<memory>786432</memory>
<currentMemory>786432</currentMemory>
<vcpu>1</vcpu>
<os>
<type arch='x86_64' machine='pc-0.14'>hvm</type>
<boot dev='hd'/>
</os>
<features>
<acpi/>
<apic/>
<pae/>
</features>
<clock offset='utc'/>
<on_poweroff>destroy</on_poweroff>
<on_reboot>restart</on_reboot>
<on_crash>restart</on_crash>
<devices>
<emulator>/usr/bin/qemu-kvm</emulator>
<disk type='file' device='disk'>
<driver name='qemu' type='raw' cache='none'/>
<source file='/mnt/backup01/libvirt/images/gw1.img'/>
<target dev='vda' bus='virtio'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x05' function='0x0'/>
</disk>
<interface type='bridge'>
<mac address='52:54:00:1d:db:49'/>
<source bridge='br0'/>
<model type='virtio'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/>
</interface>
<interface type='network'>
<mac address='52:54:00:81:7d:79'/>
<source network='internal'/>
<model type='virtio'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x04' function='0x0'/>
</interface>
<serial type='pty'>
<target port='0'/>
</serial>
<console type='pty'>
<target type='serial' port='0'/>
</console>
<input type='mouse' bus='ps2'/>
<graphics type='vnc' port='-1' autoport='yes'/>
<video>
<model type='cirrus' vram='9216' heads='1'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x02' function='0x0'/>
</video>
<memballoon model='virtio'>
<address type='pci' domain='0x0000' bus='0x00' slot='0x06' function='0x0'/>
</memballoon>
</devices>
</domain>
<domain type='kvm'>
<name>gw2</name>
<uuid>f8e383ec-99ae-6500-cf8d-bf0b341106f9</uuid>
<memory>786432</memory>
<currentMemory>786432</currentMemory>
<vcpu>1</vcpu>
<os>
<type arch='x86_64' machine='pc-0.14'>hvm</type>
<boot dev='hd'/>
</os>
<features>
<acpi/>
<apic/>
<pae/>
</features>
<clock offset='utc'/>
<on_poweroff>destroy</on_poweroff>
<on_reboot>restart</on_reboot>
<on_crash>restart</on_crash>
<devices>
<emulator>/usr/bin/qemu-kvm</emulator>
<disk type='file' device='disk'>
<driver name='qemu' type='raw' cache='none'/>
<source file='/mnt/backup02/libvirt/images/gw2.img'/>
<target dev='vda' bus='virtio'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x05' function='0x0'/>
</disk>
<interface type='bridge'>
<mac address='52:54:00:2f:9c:b6'/>
<source bridge='br0'/>
<model type='virtio'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/>
</interface>
<interface type='network'>
<mac address='52:54:00:57:f3:49'/>
<source network='internal'/>
<model type='virtio'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x04' function='0x0'/>
</interface>
<serial type='pty'>
<target port='0'/>
</serial>
<console type='pty'>
<target type='serial' port='0'/>
</console>
<input type='mouse' bus='ps2'/>
<graphics type='vnc' port='-1' autoport='yes'/>
<video>
<model type='cirrus' vram='9216' heads='1'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x02' function='0x0'/>
</video>
<memballoon model='virtio'>
<address type='pci' domain='0x0000' bus='0x00' slot='0x06' function='0x0'/>
</memballoon>
</devices>
</domain>
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