Hi,
I've set up a KVM hypervisor for testing purposes. It's running on an
AMD Quad-Core Phenom with a local RAID5 which is made of three disks
connected to a hardware RAID controller. On top of this, there an LVM
volume.
dd'ing /dev/zero to a testfile gives me a throughput of about 400MB/s
when done directly on the hypervisor. If I try this from within a
virtual guest, it's only 19MB/s to 24MB/s if the guest is on the LVM
volume (raw device, not qcow2 or something, no filesystem on top of the
LVM).
Even if I place the guest on an iSCSI target on my desktop PC (which
has one cheap SATA drive only), I get a write throughput of about
50MB/s to 60MB/s from within the virtual machine. What could be the
reason for the slow disk IO if the guests are stored locally on the
hypervisor?
The setup is as follows:
1. Package versions:
kvm-qemu-img-83-164.el5_5.25
kmod-kvm-83-164.el5_5.25
kvm-83-164.el5_5.25
libvirt-0.6.3-33.el5_5.3
2. Hypervisor kernel version: 2.6.18-194.32.1.el5
3. kvm_amd is loaded on the hypervisor, and the virtio stuff is loaded
on the guest:
virtio_net 47425 0
virtio_balloon 39877 0
virtio_blk 41417 3
virtio_pci 41417 0
virtio_ring 37441 1 virtio_pci
virtio 39365 4
virtio_net,virtio_balloon,virtio_blk,virtio_pci
4. Guest has been installed as follows:
virt-install --name spacewalk --vcpus 4 -r 2048 -u `uuidgen` --os-type
linux --os-variant virtio26 -b kvmbr0 -v --accelerate -c
/root/CentOS-5.5-x86_64-netinstall.iso --disk
path=/dev/vg0/lvspace,bus=virtio,sparse=false
5. Guest XML dump:
<domain type='kvm' id='11'>
<name>spacewalk</name>
<uuid>a6e2b1f7-d3eb-4e91-a1db-4e248a395b25</uuid>
<memory>2097152</memory>
<currentMemory>2097152</currentMemory>
<vcpu>4</vcpu>
<os>
<type arch='x86_64' machine='rhel5.4.0'>hvm</type>
<boot dev='cdrom'/>
</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/libexec/qemu-kvm</emulator>
<disk type='block' device='disk'>
<driver name='qemu' cache='none'/>
<source dev='/dev/vg0/lvspace'/>
<target dev='vda' bus='virtio'/>
</disk>
<disk type='file' device='cdrom'>
<source file='/root/CentOS-5.5-x86_64-netinstall.iso'/>
<target dev='hdc' bus='ide'/>
<readonly/>
</disk>
<interface type='bridge'>
<mac address='54:52:00:13:cf:58'/>
<source bridge='kvmbr0'/>
<target dev='vnet0'/>
<model type='virtio'/>
</interface>
<serial type='pty'>
<source path='/dev/pts/2'/>
<target port='0'/>
</serial>
<console type='pty' tty='/dev/pts/2'>
<source path='/dev/pts/2'/>
<target port='0'/>
</console>
<input type='mouse' bus='ps2'/>
<graphics type='vnc' port='5900' autoport='yes' listen='0.0.0.0'
keymap='de'/>
</devices>
</domain>
Thanks in advance,
--
Thomas
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