Slow disk IO on virtio kvm guests with Centos 5.5 as hypervisor

* Slow disk IO on virtio kvm guests with Centos 5.5 as hypervisor
@ 2011-02-14 18:15 Thomas Broda
  2011-02-15  9:19 ` Stefan Hajnoczi
  0 siblings, 1 reply; 8+ messages in thread
From: Thomas Broda @ 2011-02-14 18:15 UTC (permalink / raw)
  To: kvm

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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