I have a CentOS 6 host system running a CentOS 6 KVM guest and the
guest seems to get about 5x slower disk throughput than the host:
host $ sudo dd of=/dev/datavol/disk1 if=/dev/zero bs=1M oflag=direct count=50000
50000+0 records in
50000+0 records out
52428800000 bytes (52 GB) copied, 150.36 s, 349 MB/s
host $ ssh guest dd of=/dev/vdb if=/dev/zero bs=1M oflag=direct count=50000
50000+0 records in
50000+0 records out
52428800000 bytes (52 GB) copied, 731.007 s, 71.7 MB/s
This is actually a vast improvement over earlier tests where it was a
factor of 40x difference. But even still, I thought virtio disks were
supposed to be much closer to native host speed than 5x.
/dev/datavol/disk1 is a RAIDed LV in an LVM volume group consisting of 5 disks:
$ sudo pvs
PV VG Fmt Attr PSize PFree
/dev/sda3 lvm2 a- 420.16g 420.16g
/dev/sdb datavol lvm2 a- 465.76g 227.66g
/dev/sdc datavol lvm2 a- 465.76g 245.76g
/dev/sdd datavol lvm2 a- 465.76g 245.76g
/dev/sde datavol lvm2 a- 465.76g 245.76g
/dev/sdf datavol lvm2 a- 465.76g 245.76g
$ sudo lvdisplay --maps /dev/datavol/disk1
--- Logical volume ---
LV Name /dev/datavol/disk1
VG Name datavol
LV UUID 3gD1N3-ybAU-GzUO-8QBV-b6op-lsK9-GMNm3w
LV Write Access read/write
LV Status available
# open 1
LV Size 500.00 GiB
Current LE 128000
Segments 1
Allocation inherit
Read ahead sectors auto
- currently set to 256
Block device 253:1
--- Segments ---
Logical extent 0 to 127999:
Type striped
Stripes 5
Stripe size 4.00 KiB
Stripe 0:
Physical volume /dev/sdb
Physical extents 5120 to 30719
Stripe 1:
Physical volume /dev/sdc
Physical extents 5120 to 30719
Stripe 2:
Physical volume /dev/sdd
Physical extents 5120 to 30719
Stripe 3:
Physical volume /dev/sde
Physical extents 5120 to 30719
Stripe 4:
Physical volume /dev/sdf
Physical extents 5120 to 30719
The KVM command line is as follows:
$ tr '\0' '\n' < /proc/9409/cmdline
/usr/libexec/qemu-kvm
-S
-M
rhel6.0.0
-enable-kvm
-m
1024
-smp
1,sockets=1,cores=1,threads=1
-name
guest
-uuid
e2bf97e0-cfb7-444c-abc3-9efe262d8efe
-nodefconfig
-nodefaults
-chardev
socket,id=monitor,path=/var/lib/libvirt/qemu/guest.monitor,server,nowait
-mon
chardev=monitor,mode=control
-rtc
base=utc
-boot
c
-drive
file=/var/lib/libvirt/images/cdrom.iso,if=none,media=cdrom,id=drive-ide0-1-0,readonly=on,format=raw
-device
ide-drive,bus=ide.1,unit=0,drive=drive-ide0-1-0,id=ide0-1-0
-drive
file=/var/lib/libvirt/images/guest.qcow2,if=none,id=drive-virtio-disk0,boot=on,format=qcow2
-device
virtio-blk-pci,bus=pci.0,addr=0x4,drive=drive-virtio-disk0,id=virtio-disk0
-drive
file=/dev/datavol/disk1,if=none,id=drive-virtio-disk1,format=raw
-device
virtio-blk-pci,bus=pci.0,addr=0x6,drive=drive-virtio-disk1,id=virtio-disk1
-netdev
tap,fd=23,id=hostnet0
-device
virtio-net-pci,netdev=hostnet0,id=net0,mac=52:54:00:ec:8d:47,bus=pci.0,addr=0x3
-chardev
pty,id=serial0
-device
isa-serial,chardev=serial0
-usb
-vnc
127.0.0.1:2
-vga
cirrus
-device
virtio-balloon-pci,id=balloon0,bus=pci.0,addr=0x5
That is started from virt-manager given the following configuration:
<domain type='kvm' id='18'>
<name>guest</name>
<uuid>e2bf97e0-cfb7-444c-abc3-9efe262d8efe</uuid>
<memory>1048576</memory>
<currentMemory>1048576</currentMemory>
<vcpu>1</vcpu>
<os>
<type arch='x86_64' machine='rhel6.0.0'>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/libexec/qemu-kvm</emulator>
<disk type='file' device='cdrom'>
<driver name='qemu' type='raw'/>
<source file='/var/lib/libvirt/images/cdrom.iso'/>
<target dev='hdc' bus='ide'/>
<readonly/>
<alias name='ide0-1-0'/>
<address type='drive' controller='0' bus='1' unit='0'/>
</disk>
<disk type='file' device='disk'>
<driver name='qemu' type='qcow2'/>
<source file='/var/lib/libvirt/images/guest.qcow2'/>
<target dev='vda' bus='virtio'/>
<alias name='virtio-disk0'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x04' function='0x0'/>
</disk>
<disk type='block' device='disk'>
<driver name='qemu' type='raw'/>
<source dev='/dev/datavol/disk1'/>
<target dev='vdb' bus='virtio'/>
<alias name='virtio-disk1'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x06' function='0x0'/>
</disk>
<controller type='ide' index='0'>
<alias name='ide0'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x01' function='0x1'/>
</controller>
<interface type='network'>
<mac address='52:54:00:ec:8d:47'/>
<source network='default'/>
<target dev='vnet2'/>
<model type='virtio'/>
<alias name='net0'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/>
</interface>
<serial type='pty'>
<source path='/dev/pts/3'/>
<target port='0'/>
<alias name='serial0'/>
</serial>
<console type='pty' tty='/dev/pts/3'>
<source path='/dev/pts/3'/>
<target port='0'/>
<alias name='serial0'/>
</console>
<input type='mouse' bus='ps2'/>
<graphics type='vnc' port='5902' autoport='yes'/>
<video>
<model type='cirrus' vram='9216' heads='1'/>
<alias name='video0'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x02' function='0x0'/>
</video>
<memballoon model='virtio'>
<alias name='balloon0'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x05' function='0x0'/>
</memballoon>
</devices>
</domain>
Any ideas why the huge performance difference?
Cheers,
b.
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