can a VM's harddisk be faster than the physical disk?

Stefan Sticht sticht at sticht.com
Mon Jan 10 20:11:22 UTC 2011


Dear Ubuntu friends,

I have a little home-grown box whose VM's harddisk seems to be faster than the host's harddisk:

My Maverick VM:

root at keira:~# hdparm -t /dev/sda
/dev/sda:
 Timing buffered disk reads:  1184 MB in  3.00 seconds = 394.25 MB/sec
root at keira:~# hdparm -t /dev/sda
/dev/sda:
 Timing buffered disk reads:  1250 MB in  3.01 seconds = 414.66 MB/sec
root at keira:~# hdparm -t /dev/sda
/dev/sda:
 Timing buffered disk reads:  1130 MB in  3.00 seconds = 376.05 MB/sec

My Maverick host:

root at black:~# hdparm -t /dev/vg0/vm 
/dev/vg0/vm:
 Timing buffered disk reads:  840 MB in  3.01 seconds = 279.41 MB/sec
root at black:~# hdparm -t /dev/vg0/vm 
/dev/vg0/vm:
 Timing buffered disk reads:  862 MB in  3.00 seconds = 287.06 MB/sec

The VM's disk is a qcow2 file on a LVM volume on a RAID-0 over 3 SATA disks.  CPU is i7-920. Running Maverick with qemu-kvm 0.13.

Possible explanations? Is the time in my VM not accurate?

Regards,
Stefan



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