On Mar 25, 2009, at 1:55 PM, Avi Kivity wrote:
Kurt Yoder wrote:
What do you mean about the cache? Is my test fundamentally flawed?
I *thought* I was testing write speed on the disk...
'dd', without further arguments, will write to the page cache and
let the kernel write the data back at a later time. If you increase
the block size (bs=1M count=1000) you should see much faster times
on the host, I wouldn't be surprised to see 1GB/s.
If you want to test disk speed, use of=/dev/blah oflag=direct.
Beware of destroying your data disk.
I see. I looked up another test: using hdparm -t. It doesn't show the
situation as quite so bad, but the guest is still a little over half
the speed of the host:
me@host:~$ sudo hdparm -t /dev/mapper/HW_RAID-ROOT
/dev/mapper/HW_RAID-ROOT:
Timing buffered disk reads: 282 MB in 3.00 seconds = 93.92 MB/sec
me@guest:~# hdparm -t /dev/vda
/dev/vda:
Timing buffered disk reads: 156 MB in 3.03 seconds = 51.56 MB/sec
Something weird is happening with your system. If you extend the
test, what does 'top' show? On both guest and host.
If I extend the test thusly on the guest:
dd if=/dev/zero of=/bigfile count=10000000
I see 100% CPU utilization on the guest, and 100% CPU utilization on
one of the host cores.
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