Win2k8 guest with very poor disk performance on KVM host

Win2k8 guest with very poor disk performance on KVM host

[Daniel Tschritter]

Hi everybody,

I got a server with CentOS 6.3 and KVM as a host and a windows 2k8 
guest.

The windows machine's disk performance is very poor.
The windows guest uses VirtIO disk drivers, no cache and uses a LVM 
partition on a Raid1.

atop shows 100% disk utilization as soon as the windows guest accesses 
the HDD, data transfers figures given are most times less than 1MB/s 
r/w, peaks are around 3MB/s r/w.

I've run a few tests to see what's going on:

Creating a 10GB test file on CentOS (guests switched off):
 	time dd if=/dev/random of=testfile bs=1 count=0 seek=10G
	0+0 records in
	0+0 records out
	0 bytes (0 B) copied, 5.777e-06 s, 0.0 kB/s
	real 0m0.001s
	user 0m0.000s
	sys 0m0.000s
 
Create copy of test file created above:
	time cp testfile testfile2
	real 0m3.136s
	user 0m0.440s
	sys 0m2.693s
That looks ok to me. According to atop data transfer rates are between 
130 and 180MB/s.

Create copy of test file above while Windows guest boots up:
	time cp testfile testfile2
	real 0m3.367s
	user 0m0.515s
	sys 0m2.826s
not much different...

Creating copy of test file above within the Win2k8R2 guest:
	Current Time: 20:12:32,41
	copy testfile testfile2
	1 file(s) copied
	Current Time: 20:22:08,64
	576,23s
That takes about 170 time longer than the copy unter CentOS!

I've run the same test on a CentOS6.3 guest with the following results:
	time cp testfile testfile2
	real 0m3.950s
	user 0m0.470s
	sys 0m3.383s
that's almost as quick as the host...

I've run these tests a few times, always giving about the same result.

Why is the disk performance in the Win guest that poor?

What can be done to improve things?

Comments and ideas welcome!

Re: Win2k8 guest with very poor disk performance on KVM host

[Avi Kivity]

On 09/10/2012 09:33 PM, Daniel Tschritter wrote:
> Hi everybody,
> 
> I got a server with CentOS 6.3 and KVM as a host and a windows 2k8 
> guest.
> 
> The windows machine's disk performance is very poor.
> The windows guest uses VirtIO disk drivers, no cache and uses a LVM 
> partition on a Raid1.
> 
> atop shows 100% disk utilization as soon as the windows guest accesses 
> the HDD, data transfers figures given are most times less than 1MB/s 
> r/w, peaks are around 3MB/s r/w.
> 
> I've run a few tests to see what's going on:
> 
> Creating a 10GB test file on CentOS (guests switched off):
>  	time dd if=/dev/random of=testfile bs=1 count=0 seek=10G
> 	0+0 records in
> 	0+0 records out
> 	0 bytes (0 B) copied, 5.777e-06 s, 0.0 kB/s
> 	real 0m0.001s
> 	user 0m0.000s
> 	sys 0m0.000s
>  
> Create copy of test file created above:
> 	time cp testfile testfile2
> 	real 0m3.136s
> 	user 0m0.440s
> 	sys 0m2.693s
> That looks ok to me. According to atop data transfer rates are between 
> 130 and 180MB/s.
> 
> Create copy of test file above while Windows guest boots up:
> 	time cp testfile testfile2
> 	real 0m3.367s
> 	user 0m0.515s
> 	sys 0m2.826s
> not much different...
> 
> Creating copy of test file above within the Win2k8R2 guest:
> 	Current Time: 20:12:32,41
> 	copy testfile testfile2
> 	1 file(s) copied
> 	Current Time: 20:22:08,64
> 	576,23s
> That takes about 170 time longer than the copy unter CentOS!
> 
> I've run the same test on a CentOS6.3 guest with the following results:
> 	time cp testfile testfile2
> 	real 0m3.950s
> 	user 0m0.470s
> 	sys 0m3.383s
> that's almost as quick as the host...
> 
> I've run these tests a few times, always giving about the same result.
> 
> Why is the disk performance in the Win guest that poor?
> 
> What can be done to improve things?

Your tests are invalid.  On Linux the files you create are sparse, on
Windows they are not.


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