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

[Avi Kivity <avi@redhat.com>]

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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