qemu-kvm -snapshot

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Hi,
I am trying to benchmark disk I/O performance on VM running with 
"-snapshot" option enabled.
In order to do that I specify cache=none in the -drive parameter (yes I 
am running qemu-kvm at command line0.
The problem is that if I use this option kvm seems to ignore the 
cache=none directive and I get weird output from iozone (I/O values are 
better than on the real machine, indicating that caching is enabled). 
The same command line, without "-snapshot" gives a correct output.

Has anyone a clue about that? I succeeded in disabling cache in the past 
(with -snapshot enabled), but apparently this is not working anymore and 
I can't explain why.

If this can help, here is the command line I am using:

/usr/libexec/qemu-kvm  -m 2048 -smp 1 -name test-1 \
-boot c -drive 
file=/var/lib/libvirt/images/test-1.img,if=virtio,index=0,boot=on,cache=none 
\
-net nic,macaddr=54:52:00:37:a9:eb,model=virtio -net tap \
-vnc 127.0.0.1:10 -k en-us -snapshot &

the host is running centos5 and all latest updates are applied.

Cheers,

Andrea

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Andrea Chierici - INFN-CNAF 			
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Office Tel: +39 051 6092809	
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