kvm io performance after kernel 3.2/3.3

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Hi All,

I found after update my kernel from 3.0.25 to 3.2.12 or same with kernel 3.3
Disk performance inside VM reduced by 10x times!!! ( for both kvm
0.12.5 and qemu 1.0.1)

For example restore postgresql database in kernel 3.0.25 took only
48sec, while in 3.2.12 or 3.3 more than 9m

command to launch VM:
qemu-system-x86_64 -m 512 -drive
file=/Virtual/sql.raw,if=virtio,cache=none -enable-kvm
or raw lvm disk:
qemu-system-x86_64 -m 512 -drive
file=/dev/V1/sqltest,if=virtio,cache=none -enable-kvm

than in VM:
time cat 20120323.sql | psql dbtest -U dbtest

results:

kernel 3.0.25
real	0m53.659s
user	0m1.587s
sys	0m2.362s

kernel 3.2.12 ( or 3.3 - similar results )
real    9m6.563s
user    0m4.508s
sys     0m1.752s

in vmware 8 ws (kernel 3.3):
real    0m41.765s
user    0m2.468s
sys     0m1.068s


Any explanation for that? And how bring kvm back to speed?

one more interesting test on kernel 3.2.12
assign lvm to iscsi ( using tgt)
qemu-system-x86_64 -m 512 -drive file=/dev/sdg,if=virtio,cache=none -enable-kvm

result:
real    1m44.862s
user    0m1.392s
sys     0m0.612s


-- 
--
Michael
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