Ubuntu
kvm package

KVM and disk flush on guest = 100% cpu

Asked by Roman R

Hello,

I'll start from that I've searched the forums for the problem solution, but wasn't able to find.
so here it is:
host:
ubuntu 10.04.1 LTS
2.6.32-27-server x86_64
2x Intel(R) Xeon(R) CPU E5620 (total of 16 cpus for os)
12GB RAM
2x software raids:
160GB and 2TB
the 160GB one is for system and 2TB one is for kvm images

guest:
Ubuntu 10.04.1 LTS
Disk: virtio
network: virtio

Problem:
the problem starts when the flush process starting.
it becomes using 100% of cpu for a while and then ok. and this happens on random periods of time

PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND

 869 root 20 0 0 0 0 S 117 0.0 0:17.80 flush-252:0
252 is the vda disk:
monitor@monitor:~$ ls -l /dev/vda
brw-rw---- 1 root disk 252, 0 2011-01-13 16:44 /dev/vda

and this feels bad: guest responses very slowly for the time flush process is active. it doesnt seem to affect host.
the server is just bought. no errors in dmesg about SW raid.

any ideas to solve it? ot mby turn off that sick flushing on the guest?

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Roman R (romeo-r) said : 		#1

============!!!!============
update
============!!!!============
update:
seems like this problem exists only with virtio block driver! (virtio hdd). made it ide and no more such problems. a bug?

i'm installing 10.10 guest now with virtio block drive, gonna see if the problem exists there.

confirmed. with virtio driver same problem. flush process takes up to 100 and more cpu %.

if some1 interested in debugging, answer please. as time is critical to me, i will install ubuntu 10.10 server on the kvm host tomorrow.

Revision history for this message
Roman R (romeo-r) said : 		#2

10.10 same problem with virtio disk. kind of a bug. where to report?

Revision history for this message
Eliah Kagan (degeneracypressure) said : 		#3

To report bugs in Ubuntu, follow these guidelines:
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By reporting bugs, you help to improve Ubuntu, so thank you for doing this!

Please feel free to post again if you have any questions about those guidelines, or otherwise about this issue. Once you report the bug successfully, discussion of the bug will probably move to the bug tracker.

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