Re: Poor Write I/O Performance on KVM-79

Rodrigo Campos
2009-01-04T18:07:54Z

On Sun, Jan 4, 2009 at 11:24 AM, Avi Kivity <avi@redhat.com> wrote:
> Alexander Atticus wrote:
>>
>> Hello!
>>
>> I have been experimenting with KVM and have been experiencing poor write
>> I/O
>> performance. I'm not sure whether I'm doing something wrong or if this is
>> just the current state of things.
>>
>> While writing to the local array on the node running the guests I get
>> about
>> 200MB/s from dd (bs=1M count=1000) or about 90MB/s write performance from
>> iozone (sequencial) when I write to a 2G file with a 16M record length.
>> The
>> node is an 8 disk system using 3ware in a RAID50 configuration. It has
>> 8GB
>> of RAM.
>>
>> The guests get much slower disk access. The guests are using file based
>> backends (tried both qcow2 and raw) with virtio support. With no other
>> activity on the machine, I get about 6 to 7MB/s write performance from
>> iozone with the same test. Guests are running Debian lenny/sid with
>> 2.6.26-1-686.
>>
>
> qcow2 will surely lead to miserable performance. raw files are better.
> best is to use lvm.
>

What do you mean with best is to use lvm ?
You just say to use raw images on an lvm partition because you can
easily resize it ? Or somehow images only use the used space of the
raw file when used with lvm ? Or there's some trick to make it ?

Thanks a lot,

Rodrigo