Re: Very poor write performance

[Richard Weinberger <richard@nod.at>]

Am 19.04.2014 15:50, schrieb Paolo Bonzini:
> Il 19/04/2014 08:04, Richard Weinberger ha scritto:
>> Hi!
>>
>> I hope this is the right place to ask. :)
>>
>> On a rather recent x86_64 server I'm facing very bad write performance.
>> The Server is a 8 Core Xeon E5 with 64GiB ram.
>>
>> Storage is a ext4 filesystem on top of LVM which is backed by DRBD.
>> On the host side dd can easily write with 100MiB/s to the ext4.
>> OS is Centos6 with kernel 3.12.x.
>>
>> Within a KVM Linux guest the seq write throughput is always only
>> between 20 and 30MiB/s.
>> The guest OS is Centos6, it uses virtio-blk, cache=none, io=natvie and
>> the deadline IO scheduler.
>>
>> The worst thing is that the total IO bandwidth of KVM seems to 30MiB/s.
>> If I run the same write benchmark within 5 guests each one achieves
>> only 6 or 7 MiB/s.
>> I see the same values also if the guest writes directly to a disk like vdb.
>> Having the guest disk directly on LVM instead of a ext4 file also didn't help.
>> It really looks like 30MiB/s is the upper bound for KVM disk IO.
> 
> As a first guess, can you try XFS or direct DRBD?  There seems to be a bug in ext4 that limits queue depth to a very low value.

Problem solved. I was hunting a non-issue.
The guests had swap enabled. m(
Therefore each guest was busy with reading/writing pages do disk.

Now I see write rates between 90 and 100 MiB/s within my guests. :-)

Thanks,
//richard