Re: Very poor write performance

[Paolo Bonzini <pbonzini@redhat.com>]

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.

Paolo