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.
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