From mboxrd@z Thu Jan 1 00:00:00 1970 From: Paolo Bonzini Subject: Re: Very poor write performance Date: Sat, 19 Apr 2014 09:50:50 -0400 Message-ID: <53527F3A.4010705@redhat.com> References: Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit To: Richard Weinberger , kvm , Stefan Hajnoczi Return-path: Received: from mail-qa0-f47.google.com ([209.85.216.47]:64411 "EHLO mail-qa0-f47.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751452AbaDSNu5 (ORCPT ); Sat, 19 Apr 2014 09:50:57 -0400 Received: by mail-qa0-f47.google.com with SMTP id m5so2450944qaj.34 for ; Sat, 19 Apr 2014 06:50:56 -0700 (PDT) In-Reply-To: Sender: kvm-owner@vger.kernel.org List-ID: 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