From mboxrd@z Thu Jan 1 00:00:00 1970 From: Avi Kivity Subject: Re: linux-aio usable? Date: Mon, 08 Mar 2010 18:36:37 +0200 Message-ID: <4B952795.2040202@redhat.com> References: <4B94BF05.20609@redhat.com> <20100308094801.GA24943@schleppi.birkenwald.de> <4B94C806.9000209@redhat.com> <4B95091D.4020307@redhat.com> <4B95259F.7060505@codemonkey.ws> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Cc: Dustin Kirkland , Bernhard Schmidt , kvm@vger.kernel.org To: Anthony Liguori Return-path: Received: from mx1.redhat.com ([209.132.183.28]:33189 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755333Ab0CHQg5 (ORCPT ); Mon, 8 Mar 2010 11:36:57 -0500 In-Reply-To: <4B95259F.7060505@codemonkey.ws> Sender: kvm-owner@vger.kernel.org List-ID: On 03/08/2010 06:28 PM, Anthony Liguori wrote: >> I thought there was some autodetection involved, but perhaps I just >> imagined it. > > > There's no autodetection. > > linux-aio support in the kernel downgrades to synchronous IO if the > underlying storage does not support linux-aio. There is no indication > to userspace that this has happened. > > If this happens, besides having a slow guest, the guest VCPU will be > starved during the I/O requests potentially resulting in things like > soft lockups and time drift. > > Generally, speaking, linux-aio will work well under the following > circumstances: > > - cache=off is specified > - the underlying file system is XFS or you are using a block device > > We cannot detect this reliably though so it's really up to the user to > decide whether to use it. We're working on improving the linux-aio > kernel interface though to eliminate this detectability problem after > which, we can enable it in a more automatic fashion. Well, the common case of cache=none on a block device certainly can be autodetected. -- error compiling committee.c: too many arguments to function