From mboxrd@z Thu Jan 1 00:00:00 1970 From: Avi Kivity Subject: Re: sync guest calls made async on host - SQLite performance Date: Thu, 15 Oct 2009 08:37:21 +0900 Message-ID: <4AD660B1.4090300@redhat.com> References: <4ACE0196.9010904@gmail.com> <4ACF89CB.5020406@gmail.com> <4AD1A27A.4060307@redhat.com> <20091013223714.GB16152@lst.de> <4AD5B00D.102@redhat.com> <20091014134122.GA14235@lst.de> <4AD602C8.4090603@redhat.com> <20091014170215.GA22621@lst.de> <4AD6569F.80803@codemonkey.ws> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: Christoph Hellwig , Matthew Tippett , Dustin Kirkland , RW , kvm@vger.kernel.org To: Anthony Liguori Return-path: Received: from mx1.redhat.com ([209.132.183.28]:57016 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1760723AbZJNXiG (ORCPT ); Wed, 14 Oct 2009 19:38:06 -0400 In-Reply-To: <4AD6569F.80803@codemonkey.ws> Sender: kvm-owner@vger.kernel.org List-ID: On 10/15/2009 07:54 AM, Anthony Liguori wrote: > Christoph Hellwig wrote: >> On Thu, Oct 15, 2009 at 01:56:40AM +0900, Avi Kivity wrote: >>> Does virtio say it has a write cache or not (and how does one say it?)? >> >> Historically it didn't and the only safe way to use virtio was in >> cache=writethrough mode. It didn't say? So it's up to the default, which is what? > > Which should be the default on Ubuntu's kvm that this report is > concerned with so I'm a bit confused. > > Avi's patch is a performance optimization, not a correctness issue? If filesystems do drain by default, it should be a no-op on cache!=writeback. However if lseek(0); write(1); fdatasync(); are faster than disk speed, then something in our assumptions has to be wrong. -- I have a truly marvellous patch that fixes the bug which this signature is too narrow to contain.