From mboxrd@z Thu Jan 1 00:00:00 1970 From: Kevin Wolf Subject: Re: [PATCH] qemu-img: Add nocache command line option Date: Wed, 15 Jun 2011 12:50:21 +0200 Message-ID: <4DF88E6D.2000304@redhat.com> References: <1308053382-9771-1-git-send-email-fsimonce@redhat.com> <4DF75E61.2010500@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Cc: Federico Simoncelli , kvm@vger.kernel.org To: Avi Kivity Return-path: Received: from mx1.redhat.com ([209.132.183.28]:49556 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751680Ab1FOKr2 (ORCPT ); Wed, 15 Jun 2011 06:47:28 -0400 Received: from int-mx09.intmail.prod.int.phx2.redhat.com (int-mx09.intmail.prod.int.phx2.redhat.com [10.5.11.22]) by mx1.redhat.com (8.14.4/8.14.4) with ESMTP id p5FAlSKM007472 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-SHA bits=256 verify=OK) for ; Wed, 15 Jun 2011 06:47:28 -0400 In-Reply-To: <4DF75E61.2010500@redhat.com> Sender: kvm-owner@vger.kernel.org List-ID: Am 14.06.2011 15:13, schrieb Avi Kivity: > On 06/14/2011 03:09 PM, Federico Simoncelli wrote: >> qemu-img currently writes images using writeback and filling up >> the cache buffers which are then flushed by the kernel preventing >> other processes from accessing the storage. >> This is particularly bad in cluster environments where time-based >> algorithms might be in place and accessing the storage within >> certain timeouts is critical. >> This patch adds the option to use nocache when other solutions >> (eg: cgroups) are not available. Your patch confuses a write-through cache (cache=writethrough == O_SYNC) with bypassing the cache (cache=none == O_DIRECT), which are entirely different. > Please copy qemu-devel@nongnu.org as this is not a kvm-specific patch. > >> diff --git a/qemu-img.c b/qemu-img.c >> index 4f162d1..c10e4a6 100644 >> --- a/qemu-img.c >> +++ b/qemu-img.c >> @@ -78,6 +78,7 @@ static void help(void) >> " rebasing in this case (useful for renaming the backing file)\n" >> " '-h' with or without a command shows this help and lists the supported formats\n" >> " '-p' show progress of command (only certain commands)\n" >> + " '-t' use write-through (no cache), valid with: convert, commit and rebase\n" >> "\n" >> "Parameters to snapshot subcommand:\n" >> " 'snapshot' is the name of the snapshot to create, apply or delete\n" > > IMO better to use the existing qemu option format, say -o > cache=writeback|writethrough|none|volatile. No, -o is used with qemu-img create for options that change what the image will look like (e.g. setting flags in the image header). This options is about the cache mode that is used for various operations. Kevin