From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from [140.186.70.92] (port=47094 helo=eggs.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1P623H-0005Gm-75 for qemu-devel@nongnu.org; Wed, 13 Oct 2010 10:12:07 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1P622O-0004US-5u for qemu-devel@nongnu.org; Wed, 13 Oct 2010 10:11:25 -0400 Received: from mx1.redhat.com ([209.132.183.28]:43107) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1P622N-0004UN-VE for qemu-devel@nongnu.org; Wed, 13 Oct 2010 10:10:36 -0400 Message-ID: <4CB5BDD1.8050409@redhat.com> Date: Wed, 13 Oct 2010 16:10:25 +0200 From: Avi Kivity MIME-Version: 1.0 Subject: Re: [Qemu-devel] Re: [PATCH v2 6/7] qed: Read/write support References: <1286552914-27014-7-git-send-email-stefanha@linux.vnet.ibm.com> <4CB479D2.7030901@redhat.com> <4CB47D38.3060602@linux.vnet.ibm.com> <4CB48144.9030607@redhat.com> <20101012155953.GA13872@stefan-thinkpad.transitives.com> <4CB489D1.3050204@linux.vnet.ibm.com> <20101013121328.GB8998@stefan-thinkpad.transitives.com> <4CB5AF0D.9000800@redhat.com> <4CB5B2FD.9030205@linux.vnet.ibm.com> <4CB5B908.1020406@redhat.com> <20101013140715.GF8998@stefan-thinkpad.transitives.com> In-Reply-To: <20101013140715.GF8998@stefan-thinkpad.transitives.com> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit List-Id: qemu-devel.nongnu.org List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Stefan Hajnoczi Cc: Kevin Wolf , Anthony Liguori , qemu-devel@nongnu.org, Christoph Hellwig On 10/13/2010 04:07 PM, Stefan Hajnoczi wrote: > On Wed, Oct 13, 2010 at 03:50:00PM +0200, Avi Kivity wrote: > > On 10/13/2010 03:24 PM, Anthony Liguori wrote: > > >On 10/13/2010 08:07 AM, Kevin Wolf wrote: > > >>Am 13.10.2010 14:13, schrieb Stefan Hajnoczi: > > >>>We can avoid it when a backing image is not used. Your idea to check > > >>>for zeroes in the backing image is neat too, it may well reduce the > > >>>common case even for backing images. > > >>The additional requirement is that we're extending the file and not > > >>reusing an old cluster. (And bdrv_has_zero_init() == true, but QED > > >>doesn't work on host_devices anyway) > > > > > >Yes, that's a good point. > > > > > >BTW, I think we've decided that making it work on host_devices is > > >not that bad. > > > > > >We can add an additional feature called QED_F_PHYSICAL_SIZE. > > > > > >This feature will add another field to the header that contains an > > >offset immediately following the last cluster allocation. > > > > > >During a metadata scan, we can accurately recreate this field so > > >we only need to update this field whenever we clear the header > > >dirty bit (which means during an fsync()). > > > > If you make QED_F_PHYSICAL_SIZE an autoclear bit, you don't need the > > header dirty bit. > > Do you mean we just need to check the physical size header field against > the actual file size? If the two are different, then a consistency > check is forced. I thought you'd only use a header size field when you don't have a real file size. Why do you need both? > > > > > >That means we can maintain the physical size without introducing > > >additional fsync()s in the allocation path. Since we're already > > >writing out the header anyway, the write operation is basically > > >free too. > > > > I don't see how it is free. It's an extra write. The good news is > > that it's very easy to amortize. > > We only need to update the header field on disk when we're already > updating the header, so it's not even an extra write operation. Why would you ever update the header, apart from relocating L1 for some reason? -- I have a truly marvellous patch that fixes the bug which this signature is too narrow to contain.