From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from [140.186.70.92] (port=47043 helo=eggs.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1P6232-0005Er-P1 for qemu-devel@nongnu.org; Wed, 13 Oct 2010 10:11:56 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1P6227-0004Sn-8n for qemu-devel@nongnu.org; Wed, 13 Oct 2010 10:11:15 -0400 Received: from mail-gw0-f45.google.com ([74.125.83.45]:57214) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1P6227-0004SX-5w for qemu-devel@nongnu.org; Wed, 13 Oct 2010 10:10:19 -0400 Received: by gwj15 with SMTP id 15so56205gwj.4 for ; Wed, 13 Oct 2010 07:10:18 -0700 (PDT) Message-ID: <4CB5BDC2.9090105@codemonkey.ws> Date: Wed, 13 Oct 2010 09:10:10 -0500 From: Anthony Liguori MIME-Version: 1.0 Subject: Re: [Qemu-devel] Re: [PATCH v2 6/7] qed: Read/write support References: <1286552914-27014-1-git-send-email-stefanha@linux.vnet.ibm.com> <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> In-Reply-To: <4CB5B908.1020406@redhat.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: Avi Kivity Cc: Kevin Wolf , Anthony Liguori , qemu-devel@nongnu.org, Christoph Hellwig , Stefan Hajnoczi On 10/13/2010 08:50 AM, 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. Yes, autoclear bits are essentially granular header dirty bits. Regards, Anthony Liguori > >> >> 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. >