From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from [140.186.70.92] (port=36782 helo=eggs.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1P61ic-0006Vn-On for qemu-devel@nongnu.org; Wed, 13 Oct 2010 09:50:11 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1P61ib-00085u-JE for qemu-devel@nongnu.org; Wed, 13 Oct 2010 09:50:10 -0400 Received: from mx1.redhat.com ([209.132.183.28]:21931) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1P61ib-00085d-Cy for qemu-devel@nongnu.org; Wed, 13 Oct 2010 09:50:09 -0400 Message-ID: <4CB5B908.1020406@redhat.com> Date: Wed, 13 Oct 2010 15:50:00 +0200 From: Avi Kivity 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> In-Reply-To: <4CB5B2FD.9030205@linux.vnet.ibm.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: Anthony Liguori Cc: Kevin Wolf , Christoph Hellwig , Stefan Hajnoczi , qemu-devel@nongnu.org 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. > > 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. -- I have a truly marvellous patch that fixes the bug which this signature is too narrow to contain.