From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:42207) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1cpwiq-0006Jm-Hi for qemu-devel@nongnu.org; Mon, 20 Mar 2017 08:47:41 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1cpwil-0000dY-Mf for qemu-devel@nongnu.org; Mon, 20 Mar 2017 08:47:40 -0400 Received: from mx-v6.kamp.de ([2a02:248:0:51::16]:58648 helo=mx01.kamp.de) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1cpwil-0000cR-DC for qemu-devel@nongnu.org; Mon, 20 Mar 2017 08:47:35 -0400 References: <82d604c8-068a-aff4-6037-e3cd247b3588@redhat.com> <819af057-3777-dffc-4670-895b8265fd01@kamp.de> <32e1e781-f0b0-ac67-a5ce-74ccc64071a0@kamp.de> <20170320024649.GA18938@lemon.lan> <37879546-cf6e-01fa-adc6-c777e14eab0e@redhat.com> <20170320114926.GB17020@lemon.lan> From: Peter Lieven Message-ID: Date: Mon, 20 Mar 2017 13:47:31 +0100 MIME-Version: 1.0 In-Reply-To: <20170320114926.GB17020@lemon.lan> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] callout to *file in bdrv_co_get_block_status List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Fam Zheng , Paolo Bonzini Cc: "qemu-devel@nongnu.org" , qemu block Am 20.03.2017 um 12:49 schrieb Fam Zheng: > On Mon, 03/20 12:21, Paolo Bonzini wrote: >> >> On 20/03/2017 03:46, Fam Zheng wrote: >>> On Fri, 03/17 12:20, Peter Lieven wrote: >>>> Am 17.03.2017 um 12:16 schrieb Paolo Bonzini: >>>>> On 17/03/2017 12:11, Peter Lieven wrote: >>>>>>>> like VMDK or QCOW2 shouldn't we trust the information from the l2 tables in the VMDK or QCOW2? >>>>>>> It provides additional information, for example it works better with >>>>>>> prealloc=metadata. >>>>>> Okay, understood. Can you imagine of a away to conditionally avoid this second callout? In my case we have an additional >>>>>> lseek for each cluster. For a 20GB file this are approx. 327k calls to lseek. And if the file has no preallocated metadata >>>>>> it will likely not improve anything. And even if the metadata is prealloced what is the allocation status of the clusters? >>>>> If the metadata is preallocated, cluster will (or should) show up as >>>>> zero, speeding up the copy. >>>> Okay, in this case the second call out to *file will not happen. It only happens if the metadata says it contains data. >>>> So where does it actually help? >>>> >>>> The condition is: (ret & BDRV_BLOCK_DATA) && !(ret & BDRV_BLOCK_ZERO) && (ret & BDRV_BLOCK_OFFSET_VALID)) >>>> >>>> So from my view it can only have any effect if the metadata returns BDRV_BLOCK_DATA, but the protocol driver returns >>>> BDRV_BLOCK_ZERO. >>>> >>>> This can only happen if I partially write to a cluster, or am I wrong here? >>> I think you have a point. The metadata should have said BDRV_BLOCK_ZERO if >>> protocol would say BDRV_BLOCK_ZERO - there is no reason the format driver cannot >>> know. >> That's true of qcow2, but many formats (including raw :)) don't know >> about BDRV_BLOCK_ZERO. > Raw is a little special, it could have forwarded the call to *file in its > BlockDriver callback. Most formats with metadata stores zero/nonzero information > in L1/L2 tables. For qcow2 and VMDK I think it's okay to just trust meta data on > zero/nonzero. > > Fam BTW, the extra check was added in commit 5daa74a6ebce7543aaad178c4061dc087bb4c705 Author: Paolo Bonzini Date: Wed Sep 4 19:00:38 2013 +0200 block: look for zero blocks in bs->file Reviewed-by: Eric Blake Signed-off-by: Paolo Bonzini Signed-off-by: Stefan Hajnoczi It was introduced while introducing bdv_get_block_status. I don't know what the real issue was that was addressed with this patch? Peter