From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:49757) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1cpx7h-0004mm-M3 for qemu-devel@nongnu.org; Mon, 20 Mar 2017 09:13:22 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1cpx7e-00031T-FD for qemu-devel@nongnu.org; Mon, 20 Mar 2017 09:13:21 -0400 Received: from mx-v6.kamp.de ([2a02:248:0:51::16]:38393 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 1cpx7e-00030u-5h for qemu-devel@nongnu.org; Mon, 20 Mar 2017 09:13:18 -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 14:13:13 +0100 MIME-Version: 1.0 In-Reply-To: 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 13:47 schrieb Peter Lieven: > 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? Is it possible that this optimization was added especially for RAW? I was believing that raw would forward the get_block_status call to bs->file, but it looks it doesn't. If this one here was for RAW would it be an option to move this callout to the raw-format driver and remove it from the generic code? Peter