From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:45835) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1cpvNd-0005xt-IE for qemu-devel@nongnu.org; Mon, 20 Mar 2017 07:21:42 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1cpvNc-0001b7-PH for qemu-devel@nongnu.org; Mon, 20 Mar 2017 07:21:41 -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> From: Paolo Bonzini Message-ID: <37879546-cf6e-01fa-adc6-c777e14eab0e@redhat.com> Date: Mon, 20 Mar 2017 12:21:32 +0100 MIME-Version: 1.0 In-Reply-To: <20170320024649.GA18938@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 , Peter Lieven Cc: qemu block , "qemu-devel@nongnu.org" 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. Paolo