From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:53370) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1cpvod-0000zm-Vj for qemu-devel@nongnu.org; Mon, 20 Mar 2017 07:49:36 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1cpvod-0002uw-3I for qemu-devel@nongnu.org; Mon, 20 Mar 2017 07:49:36 -0400 Date: Mon, 20 Mar 2017 19:49:26 +0800 From: Fam Zheng Message-ID: <20170320114926.GB17020@lemon.lan> 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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <37879546-cf6e-01fa-adc6-c777e14eab0e@redhat.com> 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: Paolo Bonzini Cc: Peter Lieven , "qemu-devel@nongnu.org" , qemu block 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