From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:54200) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1aj0pv-00034T-UP for qemu-devel@nongnu.org; Thu, 24 Mar 2016 04:41:48 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1aj0pq-0003z9-Rz for qemu-devel@nongnu.org; Thu, 24 Mar 2016 04:41:47 -0400 Received: from barbershop.grep.be ([89.106.240.122]:44731) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1aj0pq-0003yN-Lx for qemu-devel@nongnu.org; Thu, 24 Mar 2016 04:41:42 -0400 Date: Thu, 24 Mar 2016 09:41:29 +0100 From: Wouter Verhelst Message-ID: <20160324084129.GG1590@grep.be> References: <1458742562-30624-1-git-send-email-den@openvz.org> <1458742562-30624-3-git-send-email-den@openvz.org> <20160323175834.GC2467@grep.be> <20160323181454.GI4126@noname.redhat.com> <20160324082552.GB24831@phobos.sw.ru> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20160324082552.GB24831@phobos.sw.ru> Subject: Re: [Qemu-devel] [Nbd] [PATCH 2/2] NBD proto: add GET_LBA_STATUS extension List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Pavel Borzenkov Cc: Kevin Wolf , nbd-general@lists.sourceforge.net, qemu-devel@nongnu.org, Stefan Hajnoczi , "Denis V. Lunev" , Paolo Bonzini On Thu, Mar 24, 2016 at 11:25:52AM +0300, Pavel Borzenkov wrote: > On Wed, Mar 23, 2016 at 07:14:54PM +0100, Kevin Wolf wrote: > > Am 23.03.2016 um 18:58 hat Wouter Verhelst geschrieben: > > > On Wed, Mar 23, 2016 at 05:16:02PM +0300, Denis V. Lunev wrote: > > > > + the provisioning state of the device. The following provisionnig states > > > > + are defined for the command: > > > > + > > > > + - `NBD_STATE_ALLOCATED` (0x0), LBA extent is present on the block device; > > > > + - `NBD_STATE_ZEROED` (0x1), LBA extent is present on the block device > > > > + and contains zeroes; > > > > > > Presumably this should be "contains only zeroes"? > > > > > > Also, this may end up being a fairly expensive call for the server to > > > process. Is it really useful? > > > > I think we need to make clear that this is meant as an optimisation and > > it's always a valid option for a server to return NBD_STATE_ALLOCATED > > even if the contents is zeroed. > > > > It is definitely useful if the server has a means to efficiently find > > out the allocation status (e.g. SEEK_HOLE). In that case the client may > > be able to avoid reading the block and sending it over the network, or > > when making a copy, it could use it to keep the target file sparse. If > > the client can't take advantage, we didn't have much overhead, so it's > > fine. > > Yes, that was the idea. I'll add a note that the server may return > NBD_STATE_ALLOCATED instead of NBD_STATE_ZEROED if it has not means to > efficiently differentiate allocated blocks with zeroes from allocated > blocks with non-zeroed content. Okay, that alleviates my concerns. In that case it might be useful if the server could say something along the lines of "I know it's allocated, but I didn't check whether there's anything non-zero in there"? The client can then decide to do nothing with that information; but the more useful information is sent along, the better... -- < ron> I mean, the main *practical* problem with C++, is there's like a dozen people in the world who think they really understand all of its rules, and pretty much all of them are just lying to themselves too. -- #debian-devel, OFTC, 2016-02-12