From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:46008) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1aZPCL-0002pY-0W for qemu-devel@nongnu.org; Fri, 26 Feb 2016 15:41:13 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1aZPCG-00082n-Vj for qemu-devel@nongnu.org; Fri, 26 Feb 2016 15:41:12 -0500 Received: from mx2.parallels.com ([199.115.105.18]:45510) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1aZPCG-00082Y-Pu for qemu-devel@nongnu.org; Fri, 26 Feb 2016 15:41:08 -0500 References: <1454151394-52320-1-git-send-email-vsementsov@virtuozzo.com> <20160203081418.GC25746@ad.usersys.redhat.com> <56B45D3A.405@virtuozzo.com> <20160209142852.GA13149@stefanha-x1.localdomain> <56B9FAAE.8040503@virtuozzo.com> <20160210101004.GB7317@stefanha-x1.localdomain> <20160216170943.GA31393@stefanha-x1.localdomain> <56C4B21F.9030006@virtuozzo.com> <20160218121114.GC12470@redhat.com> <20160218164148.GB13271@stefanha-x1.localdomain> <20160219020826.GA23506@ad.usersys.redhat.com> <87povtulpt.fsf@blackfin.pond.sub.org> <56D0ADBD.20000@redhat.com> From: "Denis V. Lunev" Message-ID: <56D0B858.6010303@virtuozzo.com> Date: Fri, 26 Feb 2016 23:40:56 +0300 MIME-Version: 1.0 In-Reply-To: <56D0ADBD.20000@redhat.com> Content-Type: text/plain; charset="windows-1252"; format=flowed Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [PATCH v2 0/6] external backup api List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Paolo Bonzini , Markus Armbruster , Fam Zheng Cc: kwolf@redhat.com, Stefan Hajnoczi , Vladimir Sementsov-Ogievskiy , jsnow@redhat.com, qemu-devel@nongnu.org On 02/26/2016 10:55 PM, Paolo Bonzini wrote: > > On 19/02/2016 09:51, Markus Armbruster wrote: >>> Is it an abuse to "Get LBA Status" to return dirty information? Because in SCSI >>> the command reports "mapped", "allocated" and "anchored" statuses. Does that >>> mean NBD will use a different status set? >> Perhaps some conceptual gymnastics can get us to standard semantics. >> >> Incremental backup wants to copy out an image's "dirty" blocks. >> >> We can view this as a bitmap telling us which of the image's blocks are >> dirty. >> >> An alternative view would be base image + dirty delta image. In the the >> dirty delta, exactly the dirty blocks are allocated. The delta image >> may be conceptual. > I see a problem here. On one hand I agree that the "GET LBA STATUS" is > a natural extension to the NBD protocol. > > On the other hand, the Get LBA Status command in SCSI reflects the > state over the whole chain, not only the top element. It is the > equivalent of bdrv_get_block_status_above(bs, NULL, ...), rather than > bdrv_get_block_status(bs, ...). My understanding is that the dirty > block information would require the latter, especially in the > "conceptual delta image" model that Markus describes above. > > Having NBD implement subtly different semantics compared to SCSI is a > bad idea in my opinion. > > Of course if we call it "READ DIRTY BLOCKS" that would work, but I > don't think such a command would be something that it makes sense to > have in the general purpose NBD spec, so you would need a mechanism > for vendor-specific extensions. > > Paolo > In general, the idea to bind DIRTY BITMAP to GET STATUS command is not that bad. First of all, NBD has no relation to the SCSI at all thus we are not bound to the SCSI protocol. This is good thing. Next, it is generally good to query state of the data block before reading to reduce amount of transfers over the network. This is useful even for a full backup operation to avoid zero block transfers. Thus, if we have created special NBD share, we can provide STATUS bitmap in the way we want, f.e. with the proposed semantics. This will not be a violation of the protocol. Den