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Tue, 22 Jun 2021 09:56:48 -0700 (PDT) X-Google-Smtp-Source: ABdhPJwKOVuUkX+lgTxAnFOs52uKBvc1lMye7uAJsKvFhTF/39sBLGDsqA/NZtAqFaKxIOyrLoqj0h5KJoFpL5xclec= X-Received: by 2002:a9d:6250:: with SMTP id i16mr3893036otk.170.1624381008551; Tue, 22 Jun 2021 09:56:48 -0700 (PDT) MIME-Version: 1.0 References: <20210611140157.1366738-1-eblake@redhat.com> <20210611190316.1424729-1-eblake@redhat.com> In-Reply-To: From: Nir Soffer Date: Tue, 22 Jun 2021 19:56:32 +0300 Message-ID: Subject: Re: [PATCH v2 2/1] qemu-img: Add "backing":true to unallocated map segments To: Kevin Wolf Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=nsoffer@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset="UTF-8" Received-SPF: pass client-ip=170.10.133.124; envelope-from=nsoffer@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -29 X-Spam_score: -3.0 X-Spam_bar: --- X-Spam_report: (-3.0 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.223, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Vladimir Sementsov-Ogievskiy , "open list:Block layer core" , Markus Armbruster , QEMU Developers , Max Reitz , Eric Blake Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Tue, Jun 22, 2021 at 6:38 PM Kevin Wolf wrote: > > Am 11.06.2021 um 21:03 hat Eric Blake geschrieben: > > To save the user from having to check 'qemu-img info --backing-chain' > > or other followup command to determine which "depth":n goes beyond the > > chain, add a boolean field "backing" that is set only for unallocated > > portions of the disk. > > > > Signed-off-by: Eric Blake > > --- > > > > Touches the same iotest output as 1/1. If we decide that switching to > > "depth":n+1 is too risky, and that the mere addition of "backing":true > > while keeping "depth":n is good enough, then we'd have just one patch, > > instead of this double churn. Preferences? > > I think the additional flag is better because it's guaranteed to be > backwards compatible, and because you don't need to know the number of > layers to infer whether a cluster was allocated in the whole backing > chain. And by exposing ALLOCATED we definitely give access to the whole > information that exists in QEMU. > > However, to continue with the bike shedding: I won't insist on > "allocated" even if that is what the flag is called internally and > consistency is usually helpful, but "backing" is misleading, too, > because intuitively it doesn't cover the top layer or standalone images > without a backing file. How about something like "present"? Looks hard to document: # @present: if present and false, the range is not allocated within the # backing chain (since 6.1) And is not consistent with "offset". It would work better as: # @present: if present, the range is allocated within the backing # chain (since 6.1) Or: # @absent: if present, the range is not allocated within the backing # chain (since 6.1) This is used by libnbd now: https://github.com/libguestfs/libnbd/commit/1d01d2ac4f6443b160b7d81119d555e1aaedb56d But I'm fine with "backing", It is consistent with BLK_BACKING_FILE, meaning this area exposes data from a backing file (if one exists). We use "backing" internally to be consistent with future qemu-img.