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Mon, 4 Nov 2019 11:21:24 +0000 (UTC) Subject: Re: bitmaps -- copying allocation status into dirty bitmaps To: John Snow , Qemu-block References: From: Max Reitz Autocrypt: addr=mreitz@redhat.com; prefer-encrypt=mutual; keydata= mQENBFXOJlcBCADEyyhOTsoa/2ujoTRAJj4MKA21dkxxELVj3cuILpLTmtachWj7QW+TVG8U /PsMCFbpwsQR7oEy8eHHZwuGQsNpEtNC2G/L8Yka0BIBzv7dEgrPzIu+W3anZXQW4702+uES U29G8TP/NGfXRRHGlbBIH9KNUnOSUD2vRtpOLXkWsV5CN6vQFYgQfFvmp5ZpPeUe6xNplu8V mcTw8OSEDW/ZnxJc8TekCKZSpdzYoxfzjm7xGmZqB18VFwgJZlIibt1HE0EB4w5GsD7x5ekh awIe3RwoZgZDLQMdOitJ1tUc8aqaxvgA4tz6J6st8D8pS//m1gAoYJWGwwIVj1DjTYLtABEB AAG0HU1heCBSZWl0eiA8bXJlaXR6QHJlZGhhdC5jb20+iQFTBBMBCAA9AhsDBQkSzAMABQsJ CAcCBhUICQoLAgQWAgMBAh4BAheABQJVzie5FRhoa3A6Ly9rZXlzLmdudXBnLm5ldAAKCRD0 B9sAYdXPQDcIB/9uNkbYEex1rHKz3mr12uxYMwLOOFY9fstP5aoVJQ1nWQVB6m2cfKGdcRe1 2/nFaHSNAzT0NnKz2MjhZVmcrpyd2Gp2QyISCfb1FbT82GMtXFj1wiHmPb3CixYmWGQUUh+I AvUqsevLA+WihgBUyaJq/vuDVM1/K9Un+w+Tz5vpeMidlIsTYhcsMhn0L9wlCjoucljvbDy/ 8C9L2DUdgi3XTa0ORKeflUhdL4gucWoAMrKX2nmPjBMKLgU7WLBc8AtV+84b9OWFML6NEyo4 4cP7cM/07VlJK53pqNg5cHtnWwjHcbpGkQvx6RUx6F1My3y52vM24rNUA3+ligVEgPYBuQEN BFXOJlcBCADAmcVUNTWT6yLWQHvxZ0o47KCP8OcLqD+67T0RCe6d0LP8GsWtrJdeDIQk+T+F xO7DolQPS6iQ6Ak2/lJaPX8L0BkEAiMuLCKFU6Bn3lFOkrQeKp3u05wCSV1iKnhg0UPji9V2 W5eNfy8F4ZQHpeGUGy+liGXlxqkeRVhLyevUqfU0WgNqAJpfhHSGpBgihUupmyUg7lfUPeRM DzAN1pIqoFuxnN+BRHdAecpsLcbR8sQddXmDg9BpSKozO/JyBmaS1RlquI8HERQoe6EynJhd 64aICHDfj61rp+/0jTIcevxIIAzW70IadoS/y3DVIkuhncgDBvGbF3aBtjrJVP+5ABEBAAGJ ASUEGAEIAA8FAlXOJlcCGwwFCRLMAwAACgkQ9AfbAGHVz0CbFwf9F/PXxQR9i4N0iipISYjU sxVdjJOM2TMut+ZZcQ6NSMvhZ0ogQxJ+iEQ5OjnIputKvPVd5U7WRh+4lF1lB/NQGrGZQ1ic alkj6ocscQyFwfib+xIe9w8TG1CVGkII7+TbS5pXHRxZH1niaRpoi/hYtgzkuOPp35jJyqT/ /ELbqQTDAWcqtJhzxKLE/ugcOMK520dJDeb6x2xVES+S5LXby0D4juZlvUj+1fwZu+7Io5+B bkhSVPb/QdOVTpnz7zWNyNw+OONo1aBUKkhq2UIByYXgORPFnbfMY7QWHcjpBVw9MgC4tGeF R4bv+1nAMMxKmb5VvQCExr0eFhJUAHAhVg== Message-ID: Date: Mon, 4 Nov 2019 12:21:23 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.1.1 MIME-Version: 1.0 In-Reply-To: X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 X-Mimecast-Spam-Score: 0 Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="JiDecQvIuvsyrX3r54DYFeLVj4LTp1TbD" X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 205.139.110.120 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: Kevin Wolf , "Denis V. Lunev" , Vladimir Sementsov-Ogievskiy , qemu-devel Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" This is an OpenPGP/MIME signed message (RFC 4880 and 3156) --JiDecQvIuvsyrX3r54DYFeLVj4LTp1TbD Content-Type: multipart/mixed; boundary="HAXhhUs2VADnhvrfgcI6W6218e0q6ejHl" --HAXhhUs2VADnhvrfgcI6W6218e0q6ejHl Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: quoted-printable On 01.11.19 16:42, John Snow wrote: > Hi, in one of my infamously unreadable and long status emails, I > mentioned possibly wanting to copy allocation data into bitmaps as a way > to enable users to create (external) snapshots from outside of the > libvirt/qemu context. >=20 > (That is: to repair checkpoints in libvirt after a user extended the > backing chain themselves, you want to restore bitmap information for > that node. Conveniently, this information IS the allocation map, so we > can do this.) >=20 > It came up at KVM Forum that we probably do want this, because oVirt > likes the idea of being able to manipulate these chains from outside of > libvirt/qemu. >=20 > Denis suggested that instead of a new command, we can create a special > name -- maybe "#ALLOCATED" or something similar that can never be > allocated as a user-defined bitmap name -- as a special source for the > merge command. >=20 > You'd issue a merge from "#ALLOCATED" to "myBitmap0" to copy the current > allocation data into "myBitmap0", for instance. Sounds fun, but is there actually any use for this if the only purpose is to work as a source for merge? I mean, it would be interesting if it worked exactly like a perma-RO pseudo-bitmap that whenever you try to get data from it performs a block-status call. But as you say, that would probably be too slow, and it would take a lot of code modifications, so I wonder if there is actually any purpose for this. > Some thoughts: >=20 > - The only commands where this pseudo-bitmap makes sense is merge. > enable/disable/remove/clear/add don't make sense here. >=20 > - This pseudo bitmap might make sense for backup, but it's not needed; > you can just merge into an empty/enabled bitmap and then use that. >=20 > - Creating an allocation bitmap on-the-fly is probably not possible > directly in the merge command, because the disk status calls might take > too long... >=20 > Hm, actually, I'm not sure how to solve that one. Merge would need to > become a job (or an async QMP command?) or we'd need to keep an > allocation bitmap object around and in-sync. I don't really want to do > either, so maybe I'm missing an obvious/better solution. All of what you wrote in this mail makes me think it would make much more sense to just add a =E2=80=9Cblock-dirty-bitmap-create-from=E2=80=9D j= ob with an enum of targets. (One of which would be =E2=80=9Callocated-blocks=E2=80=9D= .) 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