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AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 407995AFD9; Fri, 28 Jun 2019 00:25:40 +0000 (UTC) Received: from [10.18.17.47] (dhcp-17-47.bos.redhat.com [10.18.17.47]) by smtp.corp.redhat.com (Postfix) with ESMTP id 3F9161001B05; Fri, 28 Jun 2019 00:25:35 +0000 (UTC) To: Vladimir Sementsov-Ogievskiy , "qemu-devel@nongnu.org" , "qemu-block@nongnu.org" References: <20190603120005.37394-1-vsementsov@virtuozzo.com> <54598cf4-a326-5385-2d86-d2dd43f26f47@redhat.com> <125166e6-2983-aaa8-2dad-14c5c7081fd8@virtuozzo.com> From: John Snow Openpgp: preference=signencrypt Autocrypt: addr=jsnow@redhat.com; prefer-encrypt=mutual; keydata= mQINBFTKefwBEAChvwqYC6saTzawbih87LqBYq0d5A8jXYXaiFMV/EvMSDqqY4EY6whXliNO IYzhgrPEe7ZmPxbCSe4iMykjhwMh5byIHDoPGDU+FsQty2KXuoxto+ZdrP9gymAgmyqdk3aV vzzmCa3cOppcqKvA0Kqr10UeX/z4OMVV390V+DVWUvzXpda45/Sxup57pk+hyY52wxxjIqef rj8u5BN93s5uCVTus0oiVA6W+iXYzTvVDStMFVqnTxSxlpZoH5RGKvmoWV3uutByQyBPHW2U 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transaction 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: "kwolf@redhat.com" , "fam@euphon.net" , Denis Lunev , "armbru@redhat.com" , "mreitz@redhat.com" , Nikolay Shirokovskiy Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 6/17/19 7:37 AM, Vladimir Sementsov-Ogievskiy wrote: > 08.06.2019 1:26, John Snow wrote: >> >> >> On 6/3/19 8:00 AM, Vladimir Sementsov-Ogievskiy wrote: >>> Hi all! >>> >>> Here is block-dirty-bitmap-remove transaction action. >>> >>> It is used to do transactional movement of the bitmap (which is >>> possible in conjunction with merge command). Transactional bitmap >>> movement is needed in scenarios with external snapshot, when we don't >>> want to leave copy of the bitmap in the base image. >>> >> >> Oh, interesting. I see why you want this now. OK, let's do it. >> > > > Hi John! > > Hmm, could you stage it, or should I fix something? Seems I've answered all questions. > We need this for our nearest release and wanting to avoid x-vz- prefixes in the API, > I'd be very grateful if we merge it soon. > > Hi Vladimir, Sorry, I lost track of this thread. (In the future, if you have something pressing like a release and I don't seem to have noticed an email, try sending me an email directly without the word "patch" in the subject it to get my attention, or come ping me on IRC.) For this series: I think this is pretty confusing, as evidenced by how we both misunderstood what it did. So "block_dirty_bitmap_remove" now removes a bitmap from storage but might not actually release it. That's a little surprising, but I see why we want it. So what really happens is: 1. Remove a bitmap from storage, but actually don't release it; then 2. hide the bitmap we're holding onto (holding the persistence and name data aside) 3. On success, we release the bitmap. During this process, taking the bitmap's name away and marking it as non-persistent helps keep it from getting rewritten to disk or from having anything else interact with it. On Failure: 1. unhide: - Restore the persistent bit - Restore the name So we restore the persistent bit, but we don't actually go back through the trouble of making sure that there isn't a collision on the name. I suppose we are guaranteed this won't happen because if a bitmap got added, it MUST have been added AFTER this action, and if we are aborting, it MUST have been removed before we get here. .... phew, I think that this works, but isn't obvious that it does. However, if we ever use "hide" or "unhide" outside of the remove API, you might actually run into troubles where we collide on the name when you "unhide" it, and I don't like that very much. I feel like a "hidden" bitmap probably should still occupy namespace to avoid this problem. But then, it isn't truly hidden from API queries and such, and subsequent commands could interact with it... we could add a new permissions flag, but that starts to get kind of messy. Is this a problem of naming? do we need to "hide" bitmaps? Could we get away with something simpler? We could rename the "migration" bool to be something generic and then use that. This way, it keeps its persistence flag and name, but it won't get flushed back out to disk or anything at the conclusion of the transaction. This avoids the need for worrying about name collision on "unhide", and doesn't need any new fields. During this time, we can also mark the bitmap as BUSY which will prevent it from being used for any operations. The ones that we could use during this critical window are: - BACKUP - ADD - CLEAR - ENABLE - DISABLE - MERGE - REMOVE Backup, clear, enable, disable, and remove will all fail a BUSY check. Merge will fail for either the source or the destination being BUSY. So I think that it's possible to avoid the need for a hide() API right now, just by using the migration bool (renamed) and the busy status. I want to be very aware of your time constraints though, so I prepared a mock-up on top of your patches; https://github.com/jnsnow/qemu/commit/9b3434cc86bbd1cbd86f9fc845435d8d6883c205 If this seems agreeable to you I'll re-send and stage right away tomorrow. If you really DO want hide() semantics we can work on those instead. --js