On 03/01/2016 09:19 AM, Kevin Wolf wrote: > Am 01.03.2016 um 11:43 hat Markus Armbruster geschrieben: >> qemu-block@ without qemu-devel@, intentional? >> >> Kevin Wolf <[email protected]> writes: >> >>> Hi all, >>> >>> I'm currently trying to get rid of bdrv_move_feature_fields(), so we can >>> finally have more than one BB per BDS. Generally the way to do this is >>> to move features from BDS and block.c to BB and block-backend.c. >>> However, for two of the features I'm not sure about this: >>> >>> * Copy on Read: >>> >>> When Jeff introduced bdrv_append() in commit 8802d1fd, the CoR flag >>> was already moved to the new top level when taking a snapshot. Does >>> anyone remember why it works like that? It doesn't seem to make a lot >>> of sense to me. >>> >>> The use case for manually enabled CoR is to avoid reading data twice >>> from a slow remote image, so we want to save it to a local overlay, >>> say an ISO image accessed via HTTP to a local qcow2 overlay. >> >> Ignorant / forgetful question: we do that by adding a QCOW2 on top with >> COR enabled, and that makes QCOW2 copy up on read? > > It's not qcow2, but block.c that implements it, but otherwise yes. > > $ qemu-img create -f qcow2 -b http://something.slow disk.qcow2 > $ qemu-system-x86_64 -drive file=disk.qcow2,copy-on-read=on ... > >>> When >>> taking a snapshot, we end up with a backing chain like this: >>> >>> http <- local.qcow2 <- snap_overlay.qcow2 >> >> Now COR is enabled where? Just in snap_overlay.qcow2? > > Yes, that's the current behaviour. > > Of course it still copies everything that is read from the remote host > because it goes through both qcow2 layer, it just copies a bit more than > that and duplicates local data in both layers. > >>> There is no point in performing copy on read from local.qcow2 into >>> snap_overlay.qcow2, we just want to keep copying data from the remote >>> source into local.qcow2. >> >> Makes sense. >> >>> Possible caveat: We would be writing to a backing file, but that's >>> similar to what some block jobs do, so if we design our op blockers to >>> cover this case, it should be fine. >> >> COR would write to backing file local.qcow2. Doesn't change contents of >> the http <- local.qcow2 substack, though. > > Right, that's why the operation can be done in the first place. > >>> I'm actually pretty sure that simply removing COR from the list, and >>> therefore changing the behaviour to not move it to the top any more, >>> is the right thing to do and could be considered a bug fix. >> >> I'm not sure I got the relation to BBs. Perhaps its about the rule "if >> $feature sticks to the top when we put a BDS on top, it should probably >> live in the BB instead." Your point seems to be that COR shouldn't >> stick to the top. Is that roughly right? > > Not only roughly. :-) > > In order to allow multiple BBs per BDS I need to complete the split now, > so all features that currently stick to the top by using the remnants of > bdrv_swap() need to be properly converted to BB instead. The reason for > that is that if the feature is supposed to be logically part of the BB > level, different BBs on the same BDS can have different setting. > > If we decide that it shouldn't be in the BB level in the first place, > instead of converting the feature, I can simply drop it from the > bdrv_swap remnants. > >> You gave an example where COR should stay put. Do we know of any use of >> COR where sticking to the top makes sense? > > To be honest, I'm not sure if any COR users exist out there. The main > motivation for it was that the streaming block job uses it internally. > The example I gave is what Anthony used to give when the feature was > introduced. > >> In general, having the block layer move things around implicitly when >> the user adds a BDS or BB is prone to create awkward questions like "is >> this the right move for all possible user intents?" I hope that the >> ongoing rework will lead to less implicit magic and more explicit >> control. > > Implicit magic is becoming harder to implement as I remove bs->blk, so I > think we're on the right way there. > >>> * Dirty bitmaps: >>> >>> We're currently trying, and if I'm not mistaken failing, to move dirty >>> bitmaps to the top. The (back then one) bitmap was first added to the >>> list in Paolo's commit a9fc4408, with the following commit message: >>> >>> While these should not be in use at the time a transaction is >>> started, a command in the prepare phase of a transaction might have >>> added them, so they need to be brought over. >>> >>> At that point, there was no transactionable command that did this in >>> the prepare phase. Today we have mirror and backup, but op blockers >>> should prevent them from being mixed with snapshots in a single >>> transaction, so I can't see how this change had any effect. >>> >>> The reason why I think we're failing to move dirty bitmaps to the top >>> today is that we're moving the head of the list to a different object >>> without updating the prev link in the first element, so in any case >>> it's buggy today. >>> >>> I really would like to keep bitmaps on the BDS where they are, but >>> unfortunately, we also have user-defined bitmaps by now, and if we >>> change whether they stick with the top level, that's a change that is >>> visible on the QMP interface. >>> >>> On the other hand, the QMP interface clearly describes bitmaps as >>> belonging to a node rather than a BB (you can use node-name, even with >>> no BB attached), so moving them could be considered a bug, even if >>> it is the existing behaviour. >> >> You just told us moving doesn't work. Did it ever work in any release >> that also provides the QMP interface in question? > > The feature was introduced in 2.4 (commit 341ebc2f) and I think I broke > it in time for 2.5 (the bdrv_swap() removal in dd62f1ca and the > following two patches). > >> If no, existing behavior doesn't matter :) >> >> If yes, the interface might be new enough to permit incompatible design >> flaw fixes. Paolo thinks bitmaps haven't been used widely. Discuss >> with their known users? > > Who knows the known users? John? (CCed) >
Libvirt doesn't use it, I do not know of any scripts or programs that attempt to use this interface yet. I have even discouraged people from using it in production setups because of the lack of migration and persistence support. If any users do exist, I think we are within our trial period to say "Sorry, necessary bugfix." See the other tail on this thread for my thoughts on solutions. A thought: If users DID use it on 2.4, they have no way to migrate to 2.5+ anyway! We consider the existence of migration and persistence necessary to call the feature "supported." I should amend the wiki to make this very, very, very clear. --js >>> I can imagine use cases for both ways, so the interface that would >>> make the most sense to me is to generally keep BDSes at their node, >>> and to provide a QMP command to move them to a different one. >> >> Explicit control instead of implicit magic --- yes, please. >> >>> With compatibility in mind, this seems to be a reall tough one, >>> though. >>> >>> Any comments or ideas how to proceed with those two? >> >> Hope I could help a little. > > Yes, thanks. I hope my answers to your questions give you a clearer > picture, too. > > Kevin >
