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Re: [Qemu-block] block: Dirty bitmaps and COR in bdrv_move_feature_field

From: John Snow
Subject: Re: [Qemu-block] block: Dirty bitmaps and COR in bdrv_move_feature_fields()
Date: Tue, 1 Mar 2016 14:24:48 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.5.0

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 <address@hidden> 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.


>>>   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

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