[Qemu-block] block: Dirty bitmaps and COR in bdrv_move_feature_fields()

[Kevin Wolf]

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. When
  taking a snapshot, we end up with a backing chain like this:

      http <- local.qcow2 <- snap_overlay.qcow2

  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.

  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.

  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.

* 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.

  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.

  With compatibility in mind, this seems to be a reall tough one,
  though.

Any comments or ideas how to proceed with those two?

Kevin