[Qemu-block] [RFC] Intermediate block mirroring

[Alberto Garcia]

Hi,

I'm interested in adding the possibility to mirror an intermediate
node in a disk image chain, but I would like to have some feedback
before sending any patches.

The goal would be to convert this:

   [A] -> [B] -> [C] -> [D]

into this:

   [A] -> [B] -> [X] -> [D]

where [D] is the active image and [X] would be a copy of [C]. The
latter would be unlinked from the chain.

A use case would be to move disk images across different storage
backends.

My idea is to extend the drive-mirror command. Similar to what we
discussed in the case of the intermediate block streaming, I can reuse
the 'device' parameter to refer to a node name. So the API doesn't
need any changes other than the extended semantics for this parameter.

One difference with the current functionality is that once the block
job is completed, the node above the mirrored one would have to change
its backing image to point to the new one. One solution is to iterate
over all devices (bdrv_next()) and check which ones are connected
directly or indirectly to the mirrored node (bdrv_find_overlay()).

drive-mirror has three different sync modes: top, full and none. This
would be the chain from the example using each one of these modes:

  top:

     [A] -> [B] -> [X] -> [D]

  full:

     [X] -> [D]

  none:

     [A] -> [B] -> [C] -> [X] -> [D]

My understanding is that in the 'sync=full' case, [A] and [B] would
also need to be blocked during the operation since they are going to
disappear from the chain.

I have some code and in principle everything seems to be working fine,
but I'd like to test it a bit more.

What's anyway your opinion about this proposal?

Thanks,

Berto