Preserving transactions accross Xenstored Live-Update

[Julien Grall <julien@xen.org>]

Hi Juergen,

I have started to look at preserving transaction accross Live-update in 
C Xenstored. So far, I managed to transfer transaction that read/write 
existing nodes.

Now, I am running into trouble to transfer new/deleted node within a 
transaction with the existing migration format.

C Xenstored will keep track of nodes accessed during the transaction but 
not the children (AFAICT for performance reason).

Therefore we have the name of the children but not the content (i.e. 
permission, data...).

I have been exploring a couple of approaches:
    1) Introducing a flag to indicate there is a child but no content.

Pros:
   * Close to the existing stream.
   * Fairly implementation agnostic.

Cons:
   * Memory overhead as we need to transfer the full path (rather than 
the child name)
   * Checking for duplication (if the node was actually accessed) will 
introduce runtime overhead.

     2) Extend XS_STATE_TYPE_NODE (or introduce a new record) to allow 
transferring the children name for transaction

Pros:
   * The implementation is more straight forward

Cons:
    * The stream becomes implementation specific

Neither approach looks very appealing to me. So I would like to request 
some feedback for other proposals or preference between the two options.

Note that I haven't looked into much detail how transactions works on 
OCaml Xenstored.

Cheers,

-- 
Julien Grall