[Qemu-devel] [RFC] qcow2: 2 way to improve performance updating refcount

[Frediano Ziglio <freddy77@gmail.com>]

Hi,
  after a snapshot is taken currently many write operations are quite
slow due to
- refcount updates (decrement old and increment new )
- cluster allocation and file expansion
- read-modify-write on partial clusters

I found 2 way to improve refcount performance

Method 1 - Lazy count
Mainly do not take into account count for current snapshot, that is
current snapshot counts as 0. This would require to add a
current_snapshot in header and update refcount when current is changed.
So for these operation
- creating snapshot, performance are the same, just increment for old
snapshot instead of the new one
- normal write operations. As current snaphot counts as 0 there is not
operations here so do not write any data
- changing current snapshot, this is the worst case, you have to
increment for the current snapshot and decrement for the new so it will
take twice
- deleting snapshot, if is the current just set current_snapshot to a
dummy not existing value, if is not the current just decrement counters,
no performance changes

Method 2 - Read-only parent
Here parents are readonly, instead of storing a refcount store a numeric
id of the owner. If the owner is not current copy the cluster and change
it. Considering this situation

A --- B --- C

B cannot be changed so in order to "change" B you have to create a new
snapshot

A --- B --- C
         \--- D

and change D. It can take more space cause you have in this case an
additional snapshot.

Operations:
- creating snapshot, really fast as you don't have to change any
ownership
- normal write operations. If owner is not the same allocate a new
cluster and just store a new owner for new cluster. Also ownership for
past-to-end cluster could be set all to current owner in order to
collapse allocations
- changing current snapshot, no changes required for owners
- deleting snapshot. Only possible if you have no child or a single
child. Will require to scan all l2 tables and merge and update owner.
Same performance

Regards
  Frediano Ziglio