Features/QED/OutstandingWork

Outstanding features

• Zero clusters
  • Patch on mailing list, awaiting review and merge.

• Reducing time window during which dirty flag is set
  • Images with backing file don't need to set dirty bit during allocating write [merged].
  • Piggyback clearing dirty bit on guest flush operation.
  • Periodic flush in order to clear dirty bit.

• Block device support (i.e. QED on LVM volumes)
  • Solution: Keep a physical file size field in the QED header, update lazily using dirty bit logic

• Live migration on shared storage
  • Reopen file (open read-only first to prevent consistency check).

• Encryption

• Improve parallel allocating write performance
  • QED currently queues allocating write requests.
  • An initial per-L1/L2 table fine-grained locking approach improved performance somewhat but also regressed a benchmark.
  • Needs more investigation for a good scalable solution.

Possible future work

• Discard support
  • Solution: Add a freelist to the image format
  • Solution: Pass discard to underlying file. Loses compact image characteristic.

• Internal snapshots
  • Probably not worth adding since external snapshotting is being implemented

• Compression
  • No strong use case for this feature

Non-technical issues

Thanks to Kevin Wolf for contributing these points:

• Burden of maintaining an additional image format (basically doubles image format related work; both development and QA) [Update: Sharing code might reduce the burden, though I guess for QA purposes it would stay two independent formats.]
• There are concerns that starting with a simple image format core and then adding one new thing after another on top is going to make code complexity grow very fast.
• Conversion of existing images is needed to take advantage of QED.
• Code review has brought up some corruption issues that are fixed by now, but have we covered everything?
  • First dirty flag had to be added
  • Then a bdrv_flush after COW
  • What's next?