Features/QED/OutstandingWork

Thanks to Kevin Wolf for collecting these points:

• Relying on file size - have we really considered all cases with respect to data integrity?

• 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?

• Planned incompatible changes must be done as soon as possible. Checked this with Stefan, it's only zero cluster support.

• After a host crash an fsck is needed. Did anyone measure the time to start up when multiple guests on multiple hosts are recovering from a system failure? File system guys consider this a severe problem. [Update: Stefan has tested fsck time, but only with one image]

• Migration currently doesn't work with QED, likely breaks images

• QED doesn't work on block devices (required for some users)

• No support for encryption, compression, internal snapshots

• Discard support not trivial, requires addition of some data structures to the format

• Non-technical issues:
  • 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.