On Fri, Sep 10, 2010 at 08:39:21AM -0500, Anthony Liguori wrote: > You're hand waving to a dangerous degree here :-) > > TRIM in qcow2 would require the following sequence: > > 1) remove cluster from L2 table > 2) sync() > 3) reduce cluster reference count > 4) sync() > > TRIM needs to be fast so this is not going to be acceptable. How do you > solve it?
It's utterly slow in any real life SSD. > For QED, TRIM requires: > > 1) remove cluster from L2 table > 2) sync() > > In both cases, I'm assuming we lazily write the free list and have a way > to detect unclean mounts. Unclean mounts require an fsck() and both > qcow2 and qed require it. If you do proper transactional metadata updates you can completely drop the sync. TRIM / SCSI unmap are optimizations that can just be noops without compromising data integrity.
