[Qemu-devel] Interface for enabling lazy refcount updates in qcow2

[Kevin Wolf <kwolf@redhat.com>]

Hi all,

after having implemented refcount fixing in qcow2's img_check, I'm now
wondering what the best way is to allow users to optionally enable the
"QED mode" for cache=writethrough images where refcount updates aren't
written out immediately.

Basically the two options are:

1. Store it in the image with a compatible feature flag and require
   that the flag is set during image creation (and never updated)

2. Have an option on the command line and pass it each time you start
   a VM and want to enable it

I'm leaning towards option 2 because it is more flexible and consistent
with the other caching options that aren't stored in the image file
either. It's a bit ugly with -drive because it would add a new option
that is invalid/ignored for everything except qcow2.

Or maybe instead of a new option, add a cache mode almost-writethrough?
 (I'm having a hard time finding a good name...) But then, we're already
trying to get rid of the cache option and replace it by three separate
switches, so adding a fourth dimension might not be the smartest move ever.

(The correct solution is, of course, -blockdev which would allow
per-driver runtime options, but well...)

Kevin