On 22.04.2014 18:22, Max Reitz wrote:
On 22.04.2014 17:19, Eric Blake wrote:
On 04/17/2014 03:59 PM, Max Reitz wrote:
After the top image has been committed into an image in its backing
chain, all images above that base image should be emptied to restore
the
old qemu-img commit behavior.
Signed-off-by: Max Reitz <mre...@redhat.com>
---
qemu-img.c | 87
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++---
1 file changed, 84 insertions(+), 3 deletions(-)
Does emptying an image take significant time? If so, does that need to
be reflected in the progress meter?
For a 16 GB image I have here (should be nearly full) it took 1:22
min. Copying it took six minutes, so I guess committing it would take
even more. I think the ratio is small enough not to include it in the
progress meter.
Furthermore, I don't see a reasonable implementation for a make_empty
progress output: As a general implementation for all image formats
implementing discard will not work (the qcow2 implementation clearly
states that discarded sectors should read back as zero), we would need
some way of returning the progress in every format-specific make_empty
implementation. This would probably require callbacks which I don't
see feasible.
An in my opinion more reasonable approach would be to speed up the
implementation of make_empty for qcow2 - as qcow does it, we could
just create a new empty image (that is, rewrite the L1 and refcount
table and truncate the image appropriately) instead of discarding
every cluster in the existing file. This would be much faster, but the
implementation would not be as nice and easy, so I chose not to
implement it. I guess I'll have a second look, then. ;-)
Okay, after having begun to implement this, I noticed that this approach
will not work for images with internal snapshots as we probably want to
keep those during make_empty. On the other hand, I guess few people will
use internal and external snapshots at the same time; I guess I'll
implement both paths then. If there are internal snapshots, fall back to
the implementation discarding every clusters, if there aren't (which is
more likely), recreate the image.
Max
