Re: [Qemu-devel] [PATCH v5 07/12] qemu-img: Empty images after commit

[Max Reitz]

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 <address@hidden>
> > ---
> >   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. ;-)

Max