Re: [Qemu-devel] [PATCH v6 03/14] qcow2: Optimize bdrv_make_empty()

[Max Reitz]

On 29.04.2014 17:07, Max Reitz wrote:
> bdrv_make_empty() is currently only called if the current image
> represents an external snapshot that has been committed to its base
> image; it is therefore unlikely to have internal snapshots. In this
> case, bdrv_make_empty() can be greatly sped up by creating an empty L1
> table and dropping all data clusters at once by recreating the refcount
> structure accordingly instead of normally discarding all clusters.
>
> If there are snapshots, fall back to the simple implementation (discard
> all clusters).
>
> Signed-off-by: Max Reitz <address@hidden>
> ---
>   block/qcow2.c | 385 +++++++++++++++++++++++++++++++++++++++++++++++++++++++---
>   1 file changed, 370 insertions(+), 15 deletions(-)

If someone is interested in how I got to the calculation of the cluster 
size (which I hope there is, as I probably did some mistake somewhere 
and I'd be glad if someone other than me could verify the correctness 
:-P), I've attached a PDF with the transformation steps.

Max

blob-calculation.pdf
Description: Adobe PDF document