Re: [Qemu-devel] [PATCH V2] qemu-img: optimize is_allocated_sectors_min

[Max Reitz <mreitz@redhat.com>]

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On 19.01.2017 17:35, Peter Lieven wrote:
> the current implementation always splits requests if a buffer
> begins or ends with zeroes independent of the length of the
> zero area. Change this to really only split off zero areas
> that have at least a length of 'min' bytes.
> 
> Signed-off-by: Peter Lieven <pl@kamp.de>
> ---
>  qemu-img.c | 44 ++++++++++++++------------------------------
>  1 file changed, 14 insertions(+), 30 deletions(-)

That is because the idea is that you should delay the write operation as
much as possible. Say you have some empty space at the beginning of the
buffer and then some used space. Of course you should then only start
writing at the point where you actually have to write something. The
same goes for the end of the buffer. Why write zeroes when you can just
not do it?

On the other hand, it makes sense to not split a single write operation
into two just because there is some empty space in the middle. This is
because just writing that empty space may take less time than issuing
two operations, especially if that empty space is e.g. in the middle of
a qcow2 cluster anyway.

This is why empty space in the middle is treated differently than empty
space at the beginning or the end of the buffer.

Do you have a reason for changing this other than because it simplifies
the code? Which it does, admittedly, but I think the code does have a
reason for being how it is.

Max


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