Re: [Qemu-block] Performance impact of the qcow2 overlap checks

[Alberto Garcia]

On Sat 21 Jan 2017 04:27:42 PM CET, Max Reitz wrote:
>>> I have optimized these checks. See:
>>>
>>> http://lists.nongnu.org/archive/html/qemu-block/2015-08/msg00020.html
>>>
>>> Feel free to review. If you want, I can rebase the series.
>>>
>>> So far nobody has seriously done so because it wasn't considered
>>> important enough, and the patches are not exactly trivial.
>> 
>> I see. I wonder if it's worth complicating the code so much more
>> instead of simply disabling the tests.
>> 
>> For the 'refcount-block' check, which is the one that has an
>> immediate impact in all kinds of images, I was wondering if we could
>> simply use the image size to determine how many entries need to be
>> checked. This would keep things much faster unless the image grows
>> abnormally bigger.
>
> Have you tested that it helps? I would assume that checking one
> (mostly) unused cluster of the refcount table does not take longer
> than checking a single refcount block. Any entry that's 0 will just be
> skipped.

The problem is that there's 8000 entries to be skipped. In most cases it
doesn't make a difference because the bottleneck is in the hard drive,
but if it's fast enough then it starts to show. You can see it easily if
you store the qcow2 image in RAM.

Berto