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

[Alberto Garcia]

On Wed 18 Jan 2017 04:30:49 PM CET, Max Reitz wrote:

>> In other words: the vast majority of the entries in the refcount
>> table are probably not going to be used ever, but we're still
>> checking them for each write request. One user reported a >200%
>> performance increase on a fast SSD when using overlap-check=constant.
>> 
>> I think this is at least worth documenting a bit better (unless
>> there's existing documentation that I have missed), but my main
>> question is: does it make sense to try to optimize these checks, or
>> is it better to simply tell the user to disable them in these
>> scenarios?
>
> 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.

Berto