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

[Alberto Garcia]

On Tue 31 Jan 2017 01:23:33 PM CET, John Snow <address@hidden> wrote:
>> And of course another approach I already mentioned would be to scrap
>> the overlap checks altogether once we have image locking (and I guess
>> we can keep them around in their current form at least until then).
>
> I think it's worth having them. Perhaps if you use image locking they
> can be disabled by default, but I wouldn't really advocate for
> removing them.
>
> I think what you are pointing out with refcount blocks not being
> essential to protect is probably pretty sufficient as an optimization,
> too. Or I guess we can merge your ~real~ series to help optimize
> things.

I think the overhead that most checks add is negligible, so I would not
remove them. There are maybe three exceptions:

- refcount-block: this one is slow even for small images, but I think it
  should be fine after my patch.

- active-l2: this can be slow if the virtual size of the image is very
  large. It's still much faster than refcount-block and it can be made
  faster simply by increasing the cluster size (which makes sense for
  large images anyway). So I'm not too worried about it.

- inactive-l2: this is probably the slowest of them all (it even
  involves reading from disk), but it's only used if there are internal
  snapshots. I haven't looked into this one.

I think the checks as they are now are very simple and it's not worth
complicating them too much unless we have numbers that prove that
they're slowing things down. I only got those numbers for the first one.

Berto