Re: [Qemu-devel] [PATCH 00/12] qcow2: Add new overlap check functions

[Max Reitz <mreitz@redhat.com>]

On 2014-11-03 at 18:04, Max Reitz wrote:
> As has been requested, this series adds new overlap check functions to
> the qcow2 code. My local branch is called "qcow2-improved-overlap-v1",
> but I am not so sure whether it is actually an improvement; that is left
> for you to decide, dear reviewers.
>
> See patch 1 for an explanation of why this series exists and what it
> does. Patch 1 is basically the core of this series, the rest just
> employs the functions introduced there.
>
> I have yet to do benchmarks to test whether this series actually
> improves things, but judging from the iotests it at least does not slow
> things down (which it did at one time during development, particularily
> test 044 is good for testing this, so this actually has some
> significance to it).
>
> In a later patch, we may want to change the meaning of the "constant"
> overlap checking option to mean the same as "cached", which is
> everything except for inactive L2 tables. This series does make
> checking for overlaps with inactive L2 tables at runtime just as cheap
> as everything else (constant time plus caching), but using these checks
> means qemu has to read all the snapshot L1 tables when opening a qcow2
> file. This does not take long, of course, but it does result in a bit of
> overhead so I did not want to enable it by default.
>
> I think just enabling all overlap checks by default after this series
> should be fine and useful, though.

Sorry Benoît, I mistyped your address. Maybe I should learn how to use 
abbreviations with git send-email some time...

Max