Re: [Qemu-devel] [RFC V4 00/30] QCOW2 deduplication

[Benoît Canet]

> How does this code handle hash collisions, and do you have some regression
> tests that purposefully create a dedup hash collision, and verify that the
> 'right thing' happens?

The two hash function that can be used are cryptographics and not broken yet.
So nobody knows how to generate a collision.

You can do the math to calculate the probability of collision using a 256 bit
hash while processing 1EiB of data the result is so low you can consider it
won't happen.
The sha256 ZFS deduplication works the same way regarding collisions.

I currently use qemu-io-test for testing purpose and iozone with the -w flag in
the guest.
I would like to find a good deduplication stress test to run in a guest.

Regards

Benoît

> It's great that this almost works, but it seems rather dangerous to put
> something like this into the mainline code without some regression tests.
> 
> (I'm also suspecting the regression test will be a great way to find 
> flakey hardware)
> 
> --------------------------------------------------------------------------
> Troy Benjegerdes                'da hozer'                 address@hidden
> 
> Somone asked my why I work on this free (http://www.fsf.org/philosophy/)
> software & hardware (http://q3u.be) stuff and not get a real job.
> Charles Shultz had the best answer:
> 
> "Why do musicians compose symphonies and poets write poems? They do it
> because life wouldn't have any meaning for them if they didn't. That's why
> I draw cartoons. It's my life." -- Charles Shultz