Re: [Qemu-devel] [RFC PATCH 06/17] block: use bdrv_{co, aio}_discard for write_zeroes operations

[Paolo Bonzini]

Il 16/03/2012 01:47, Richard Laager ha scritto:
> On Thu, 2012-03-15 at 10:36 +0100, Paolo Bonzini wrote:
>> Changing across guest boots is a minor problem, but changing across
>> migration must be avoided at all costs.
>>
>> BTW, after this discussion I think we can instead report
>> discard_granularity = 512 and discard_zeroes_data=0 and get most of the
>> benefit, at least on file-backed storage.
> 
> Are you going to report that to guests all the time, or only when the
> host supports discard? If you don't report it all the time, you could
> still be "changing across migration". If you do report it all the time,
> then you're incurring a performance penalty on systems that don't
> support discard, as the guest will be sending discard requests that QEMU
> has to throw away (but by which time some work has been wasted).

I don't think that should be that bad.  Discard requests should be
relatively rare.

> And either way, what you're proposing means that users with
> discard_zeros_data = 1 hosts can't get the (albeit small) benefits of
> that because some other QEMU user might want to do a migration across
> heterogeneous storage.

Yes, discard_zeroes_data can be made configurable on top, and either
rejected or emulated if the storage does not support it.

> Finally, I see your proposal of advertising fixed discard support

It does not really have to be fixed, it's just a default.

Paolo