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
