Re: [Nbd] fua, trim, etc

[Paolo Bonzini <pbonzini@...696...>]

On 09/13/2011 12:49 PM, Alex Bligh wrote:
> For instance, Paul's suggestion on setting 'rotational' using /proc
> instead is definitely better than the way nbd-client does it at the
> moment.

That can go in later, right?

> > - Make NBD_CMD_TRIM cause nbd-server to call fallocate() with
> > FALLOC_FL_PUNCH_HOLE.
>
> We need to be a bit careful on these. IIRC (someone please check!)
> on Linux fallocate() is an extended call that supports various stuff,
> whereas posix_fallocate() is the normal POSIX call, and that doesn't
> support PUNCH_HOLE. On non-Linux systems, fallocate() is the normal
> POSIX call.

No, it's really called posix_fallocate elsewhere.  Non-Linux system 
might alias fallocate and posix_fallocate, but the name in POSIX is 
really posix_fallocate.

Of course, non-Linux systems will never have FALLOC_FL_PUNCH_HOLE, so we 
can assume that defined(FALLOC_FL_PUNCH_HOLE)<=>fallocate has Linux 
semantics.

> I /think/ we can just see if FALLOC_FL_PUNCH_HOLE is defined,
> and if so, make the call, and ignore the error condition. That
> way if compiled on box X and run on box Y it will still be OK.
> However, I think we ought to have a config option to turn it off.

Right now it is opt-in, via --trim.

However, flush/fua should likely _not_ be opt-in, because they are 
required for safety.

Paolo