Re: [Nbd] fua, trim, etc

[Alex Bligh <alex@...872...>]

--On 13 September 2011 12:36:07 +0200 Wouter Verhelst <w@...112...> wrote:

> I should probably make another release some time soon, there are quite a
> number of new features waiting;

Not on your list: my FUA/FLUSH stuff is in, but I must admit to having
lost track of where the ioctl() discussion got to. As there is apparently
now agreement on the ioctl(), it would be really nice if we could either
keep the current one or replace it in this version with one Paul et
al are happy with.

For instance, Paul's suggestion on setting 'rotational' using /proc
instead is definitely better than the way nbd-client does it at the
moment.

> - 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.

Secondly (a) PUNCH_HOLE is not supported on all kernel versions,
and (b) it isn't necessarily supported on all filing systems and
(c) on some filing systems it is buggy.

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.



--
Alex Bligh