Re: RFC: Use of VOP_ALLOCATE() by NFSV4.2 nfsd

On Sat, Oct 9, 2021 at 7:13 PM Rick Macklem <rmacklem@uoguelph.ca> wrote:
>
> Hi,
>
> I ran into an issue this week during the nfsv4@ietf.org's testing event.
> UFS - supports VOP_ALLOCATE() by using vop_stdallocate().
> ZFS - just return EINVAL for VOP_ALLOCATE().
>
> An NFSv4.2 server can either support Allocate or not, but it has to be
> for all exported file systems.

That seems like a protocol bug to me.  Could this be fixed in a future
NFS revision?

>
> This leads me to a couple of questions:
> - Is there a good reason for not using vop_stdallocate() for ZFS?

Yes.  posix_fallocate is supposed to guarantee that subsequent writes
to the file will not fail with ENOSPC.  But ZFS, being a copy-on-write
file system, cannot possibly guarantee that.  See SVN r325320.

> - Should I try and support both file system types via vop_stdallocate()
>   or not support Allocate at all?

Since you can't possibly support it for ZFS (not to mention other file
systems like fusefs) you'll have to not support it at all.

>
> Btw, as a bit of an aside, "cc" uses posix_fallocate() and in weird ways,
> such as offset=0, len=1. Why, I have no idea?
>
> Thanks in advance for any comments, rick
>