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

Alan Somers wrote:
>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?
Who knows. I don't see any interest in a 4.3. 4.2 is extensible, but I think
this is now "cast in stone".

>>
>> 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.
However, vop_stdallocate() just does VOP_WRITE()s to the area (with
bytes of data all zeros). Wouldn't that satisfy the criteria?

>> - 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.
It does sound like not supporting it is the best alternative.

rick

>
> 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
>