On 1/15/25 10:06 AM, Matthew Wilcox wrote:
On Tue, Jan 14, 2025 at 04:38:03PM -0500, Anna Schumaker wrote:
I've seen a few requests for implementing the NFS v4.2 WRITE_SAME [1] operation over the last few months [2][3] to accelerate writing patterns of data on the server, so it's been in the back of my mind for a future project. I'll need to write some code somewhere so NFS & NFSD can handle this request. I could keep any implementation internal to NFS / NFSD, but I'd like to find out if local filesystems would find this sort of feature useful and if I should put it in the VFS instead.
I think we need more information. I read over the [2] and [3] threads
and the spec. It _seems like_ the intent in the spec is to expose the
underlying SCSI WRITE SAME command over NFS, but at least one other
response in this thread has been to design an all-singing, all-dancing
superset that can write arbitrary sized blocks to arbitrary locations
in every file on every filesystem, and I think we're going to design
ourselves into an awful implementation if we do that.
Can we confirm with the people who actually want to use this that all
they really want is to be able to do WRITE SAME as if they were on a
local disc, and then we can implement that in a matter of weeks instead
of taking a trip via Uranus.
IME it's been very difficult to get such requesters to provide the
detail we need to build to their requirements. Providing them with a
limited prototype and letting them comment is likely the fastest way to
converge on something useful. Press the Easy Button, then evolve.
Trond has suggested starting with clone_file_range, providing it with a
pattern and then have the VFS or file system fill exponentially larger
segments of the file by replicating that pattern. The question is
whether to let consumers simply use that API as it is, or shall we
provide some kind of generic infrastructure over that that provides
segment replication?
With my NFSD hat on, I would prefer to have the file version of "write
same" implemented outside of the NFS stack so that other consumers can
benefit from using the very same implementation. NFSD (and the NFS
client) should simply act as a conduit for these requests via the
NFSv4.2 WRITE_SAME operation.
I kinda like Dave's ideas too. Enabling offload will be critical to
making this feature efficient and thus valuable.
--
Chuck Lever
