pNFS rant (was Re: [PATCH 1/8] exofs: Kbuild, Headers and osd utils)

Jeff Garzik
2009-02-16T11:05:17Z

Boaz Harrosh wrote:
> No can do. exofs is meant to be a reference implementation of a pNFS-objects
> file serving system. Have you read the spec of pNFS-objects layout? they define
> RAID 0, 1, 5, and 6. In pNFS the MDS is suppose to be able to write the data
> for its clients as NFS, so it needs to have all the infra structure and knowledge
> of an Client pNFS-object layout drive.

Yes, I have studied pNFS! I plan to add v4.1 and pNFS support to my NFS
server, once v4.0 support is working well.

pNFS The Theory: is wise and necessary: permit clients to directly
connect to data storage, rather than copying through the metadata
server(s). This is what every distributed filesystem is doing these
days -- direct to data server for bulk data read/write.

pNFS The Specification: is an utter piece of shit. I can only presume
some shady backroom deal in a smoke-filled room was the reason this saw
the light of day.

In a sane world, NFS clients would speak... NFS.

In the crazy world of pNFS, NFS clients are now forced to speak NFS,
SCSI, RAID, and any number of proprietary layout types. When will HTTP
be added to the list? :)

But anything beyond the NFS protocol for talking client <-> data servers
is code bloat complexity madness for an NFS client that wishes to be
compatible with "most of the NFS 4.1 world".

An ideal NFS client for pNFS should be asked to do these two things, and
nothing more:

1) send metadata transactions to one or more metadata servers, using
well-known NFS protocol

2) send data to one or more data servers, using well-known NFS protocol
subset designed for storage (v4.1, section 13.6)

But no.

pNFS has forced a huge complexity on the NFS client, by permitting an
unbounded number of network protocols. A "layout plugin" layer is
required. SCSI and OSD support are REQUIRED for any reasonably
compatible setup going forward.

But even more than the technical complexity, this is the first time in
NFS history that NFS has required a protocol besides... NFS.

pNFS means that a useful. compatible NFS client must know all these
storage protocols, in addition to NFS.

Furthermore, enabling proprietary layout types means that it is easy for
a "compatible" v4.1 client to be denied parallel access to data
available to other "compatible" v4.1 clients:

Client A: Linux, fully open source

Client B: Linux, with closed source module for
layout type SuperWhizBang storage

Both Client A and Client B can claim to be NFS v4.1 and pNFS
compatible,
yet Client A must read data through the metadata
server because it lacks the SuperWhizBang storage plugin.

pNFS means a never-ending arms race for the best storage layout, where
NFS clients are inevitably compatibly with a __random subset__ of total
available layout types. pNFS will be a continuing train wreck of
fly-by-night storage companies, and their pet layout types & storage
protocols.

It is a support nightmare, an admin nightmare, a firewall nightmare, a
client implementor's nightmare, but a storage vendor's wet dream.

NFS was never beautiful, but at least until v4.0 it was well known and
widely cross-compatible. And only required one network protocol: NFS.

Jeff