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

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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



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