Subject: [nfsv4] pNFS block layout & layout_hint attribute

From: [email-masked]
Date: Wed, 25 Mar 2009 16:37:17 -0400

This message summarizes the problem and the results of discussion
in the nfsv4 WG meeting this morning in San Francisco. The slides
should appear in the online meeting materials soon - both the
Problem and Possible Approaches material below is from the slides.

Problem:
- layout_hint attribute is pNFS-layout-specific
- File and Object layouts:
  - Tell server how to create file (e.g., striping)
- Block layout:
  - Tell server what client's max I/O time is
- Problem discovered by implementers:
  - Client max I/O time is per-client
  - layout_hint is specified as per-file
  - Setting this on every file is overkill for block layout

The first outcome of the discussion was that something needs
to be done, because what the pNFS block layout draft currently
says is not consistent with the main NFSv4.1 draft.

Possible Approaches:
1. Tweak default behavior if attribute not set
   - Use most recent value provided by same client
2. Make attribute scope layout-dependent
   - File and object layouts: per-file
   - Block layout: per-filesystem (track per client)
3. Define a new attribute

The sense of the discussion was that the above list appears
to be in order of preference, as a new attribute is clearly
NFSv4.2 territory, not 4.1, and it's preferable to keep the
changes limited to the block layout draft.  That makes
approach 1 preferable, or even just enough of a change to
align the block draft with the main NFSv4.1 draft.  One
consequence of this discussion is that the attribute will
remain per-file in the main NFSv4.1 draft.

Plan:
(A) The pNFS block draft authors will write up text changes
	for both the minimum required correction and the default
	behavior (i.e., what the server is supposed to do if a
	client doesn't specify this attribute on a file for which
	a block layout is requested).  That text will be sent to
	this list by a week from this Friday (i.e., by Friday,
	April 3).
(B) The NFSv4 WG chairs plan to conduct a 1-week review of this
	text.  The final text changes will be sent to the RFC
	Editor for incorporation into the pNFS block draft before
	it's published as an RFC.

Thanks,
--David
----------------------------------------------------
David L. Black, Distinguished Engineer
EMC Corporation, 176 South St., Hopkinton, MA  01748
+1 (508) 293-7953             FAX: +1 (508) 293-7786
[email-masked]        Mobile: +1 (978) 394-7754
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