Subject: Re: nfs servers and 5 minute VOP_READ's
To: Bill Studenmund <>
From: Roger Brooks <>
List: tech-kern
Date: 03/16/1999 17:46:52
On Mon, 15 Mar 1999, Bill Studenmund wrote:

>On Sat, 13 Mar 1999, Bill Sommerfeld wrote:
>> And how does an NFS client tell the difference between a server which
>> is off the air, and a server which is frantically hunting for the
>> right tape?  You might be able to do something using NQNFS leases here
>> so that clients which support NQNFS are in a little better shape...
>Not sure. Is there a way that the server can say, "I got your request,
>but I'm too busy now, try again in a little bit." ??

Isn't this what NFSERR_JUKEBOX is for?

AFAIK, the protocol goes something like this:

Client sends a request.
Server starts loading tape/optical disk/whatever.
Client resends request.
Server notices that this is a repeat of an earlier request which is already
in the "slow queue", and replies NFSERR_JUKEBOX (= "be patient, I'll send
the response eventually").
Client shuts up and waits.
Server completes request and sends response to client.

Though it occurs to me that maybe the client should check every so often
that the server is still there, because the media load might take some time.

We used to have an abortion of a two-level filestore, where the backing
store was MO disks.  Any given file had to fit on a single MO disk (512 MB?),
but there was nothing to stop you creating a larger file on the front
layer!  Then when it decided to kick the file off onto backing store it
would frantically load every MO disk in the jukebox, trying to find one
with enough free space.  During this time, it couldn't do much of anything


Roger Brooks (Systems Programmer),          |  Email:
Computing Services Dept,                    |  Tel:   +44 151 794 4441
The University of Liverpool,                |  Fax:   +44 151 794 4442
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