From: "Ara.T.Howard" <Ara.T.Howard@noaa.gov>
Subject: reliable reading
Date: Mon, 28 Jun 2004 09:13:29 -0600 (MDT)
Sender: nfs-admin@lists.sourceforge.net
Message-ID: <Pine.LNX.4.60.0406280901350.3356@harp.ngdc.noaa.gov>
Reply-To: "Ara.T.Howard" <Ara.T.Howard@noaa.gov>
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nfs'rs-

i'm in the middle of implementing a job distribution mechanism that uses an
nfs mounted work queue.  obviously clients serialize their access to this
queue using locking.  i'm using the double lock technique of lockfile + fcntl
lock on lockfile.  the code implements crude transactions using the following
logic:

   obtain_lock

   read_queue

   do_work_with_queue

   if changes_were_made
     copy queue, backup
     err = write queue
     if err
       copy backup, queue
     end
   end

   release_lock

in otherwords, all or none wholesale replacement of queue.  in order for this
sort of thing to work i need to be able to do a few things:

   - reliable lock (done)
   - reliable read
   - reliable write

now, i undertand the word 'reliable' is subjective here when we're talking
about networking but the inode caching of nfs makes the read/write bit more
that little problematic.  my questions are:

   - what is the 'safest' way read the most recent contents of an nfs mounted
     file accounting for attribute caching?

   - same question but for writing

i am not concerned with speed since the cost of accessing the queue is grossly
outweighed by the time to execute a job taken from it in almost all cases - i
simply want to make the absolutely most reliable read/write __possible__.  i
am open to mount option suggestions but want to make the code reliable even in
the face of the most aggressive attribute caching - if possible.

thanks in advance for any suggestions.

-a
--
===============================================================================
| EMAIL   :: Ara [dot] T [dot] Howard [at] noaa [dot] gov
| PHONE   :: 303.497.6469
| A flower falls, even though we love it;
| and a weed grows, even though we do not love it. 
|   --Dogen
===============================================================================


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