[daip] tape drives

[Patrick P Murphy]

On Sat, 16 Sep 2006 06:43:56 -0700 (PDT), "robert becker"
   <bob at igpp.ucllnl.org> said: 

> hi, i have a tape drive question.
> i am using a redhat operating system. initially i was using
> /dev/sto,

I assume you mean /dev/st0 (zero, not oh).  This is the flavour of tape
device that will rewind immediately when closed, so it's not useful for
our type of work.

> i was able to read image file (1550 by 1550) at 6/minute but the tape
> would rewind after each use.

Right, as it should.  See "man 4 st" from the Unix command line:

   "Devices opened using the principal device number will be sent a
    REWIND command when they are closed"

> i switched to /dev/nsto which solved the rewing problem but reading
> slowed down to 1/minute.

Is this repeatable?  I would be very surprised if there's a causal
relationship between these two factors.  And I'd try using "dd" to
eliminate any other causes, e.g. (assuming the data is blocked by a
factor of 10, i.e., 28800 bytes):

  time dd if=/dev/nst0 of=/dev/null bs=28800

And repeat this for the 2nd, 3rd file, etc., to see how fast it is.

Other possible causes for the slowdown:

  - Are the image files really all the same size?  
  - Could the tape be "dirty" causing re-reads on segments after the
      first file?
  - Are there any error messages in /var/log/messages about st0?
  - What parameters does "mt -f /dev/nst0 status" show?  Ours typically
      look like this:
      SCSI 2 tape drive:
      File number=-1, block number=-1, partition=0.
      Tape block size 0 bytes. Density code 0x0 (default).
      Soft error count since last status=0
      General status bits on (50000):
       DR_OPEN IM_REP_EN

Hope some of this helps.

 - Pat