Re: Problem with scsi tape drives (2.4.18) and soft error count (BusLogic, AIC7xxx)

[Richard B. Johnson]

On Tue, 2 Apr 2002, Evan Harris wrote:

> 
> I've had a long time problem with trying to get the total soft error count
> from tape devices when using the kernel provided tape interface.
> Hopefully, someone here can shed some light on the problem.  Using several
> different DAT and DLT tape drives, the behavior seems the same.
> 
> I'm trying to figure out how to retrieve the soft error count from a tape
> drive after having performed a backup.  It helps me to gauge when a tape
> needs to be retired, and I'm used to being able to get the total soft error
> count from other backup software packages for dos/windows.
> 
> mt apparently queries the soft error count, but it always seems to be zero.
> I've dug into the problem a bit, and it seems that mt reports zero because
> the tape drive has had it checked and cleared by the kernel at every drive
> operation.  Is there any place in the kernel that this information is stored
> so that it may be retrieved?

Not really. The soft error count is preserved across the 'correct' kinds
of open/close operations. To use `mt` to get the count and, to preserve
the state of the tape machine, you need to do your open/close against
the minor number that has the high-bit set:

# file /dev/st*
st0:    character special (9/0)
st1:    character special (9/1)
st3:    character special (9/128)

Instead of using /dev/st0, you would use (on this machine) /dev/st3.

So, if you do your I/O and status through /dev/st3, you will get
meaningful information. Once you close /dev/st0, all history is
lost (correctly). Note that if you do I/O through /dev/st3, the
tape will not automatically rewind on close. You will need to
use `mt` for that.

Cheers,
Dick Johnson

Penguin : Linux version 2.4.18 on an i686 machine (797.90 BogoMips).

                 Windows-2000/Professional isn't.