Fine-Tune Friday: DDS diagnosis and tips
April 27, 2018
We have a tape device that is not responding; that is, we put the tape in, but it is not coming online. I also see that a user is logged into the system using the LDEV assigned to the tape drive. SHOWDEV TAPE also does not list the device.
Gilles Schipper replies:
I’ve seen this before for DDS drives, Probably during your most recent reboot, there was a (possibly temporary) malfunction with your tape drive’s power supply such that its existence was not recognized during the boot up process. That would normally result in a “device unavailable” condition and the subsequent disabling of that logical device number.
I have noticed instances where that LDEV number is actually made available to the logon device number pool (for subsequent assignment for logon session device numbers). Long story short, the solution appears to be a power cycle, START NORECOVERY reboot.
After shutting down and powering off the CPU and all devices, run ODE to ensure all devices are recognized before START NORECOVERY. Failure to recognize the device at that point should lead to further investigation of the power supply, SCSI device number setting, or other hardware malfunction. If this situation happens frequently, I would first suspect a problem with the power supply of that device.
Get rid of that internal DDS tape drive
By John Burke
People complain of problems with internal DDS tape drives in systems located in remote areas with little onsite expertise, problems that lead to frequent drive replacements and downtime. It reminds me of the old vaudeville joke where the patient comes to the doctor with a complaint, “Doc, it hurts when I do this.” The doctor replies, “Then don’t do that.”
HP 3000 gurus have cautioned for years that people should not use internal tape or disk drives in 9x7, 9x8 or 9x9 production systems. The most likely failure is a tape drive and the next most likely failure is a disk drive. Everything else in the system cabinet could easily run for a decade without needing service or replacing. [Editor's note: John's advice came in 2004, so a decade-plus is definitely bonus time.] When an internal tape or disk drive fails you are looking at serious downtime while the case is opened and the drive is replaced. A common urban legend says that the primary boot device (LDEV 1) and the secondary boot device (usually LDEV 7) must be internal. Not true.
Bite the bullet now. Remove, or at least disconnect (both power and data cables) all internal drives. At the least, replace the internal DDS drive with an external DDS3 or DDS4 drive. In the case of the DDS drive, you will not even need to make any configuration changes if you set the SCSI ID to 0 on the external drive.
Someone also asked whether if you changed the boot path you should immediately create a new SLT. Technically, the answer is “No” since the SLT contains no information about boot paths. However, if you have not created an SLT since the device was added (and why not?), then by all means create a new SLT. It should also be noted that DDS drives are notorious for not being able to read tapes created on other DDS drives.
So, if you do not think you have time to create a new SLT, at least use CHECKSLT to verify you can read your existing SLT on your new drive. If you cannot read your existing SLT, then make time to create a new SLT. Your standard procedures should include regularly creating and SLT and checking it.