Disconnecting a SCSI tape drive from a VAX using VMS

[blaked97]

Hello,

I have a SCSI tape drive with the label MKA500: that is being shuttled back and forth between two different VAX servers. What VMS commands (VAX-VMS v6.2) do I need to issue so that I can unhook the tape drive's cable from the SCSI port without causing a problem? The device status is currently shown as 'Online'.

Also, would it be possible to leave the tape drive hooked up to one server (named BMVAX) and have the other server (named VXRDFC) still be able to use it by issuing a command on VXRDFC such as:

$ BACK/VER/IMA/LOG DKA300: BMVAX::MKA500:DKA300.BCK/SAV/REW

Thanks for any help you can give,
-Blake

[Hoff · Accepted solution]

Officially? Shut down OpenVMS VAX, power off the boxes, disconnect and reconfigure the SCSI bus chain, and power on and reboot. Don't break the SCSI bus chain with the operating system running.

Hot-swapping in this fashion is not officially supported and can cause bus transients and data loss, and that can be bad. Does hot-swapping work? Sometimes. Does warm-swapping (powered down to the console) work? Usually. About this particular configuration? Are you feeling lucky? Is your bus isolated to (just) the tape drive?

Your second question around remote access to a tape drive is answered in the VMS Frequently Asked Questions (FAQ) document, something you might want to acquaint yourself.

The FAQ is here:

http://labs.hoffmanlabs.com/node/1

Also see this discussion:

http://labs.hoffmanlabs.com/node/598

The easiest fix is to stop swapping gear around and to spend a (usually very small) amount of money to get a second tape drive of whatever type this is, or even two "newer" and "higher density" drives as a matched set.

If this gear is of the same vintage as OpenVMS VAX V6.2, replacement DLT and newer drives are available for very little money on various used-equipment sites. (Yes, I'm guessing that this is some sort of DLT drive, but there can be anything from 9-track magtape to DAT used on a SCSI bus here, too.)

[blaked97]

Yes, the SCSI bus is just the tape drive (and yes it is a DLT). I don't know if this makes it less risky. I think I'll just wait until I have a chance to shut down the box completely.

And I'll definitely see if I can get my boss to spring for another tape drive. :)

Thank you for the help!

[GuentherF]

I mostly had been lucky disconnecting the SCSI cable from a powered-off tape drive on the SCSI bus. But as Hoff pointed out this was never supported in OpenVMS.

A few times the system RESET (aka crashed). I assumed then from some "sparkling" signals on the SCSI bus because these connectors are not designed for hot swapping and the OpenVMS PK/MK driver could not makes sense out of it.

Take Hoff's advice and get another drive from the "bay'.

/Guenther

[Peter Weaver_1]

I have always said that disconencting a SCSI tape that is on the same bus as the VAX's system drive is a risk. At one major customer 9 times out of 10 the business managers were willing to take the risk of a possible outage rather than take a planned shutdown. On V6.x it was usually safe and managers were happy they took the risk. But after we upgraded to V7 our DEC (or was it HP at the time?) Field Service person found out that there was a bug that caused VMS to bugcheck 100% of the time if a SCSI tape drive was disconnected. He was able to get a patch that we quickly applied because Field Service was in to swap out a tape drive at least once a quarter (there were a lot of VAX machines at this site).

[Hoff]

If it's not already underway (or a full port off VMS), I'd be looking to migrate this whole show to VAX emulation (or maybe to Integrity, if you still have the source code), but that's another discussion.

VAX emulation is available from various vendors, and via (free) packages such as simh.

Reasonable Integrity server boxes show up for US$200 to US$1000 on the used market, plus maybe another US$1000 to (over-)load it with extras and giblets. I don't know what the VMS licenses are going for off-hand, but they were under US$1000 per core when last I checked.

As for your current predicament, having just the tape on the SCSI with no other peripherals means you're less likely to corrupt traffic, so long as you're not doing something with the tape drive when you yank it.

Yes, there can be the odd misbehavior. Which can include crashes. Yanking the tape drive is entirely analogous to a hardware failure, and hardware failures can sometimes bring out the bugs. It's rare, but I've seen it.

Used tape drives in the DLT series are pretty cheap. I was offered DLT8000-class drives for the price of hauling them off a while back. Ultrium 2 series drives (200 GB) can show up for US$250, and DLT drives are often less.

[Bob Blunt]

Additionally... none/few of the standard SCSI giblets like tabletop tapes were treated to some of the ingenuity that DEC did build into much of the DSSI hardware. Maybe in the StorageWorks arena where a drive is in a "container" you've got some of the connecting pins adjusted in length so some of the signals are protected but it isn't likely with "just a regular SCSI cable" that connects most tapes and other external peripherals.

When I've been in a HUGE rush and required something get done where the shutdown/power cycle/reboot time was THAT problematic I have also moved a tape when pushed hard to do so by someone else who was A) demanding it and B) in a position to take the heat if something didn't work. At the very least I would power off the tape, shutdown VMS and the VAX to the console prompt, move tape and terminate as needed, then issue the following console commands (on VAX):

>>> halt
>>> unjam
>>> init

Then reboot. On Alpha you can usually get by with just "halt" and "init" and reboot but I'm less confident when doing this on an Alpha.

"Just moving the drive" is risky.

As far as remote tape access? There's an ancient DECnet hack that can be used if you *KNOW* that you're only going to use ONE tape in a saveset and this solution is free and pretty inefficient. There are other solutions. Out of curiosity, what type VAXen are being used in this picture?

bob

[tsgdavid]

Blake,
Depending on what you plan to do with your backups, you could move from doing tape backups to storing backups on disk.

Backups can be done to a local disk and/or to a saveset on a remote disk over the network.

If you had a SCSI storage shelf, you could hook it up to one of your machines and use it (more or less) the same as you do with the tape drive now. The difference being that you CAN do the backup over the network just like you were wanting to do with the tape drive. There would be no need to disconnect the hardware unless you needed to restore the system disk on the other machine.

Of course, if you need to separate the media from the machines, that makes this more of a challenge.

Dave

[Andy Bustamante]

While we're helping Blaked97 spend money, see the Symantec netbackup options. This reasonably well behaved client allows you to create backups over the network.

One caution, maintain a system disk recovery tape so you can connect to the backup server.

[Proliant VMS San Mgrs]

Blake,

I've never tried this, so I may be way off target.

There's a hardware device called a "SCSI" hub. That was intended for use with SCSI clusters so you could unplug a given CPU from the cluster without disrupting the remaining nodes. I've never tested that; it became available after we froze our SCSI configuration (and finally upgraded from HSZ to EVA). Tapes are explicitly not supported (in a cluster configuration).

I also have no idea if a SCSI hub would work (supported NOT) with a tape drive. If it is, you could at least avoid the chance that you induce unintended side effects by unplugging / plugging SCSI tapes into different processors.

The SCSI hub (DWZZH) goes in a BA356 shelf and is described in the Guide to VMS Cluster Configurations. Perhaps someone has tried this with tapes; it is normally used to connect HSZ storage systems to a 3 or 4 (CPU) node cluster.

You will also need a tape "brick" which will plug into a BA356. All ancient, and in this case completely unsupported (so you would want someone here to say they've tried this; I haven't).

Clearly a second tape drive is the best option for you (IMO).

Carl

[RBrown_1]

We used to do this when we shared an 8mm tape drive between two PDP-11s and two VAXes. As long as we turned off the tape drive before moving it, we did not detect any problems.

When we tried it on an alpha, we would see some SCSI errors, but usually (always?) the system would recover.

Obviously we have not done this in years. After all, who wants to use an 8mm tape drive?

More recently we set up a customer with a Paralan brand SCSI switch to swing a string of SCSI disk drives between two PDP-11 hosts.