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February 19, 2016

How hot plug disks can replace DDS offsite

300 GB Ultra SCSII need to find an alternative to DLT and DDS tapes for offsite storage. Sure, there's DS2100 and Jamaica drives. But a few $35 300GB Ultra SCSI drives would hold a lot more data with less points of failure. I'll set up a BACKUP_VOLUME_SET and use the internal disks to do store-to-disk backups of the system. 

I've always used my A-Class and N-Class systems with fiber-attached disk. Are the internal disk drives hot-pluggable? 

Jim Hawkins, IO maven for HP 3000 systems at HP, replies with details.

There are multiple layers of changes for actual hot plugs or swaps to work.  

  • You need the disk HDD to handle this electrically.
  • You need HDD physical carrier and physical interface to comply.
  • You need the system physical interface and receptacle to comply. 
  • You need your Host System Bus Adapter (HBA) to electrically support this.
  • You need the OS to be aware enough of the HBA to not get flustered by absence of the device and deal with any notifications from the HBA of the activity.   

Given that the N-Class disk cage has a screw-based cover and the HDD carriers have no quick release levers (as compared with HASS/Jamaica or VA7400) I would state definitively that there is no hot-plug intention.  

At the same time, the SCSI bus is pretty low power and low voltage, so it would be generally not-too-unsafe to experiment. But you're also close to AC inputs and they are not low power.

Hawkins explored the not-too-unsafe scenario with theoretical possibilities.

Might you be able to pull/push a drive where you've closed the volume?  Likely it would work, but there may be all kinds of noise and stress on the SCSI bus which may not be well handled. However, I think each disk is on its own HBA channel which isn't shared with anything else, and so unlikely to abort someone else's IO.  

This takes us to the last issue: mechanical wear.  

These connectors were likely intended for more or less permanent mating of two components. Very likely they have a limited number of cycles that they are specified to hold-up. I've seen connectors that are specified for fewer than 25 cycles before you lose gold contact material.   This is okay for normal HDD where one might replace one or two per slot in a system lifetime, but not sufficient if you're doing nightly back-ups and swaps.  Connectors, where there is an expectation of a high number of pull/replace cycles, have special designs.  

Now a little good news here is that the N-Class was still pretty much old-school HP design, so likely they didn't pick up something cheap that saved them .2 cents per unit on gold plating. No idea though if the HDD connector is a 10-, 100-, 1000-cycle part. Your system, your risk. 

Consultant Mark Ranft points out that the HP design for 3000s seems to make the servers and components good candidates for exceptional mechanical wear tolerances.

It is especially helpful to understand the concept of mechanical wear on the connectors. HP always had excellent and innovative hardware engineering on their HP 3000 servers. Remember, you can drop them off a building and still self-test them

The Unix N-Class appears to allow hot-pluggable drives.   

The actual power supply and the fans are in the front of the N-Class.  The power receptacles in the back have internal cords that lead to the front.

08:59 PM in Hidden Value, Homesteading | Permalink

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