HP 3000 Manuals

SPU Switchover/XL User's Guide

Communicating Between SPUs 

The SPU Switchover/XL product requires a communication link between the
two systems.  This communication link is used to exchange information
about locked disks, volume sets, configuration, and the system status.
The communication link used is the HP-FL (fiber link) that connects the
disk drives to both systems (see Figure 1-2 ).


          Figure 1-2.  SPU Switchover/XL Fiber Link Connection 

From each system, a fiber link cable is attached to an HP-FL device
adapter card.  A message from one SPU to another travels through an HP-FL
card, to the disk controller of the shared disk drive, and up to the
HP-FL card of the other system (see Figure 1-3 ).  Note that this is
not a direct HP-FL to HP-FL message, but a method that utilizes the fiber
link drives.  For example:

   1.  The switchover code on the home system sends a request to its
       host-to-host device manager (DM).

   2.  The host-to-host DM of the home system then sends the request to
       its HP-FL device adapter manager (DAM).

   3.  The HP-FL DAM sends the message through the shared fiber link
       disk(s) and onto the alternate system's HP-FL DAM.

   4.  The HP-FL DAM of the alternate system then sends the request to
       its host-to-host DM.

   5.  The host-to-host DM of the alternate system sends the request to
       the SPU Switchover/XL software, which takes the appropriate
       action.


          Figure 1-3.  Software Layers for Communication 

Locked and Unlocked Volume Sets 

With the SPU Switchover/XL product, switchable volume sets are connected
to each system.  To prevent data corruption, a locking mechanism gives
exclusive access of the volume set to the SPU issuing the LOCK command.
If the other SPU tries to read or write to the volume set, it receives a
status of resource "unavailable." Once this status is returned, the
locked-out SPU can send a message to the locking SPU to verify that it
has locked the volume set.  If the locking SPU fails to respond, the
inquiring SPU can then issue its own lock command.  This locking and
unlocking scheme allows for one system to maintain control over the
switchable volume set at any given time.

Switched Volume Sets 

When an SPU detects a locked disk, it receives acknowledgment and disk
volume information from the locking system.  If a hardware failure occurs
on the locking system and a SWITCHOVER command is then issued, this
volume information can be used by the alternate system to open the
switchable volume set and lock it for its own use.

When the home system comes back up, it is prevented from accessing these
disks until the operator does a VSCLOSE for the switched switchable
volume sets on the alternate system and a SWITCHBACK command on the home
system.  This unlocks the disks on the alternate system and locks them on
the home system.

MPE/iX 5.0 Documentation