[Simh] RE : How to setup a simh VAX VMS cluster that handles more than 16 disks.

[Jeremy Begg]

Hi,

Just a quick suggestion about your cluster device naming ...

>DISKS SERVING

>VMS in a cluster supports disks when they are named differently.
>
>Since you can have a cluster done with identical machines, their device
>names would be the same!  This situation would create a disk name
>collision!.

I think you mean "... identical machine configurations" rather than
"identical machines".

>To avoid this, you have to modify the ALLOCLASS parameter in MODPARAMS.DAT
>to have a different number of ALLOCLASS for each node.  For example, the
>ALLOCLASS=1 will have disks of the node having a $1 prefix added for the
>disk device.  Also, on REGIE, an ALLOCLASS=3 will add the $3 prefix etc.
>
>(define a different ALLOCLASS number for each node).

ALLOCLASS is used to great effect when disks are on shared storage bus such
as DSSI or CI or they are dual-ported (which are the only options for VAX;
on Alpha and Integrity you can also have FibreChannel and SCSI shared disks
-- but SIMH only does VAX.)  Correct use of ALLOCLASS allows a given
physical drive to be accessible to all cluster members by more than one
path, thereby improving fault-tolerance.

However if all the disks in your cluster are "local" disks (i.e. directly
attached to their respective host nodes) which is typical for the disks in a
SIMH setup, much confusion can be avoided by setting ALLOCLASS to 0 on all
nodes.  With ALLOCLASS=0 the device names change to node$DUAnnn: where
'node' is the VAX nodename (SCSNODE parameter) and 'nnn' is the unit number.

Of course, if you have a VAXcluster of SIMH instances running on a single
host computer (e.g. a single PC running Windows or Linux), it might be
possible to have each simulated VAX connect to the same disk image.  In
which case you would have shared storage and need to set ALLOCLASS to some
positive integer > 0 (the same value on each machine which shares the disk).

Regards,

        Jeremy Begg

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