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  by Bill Degnan - 11/20/2015 14:34
"..Hi Bill,

Thanks for sharing that. It's handy to have all this in one place to refer
people to when they arrive on the mailing list asking how to get into their
new VAX, how to see what disks it has and so on.

The $255 in the DIA5 device name is the device allocation class, also known as
ALLOCLASS. This is a not very well explained and rather obscure corner of VMS.
I think the original purpose of ALLOCLASS was to provide a way to ensure that
particular devices that are visible to several nodes in a cluster via different
paths can be configured so that all nodes see the same device name. This is
important because the VMS distributed lock manager requires the full name of a
file, including device name, to be the same wherever it is seen from. This
problem usually arises when disks are served to multiple cluster nodes from a
storage controller such as a HSC or a HSZ. If NODEA and NODEB can both see
DUA0 (for example) on the storage controller and they both also serve this disk
to other nodes in the cluster, the same disk could be seen from a third node as
either NODEA$DUA0 or NODEB$DUA0 and file locking would then not operate
correctly leading to possible data corruption.

Many years ago, after a lot of head scratching, my boss and I eventually
figured out that shadow sets can only be composed of members with a non zero
ALLOCLASS. This is probably why lots of the examples of using ALLOCLASSes also
invove shadow sets. I think we figured out why this made sense some time after
that but I have since forgotten again.

Your system might not have any shadow sets but it does have DSSI disks and
these behave like each disk is effectively it's own cluster node. See:

$ SHOW CLUSTER

Setting a non-zero ALLOCLASS in this case may be preferable to the semi-random
looking pseudo-node names for each disk that DSSI subsystems typically comes up
with. It looks likely that the replacement DIA5 you obtained arrived
configured with an ALLOCLASS (255) which is different to the ALLOCLASS your
other disks are configured with (1). This can probably be changed by using

$ SET HOST /DUP

to connect to it but I never got into finding my way around that as the only
DSSI disks I've ever seen died not long after I got them.

You have probably been able to omit the $1$ when referring to your original
disks because the system ALLOCLASS (set in SYSGEN) is likely also set to 1.

I was hoping I might be able to clarify some of the confusion that often
surrounds ALLOCLASS but reading back over what I wrote, it seems that I
probably haven't. It seems to be a topic that is as hard to explain as it is
to understand in the first place :-(

Regards,
P Coghlan. .."


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