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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. .." Reply |