From: kparris@my-deja.com
Sent: Monday, November 06, 2000 5:39 PM
To: Info-VAX@Mvb.Saic.Com
Subject: Re: VMS Software Mirroring?

helbig@astro.rug.nl wrote:
> Now I have a question:  WHY must the $allocation-class$ddcu[:] syntax
be
> used?  As far as I can tell (and I have just read up on node and port
> allocation classes), this is only necessary with dual-ported disks.

I haven't examined the code myself, but I asked someone in VMS
Engineering once and was told this was a consequence of the 16-byte
limit on lock value block size in the lock manager.  Volume Shadowing
apparently uses lock value blocks to pass around shadowset member
information, including device names, and at some point during the design
a decision had to be made on what could fit into a lock value block and
since an allocation class could fit into one byte, but an SCS node name
would take 6 bytes (actually, the SCS spec. in fact allows for for up to
15 characters, in theory) the nodename would not fit into whatever space
was available for it in a lock value block, so allocation classes are
required for all shadowset member device names.
-----------------------------------------------------------------------
Keith Parris|Integrity Computing,Inc.|parris@decuserve.decus.org-nospam
VMS Consulting: Clusters, Perf., Alpha porting, Storage&I/O, Internals


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