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From: carl@CITHEX.CALTECH.EDU (Carl J Lydick)
Newsgroups: mod.computers.vax
Subject: Re: Comments about volume sets
Message-ID: <860913201512.038@CitHex.Caltech.Edu>
Date: Sat, 13-Sep-86 23:16:12 EDT
Article-I.D.: CitHex.860913201512.038
Posted: Sat Sep 13 23:16:12 1986
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It's not quite true that you can't remove a disk from a volume set once
it's been added; in fact, it's not even all that difficult to do.  You do,
however, have to edit the home block (and for consistency's sake, the backup
home block) of the disk in question and possibly [000000]VOLSET.SYS on the
first disk of the set.  The relative volume number is stored in bytes 38-39
of the home block, and the number of volumes in the set is stored in bytes
40-41.  In addition, there is a two-byte checksum in bytes 58-59 for the
home block up to that point, and another in bytes 510-511 for the entire
block.  The home block is virtual block 2 of the index file, and is the
first good physical block in the sequence 1 + n * delta, where delta is
obtained from the following table:
	Geometry	delta
	s x 1 x 1:	1
	1 x t x 1:	1
	1 x 1 x c:	1
	s x t x 1:	s + 1
	s x 1 x c:	s + 1
	1 x t x c:	t + 1
	s x t x c:	(t + 1) * s + 1
where s is the number of sectors per track, t the number of tracks per
cylinder, and c the number of cylinders on the disk.

The backup home block is a block further down in the sequence, (typically
the next good block in the sequence) located in one of virtual blocks
v * 2 + 1 through v * 3, where v is the cluster factor for the disk.  The 
entire cluster is made up of duplicates of the home block.
I've succeeded in removing a disk from a volume set by using the DCL OPEN,
READ, and WRITE commands, along with the lexical functions, so it's really
not all that difficult.