4.2 Disk Partitions

[Greg Woods]

  We want to modify the partition definitions for an RM05 look-alike.
It's a Braegen disk, which I think is really a CDC 9766, and we have
a controller that looks like another Massbus (it plugs into the backplane
of a VAX 11/750). I'm not sure who makes it; if it's critical to the
answer to my questions I could find out. At any rate, we want to put
/tmp on the A partition of this drive, and swap space on the B partition
(so that an entire RA81, the C partition, can be used for user files).
Then, we want to have /usr on the rest of the Braegen disk. Unfortunately
the default partition tables have the rest of the disk split into two
partitions (one of which can be split into three sub-partitions if
desired). We want to modify the partition definitions in /etc/disktab
and /sys/vaxmba/hp.c (the device driver source) to allow use of a
larger partition on this disk other than the whole thing. I know where
to modify it and even how to change it; what I don't know is, what are
the limitations? Does a partition size have to be an even number of
cylinders? (I presume yes). In that case, how can I determine what
the size of a cylinder in sectors is? Is there any reason why I can't
eliminate some of the partitions by making both values in the rm05 size
structure zero? (I see that this is done for an RA80 disk). Is there any 
other limitation on where the partition boundaries can be (minimum size, maximum
size, etc.) that I should know about? I do know that such re-partitioning 
can be done, since our former system administrator has done it for the RA81's.
Any experience anyone has with this sort of thing that they wouldn't mind
sharing with me would be appreciated.

--Greg
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