Cylinder boundaries in 4.3BSD

[Michael I. Bushnell]

In article <8843 at eddie.MIT.EDU> nessus at athena.mit.edu (Doug Alan) writes:
>> What we do is make the disk look like an RA-81 or an RD-51.

>That will work, but you still have the problem that your partitions
>are not beginning at the beginning of cylinders.  This messes up the
>Berkeley Fast File System's perfomance fine-tuning.

That isn't too critical.  If a partition starts half-way through a
cylinder, the first cylinder group of the second partition and the
last cylinder group of the first partition will still be "close" on
the disk to themselves.  The /etc/disktab entries are correct for the
particular disk used, and so the cylinder groups are allocated at the
right places. 

>> If you make it look like an RD53, then you will still need the
>> special disktab entry, but you won't have the benifit of separate h
>> and g partitions.

>I understand, but I don't want separate g and h partitions anyway, so
>this is not a problem.

>> Yet another possibility, if you don't really care about getting
>> "standard" kernels to run on them, is to make it look like an rd51.
>> rd51 is the type the controller returns if it can't determine the
>> type.  That is what we do for almost all our large microvax disks.

>If I don't care about getting standard kernals to run on them, I'd
>just use my own customized partition table, and then I'd get the
>fine-tuning right.  The question is how much will using a standard
>partition table, which has not been customized for a nonstandard
>drive, degrade system performance?  One percent?  Then, who cares.
>Fifty percent?  Then, it's worth caring about.

It doesn't damage things that much.  The performace disadvantages are
NIL.  What you MUST do is have a custom /etc/disktab entry for the disk.



                N u m q u a m   G l o r i a   D e o 

			Michael I. Bushnell
			HASA - "A" division
14308 Skyline Rd NE				Computer Science Dept.
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mike at turing.unm.edu				Albuquerque, NM  87131
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