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From: andrew@ee.brunel.ac.uk (Andrew Findlay)
Newsgroups: net.unix-wizards
Subject: Re: disk partitioning
Message-ID: <385@brueer.ee.brunel.ac.uk>
Date: Thu, 11-Sep-86 07:06:09 EDT
Article-I.D.: brueer.385
Posted: Thu Sep 11 07:06:09 1986
Date-Received: Fri, 12-Sep-86 01:58:17 EDT
References: <3496@brl-smoke.ARPA>
Reply-To: andrew@me.brunel.ac.uk (Andrew Findlay)
Organization: Dept of M&ES, Brunel University, Uxbridge, U.K.
Lines: 71

In article <3496@brl-smoke.ARPA> smb@mimsy.umd.EDU (Steve M. Burinsky) writes:
>I have two questions regarding disk partitioning.
...
>
>I want to use use the "c" partitions of my disks for form one large file
>system per disk.
>
>1.  Under 4.2, are there advantages/disadvantages to one large file system
>versus many smaller file systems?  What about quotas and file system
>efficiency?

If you have plenty of disks, it makes sense to have some very big file
systems. The 4.2 file system will lay them out efficiently, so that is
not a worry. There are two main advantages to this:

(A)	The less filesystems you have, the simpler everything is to keep
	track of - especially quotas and dumps.

(B)	There is a limit to the number of filesystems that 4.2 will let
	you mount at any one time. With large filesystems you get more
	storage before hitting this limit (15 in the standard distribution
	- see page 33 of 'Building Systems with Config'.

The possible disadvantage of using partition C is that the disk cannot
then be used for swapping. If you have enough disks to get interleaved
swapping as well as big filesystems, so much the better. You seem to be
building an immense system...

>
>2.  If I use the "c" partition, how do I account for/leave enough space for
>the bad sector information and replacement sectors?

You can give arguments to newfs(8) - just tell it to use
(size_of_part_C - size_of_bad_block_area).

>
>3.  If I resize the partitions, how do I account for/leave enough space for
>the bad sector information and replacement sectors?

Simply make sure that the total of partition sizes other than C is less
than the disk size by the right amount. (And remember you have to change
tables in the driver as well as in /etc/disktab). The problem here is that
all disks of a given type share a single partition table, so you cannot
have wildly different layouts on each of two RA81s..

>The background info says that you can access the bad sector information and 
>replacement sectors only through the "c" partition.  Well, if I do a disk-
>to-disk copy using the "c" partition, am I copying one disk's bad sectors and
>bad sector information to the other disk?!

Sort of.. The disk driver (/sys/vaxuba/uda.c in your case) will map any
bad sectors to the appropriate replacement sectors on both the source and
destination disks. Thus, the disks appear to be perfect until you reach
the bad sector table itself. This is on the last track, which is usually
defect-free anyway. Once you start copying this, the destination disk's
bad sector info gets overwritten with that from the source disk. Even now,
things will be OK - UNTIL YOU REBOOT THE SYSTEM. The duff bad-sector info
will then be picked up and all hell will break loose.

In general, I would avoid disk-to-disk copying. If you must, use dd(1) and
set a block count so that it does not touch the bad-sector info.

Andrew

-- 

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