From: utzoo!decvax!harpo!seismo!hao!menlo70!sytek!zehntel!zinfandel!hickman
Newsgroups: net.unix-wizards
Title: Re: disk block interleave methods - (nf)
Article-I.D.: zehntel.740
Posted: Thu Feb 10 13:20:04 1983
Received: Sat Feb 12 07:01:27 1983

#R:tekmdp:-175400:zinfandel:12400014:000:1191
zinfandel!hickman    Feb 10 08:54:00 1983

You seem to be confusing the several types of interleaving available:

1) Disk format interleaving:
	- When the disk pack is formatted, an interleave is specified which
	  allows the controller to get sequential multiple sector reads in
	  the same rotation (i.e., no rotational loss because the controller
	  couldn't finish dealing with the first sector before the second
	  sector passed under the disk heads)

2) Unix filesystem interleaving:
	- on top of the normal (and hopefully transparent) disk interleaving,
	  unix allows a filesystem interleave which is valid only at the
	  time of a "mkfs".  Once the filesystem has been used, blocks
	  tend to be moved around alot, and the interleave gets lost.  This
	  is done so that files which don't move, will be accessable to unix
	  without rotational delays.  Unix attempts read ahead, and thus the
	  filesystem rotational interleave should be set for whatever time
	  is needed to send the first request to the driver and compute the
	  read ahead request.  Unfortunately, the filesystem interleave has
	  little if no effect on most unix systems (4.1, 4.2 are exceptions).

Hope this brings things into focus...
					kipp hickman