Path: utzoo!mnetor!uunet!husc6!uwvax!oddjob!ncar!ames!ucsd!sdcsvax!voysys!bruce@ucsd.edu
From: voysys!bruce@ucsd.edu (Bruce Beare)
Newsgroups: comp.os.research
Subject: Re: O/S's using bit maps for free disk block lists?
Message-ID: <4836@sdcsvax.UCSD.EDU>
Date: 5 Apr 88 15:11:47 GMT
Sender: nobody@sdcsvax.UCSD.EDU
Organization: Voysys Corporation, Sunnyvale, CA
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Approved: comp-os-research@ucsd.edu

Convergent's Mightyframe (S/320, S/640, etc) uses a combination bitmap/free 
list file system. What we did was to keep the freelist intact, and slice off a 
section of the file system at a time for bitmapping.

Advantages: Minimal changes to kernel, fsck, etc, very good performance,
	ability to switch between the two formats (free list/combo bitmap)
	at will (fsck does it).

Disadvantages: When the bitmap is exhausted, the kernel will lock the free list
	and scan the entire free list (one pass) building a new bitmap. 
	This does not happen very often, but when it does the file system will 
	block alloc/free calls (for that file system) for several seconds.
	
Other improvements:
	When an alloc calls for more then 1 block, it gets contiguous blocks 
	from the bit map (changes to bmap, etc). This coupled with a 
	'direct I/O' facility allowed us to do large file system reads/write 
	as a single DMA to the physical device, giving us a 4X speed 
	improvement. 
	
	We peaked with the Fuji SMD drive, Interphase SMD Controller
	at the 1MB/S range for file system I/O on the AIM II benchmark.
	The typical ST506 performance was 350-400 K/S with Maxtor xt1140 drives.


	Bruce Beare
	Now @ Voysys Corporation
	..!pyramid!ctnews!voysys!bruce


Non-Standard disclaimer: 
	I never did speak for Convergent Technologies, and still don't.