Path: utzoo!mnetor!uunet!lll-winken!lll-tis!ames!ucsd!sdcsvax!die@frog.UUCP
From: die@frog.UUCP ( David I. Emery)
Newsgroups: comp.os.research
Subject: Re: O/S's using bit maps for free disk block lists?
Message-ID: <4828@sdcsvax.UCSD.EDU>
Date: 31 Mar 88 06:05:03 GMT
Sender: nobody@sdcsvax.UCSD.EDU
Organization: Charles River Data Systems, Framingham MA
Lines: 26
Approved: comp-os-research@ucsd.edu


	Charles River Data Systems UNOS operating system (an independently
implemented system V UN*X kernel (passes SVVS) that contains no AT&T code) has
used bit maps for inode and free disk block allocation since the
first version of its file system was designed in 1979.

	We feel that bit map allocation significantly enhances file system
reliability and robustness.  It also permits improved performance - we use it
to allocate contiguous regions on the disk to those blocks which are written
behind when a file is being written to sequentially.  We postpone actually
binding the blocks in the cache to physical sectors until enough have
accumulated and then go find a contiguous region big enough to hold them
by searching the bit map.  Having the sectors contiguous on the disk 
permits efficient read-ahead in one disk operation and minimizes head
motion when a file is accessed.

[ You postpone writing to disk to collect enough for a contiguous region on ]
[ disk.  What metric do you use to decide when you have collected enough?   ]
[ How do you trade this off against the vulnerability to crashes?  --DL     ]

-- 
David I. Emery
Charles River Data Systems
983 Concord St.
Framingham, MA 01701
Tel: (617) 626-1102
uucp: ...!decvax!frog!die