Path: utzoo!utgpu!water!watmath!clyde!att-cb!att-ih!pacbell!ames!ucsd!sdcsvax!darrell@cs.ucsd.edu From: darrell@cs.ucsd.edu (Darrell Long) Newsgroups: comp.os.research Subject: Re: O/S's using bit maps for free disk block lists? Message-ID: <4826@sdcsvax.UCSD.EDU> Date: 31 Mar 88 21:07:50 GMT Sender: nobody@sdcsvax.UCSD.EDU Organization: UC San Diego Lines: 38 Approved: comp-os-research@ucsd.edu Melinda Shore , says... Unicos (Cray's version of SysV) uses a bitmap of disk free blocks. Earlier versions used the traditional linked lists. We never ran the old filesystem, but apparently it really was much slower than the current one. Maybe someone at Cray could give us real figures. -- Melinda Shore ..!uunet!reason.psc.edu!shore Pittsburgh Supercomputing Center shore@reason.psc.edu emory!arnold@ucsd.edu (Arnold D. Robbins {EUCC}), says... The Eighth and Ninth Edition Research Unix systems use a bit map to store the free list; blocks in the file system are all 4K bytes in size. The V8/V9 'fsck' knows how to convert an 'old' style 1K-linked-list filesystem (i.e. the 4.1 BSD file system) into a new style 4K bitmapped filesystem, in place. Peter Weinberger wrote a description of this file system in a Bell Labs Technical Journal article circa 1981, I have the journal at home, so if someone desperately needs the reference I could look it up. As I recall, the main speed-up came from the larger block sizes, although the ability to use an in-memory bit map also helped. It's been a while since I read the article. Basically, Weinberger was able to move data through the filesystem at just about maximum practical disk transfer speeds. Note also that this is still the traditional "all the i-node blocks at the front" sort of file system. A wart is that V8 uses the eigth bit in the minor device field to distinguish between file system types; the new one has the high bit in the byte on. It seems to me they could have done it better with the File System Switch, but I suspect the FSS came much later than the faster file system. Perhaps V9 does it differently than V8 did; I've only seen (someone else's) V8 manual. -- Arnold Robbins ARPA, CSNET: arnold@emory.ARPA BITNET: arnold@emory UUCP: { decvax, gatech, }!emory!arnold DOMAIN: arnold@emory.edu (soon) ``csh: just say NO!''