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From: buck@siswat.UUCP (A. Lester Buck)
Newsgroups: comp.periphs
Subject: Re: WORMs acting like R/W disks...
Message-ID: <423@siswat.UUCP>
Date: 8 Aug 89 15:26:41 GMT
References: <680@rna.UUCP> <191@arnor.UUCP> <683@rna.UUCP>
Organization: Photon Graphics,  Houston
Lines: 50

> In article <191@arnor.UUCP> uri@arnor.UUCP (Uri Blumenthal) writes:
> >From article <680@rna.UUCP>, by dan@rna.UUCP (Dan Ts'o):
> >> ......makes the WORM emulate a read/write
> > Well, maybe I'm not that smart, but how the hell can you emulate read/write
> > device on Write-Once-Read-Many? As I understand, it will be either TERRIBLY
> > non-effective (like zillions of bytes be eaten as garbage), or it will not
> > work at all (:-)! 
> > So to use it as a source archive (with modifications once in every 100 years),
> > you probably can (if you're leaving at least 35% of WORM space free for those
> > games). But that's all!
> 
> 	Well, of course it wouldn't be very good for putting your
> /usr/spool/news filesystem on. But /usr/src, probably pretty good. The point
> is that many of us have storage applications requiring random, moderately fast
> access (scratch Exabyte), large capacity and not too many changes (images,
> for example, or school records), but have not been able to use WORMS
> effectively because existing OS's and device drivers and filesystems require
> read-write semantics. Special software drivers must be written to allow
> transparent access to WORMS from UNIX, MSDOS, VMS, etc. However this OCU
> box eliminates the need for special software by putting all those functions
> and workarounds in "hardware" (host and OS independent hardware).
> 	But the write-once limitation isn't going to go away, not entirely.
> Sometimes you want 800Mb of archival info on a $100 cartridge.

If you really want to know how to make a WORM look like a magnetic disk,
get a copy of the Summer '88 Usenix paper, "A UNIX File System for a
Write-Once Optical Disk", by Terry Laskodi et al. at Tektronix.
This is a filesystem built inside a Unix device driver which presents
a read/write disk to the operating system (though, of course, with lots
of writing the free space declines).  It is based on the report
"An Efficient I/O Interface for Optical Disks" by Jeffrey Scott Vitter,
Tech Report No. CS-84-15, June 1984, CS Dept, Brown University, where
Vitter presents some data structures for using WORMs and proves that
these algorithms are optimal (under some assumptions).

I am not familiar with this OCU box, but there are many vendors offering
optical storage solutions.  They aren't really popular yet because every
vendor considers their filesystem proprietary, so you depend on your
vendor to support a given list of machines, ad infinitum.  I am a member
of the ANSI committe X3B11.1 which formed last week to do a volume and
file structure standard for WORM media.  If things go well, there should
be a non-proprietary optical filesystem that all the major OS suppliers
will support within 1.5-2 years.  Then the market might really explode,
because you can switch software vendors at the drop of a hat, and know your
investment in disks will be good for decades.  Of course, the many hardware
standards for WORM are in a very sorry state, so you will still be tied
to a physical media format for the forseeable future.

-- 
A. Lester Buck		...!texbell!moray!siswat!buck