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From: brian@casemo.UUCP (Brian Cuthie )
Newsgroups: comp.periphs,comp.unix.wizards
Subject: Re: Filesystem/archive formats for WORM optical disks
Message-ID: <212@casemo.UUCP>
Date: Tue, 11-Aug-87 10:35:20 EDT
Article-I.D.: casemo.212
Posted: Tue Aug 11 10:35:20 1987
Date-Received: Thu, 13-Aug-87 06:10:55 EDT
References: <1775@kontron.UUCP>
Organization: CASE Communications, Columbia, MD
Lines: 54
Summary: WORM disk structures
Xref: mnetor comp.periphs:510 comp.unix.wizards:3649

In article <1775@kontron.UUCP>, brad@kontron.UUCP (Brad Yearwood) writes:
> Write-once-read-mostly optical disks, while promising very large storage
> capacities economically and in a small space, do not fit well with
> the types of file allocation and directory structures used on rewritable
> magnetic disks.  Does anybody know of published papers or standards efforts
> covering filesystem/archive formats for optical disks?
> 
> Brad Yearwood
> Kontron Electronics  {voder, pyramid}!kontron!brad
> Mountain View, CA
> (415) 965-7020  (800) 227-8834


There are two major problems when using optical WORM (Write Once Read Many)
drives. First, the block size is usually 1024 bytes instead of the usual 512.
Second, when a block is written, it's ECC is also written.  Since the media
is Write Once, it is impossible to simulate a 512 byte/block file system
using the usual read block, update the half you want, write block.  Note, 
that this is impossible even if the half in question has never been written;
it's ECC has. 

The only reasonably transparent sollution I have seen is to have the controller
map the directory part of the filesystem to a magnetic drive and the data
portion to the Optical drive.  Thus part of the optical drive has been 
mapped (or overlayed) with magnetic media.  Since this is done by the controller
all the operating system must do is write data in even numbers of blocks and
limit directory blocks to the portion of the WORM drive that has been mapped
to magnetic media. When the data area (the optical drive) is full, the magnetic
disk is copied to the area on the optical it had previously overlayed.  This
freezes the filesystem onto the optical platter.  Note that the magnetic
drive only needs to be as large as the directory area of the WORM filesystem.

This scheme works well for operating systems like VMS where directory structures
may be forced to live within a specific region of the disk.  VMS also alows
the user to specify the cluster size (the number of blocks transfered) to
be even.  Once a file system has been created and frozen, it may be mounted
under VMS as READONLY without the use of a companion magnetic drive.  Thus
platters may be created and then sent to sites containing no
ability to write optical media. 

Several companies have implimented such schemes, some more succesful than
others.  US Design (Lanham Maryland, 301 577 - 2880) has a good implimentation.

One final note. WORM drives tend to be EXTREMELY slow.  They can have seek
times on the order of seconds !

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Brian Cuthie
CASE Communications Corp.
Columbia, Md 21046
(301) 290 - 7443

ARPA:	brian@umbc3.umd.edu
UUCP:	...!seismo!mimsy!aplcen!casemo!brian