Epoch like filesystem

[Rodney Peck II]

In article <12778 at vpk4.UUCP> craig at vpk4.ATT.COM (Craig Campbell) writes:
>In article <11709 at celit.fps.com> hutch at fps.com (Jim Hutchison) writes:
>
>>With the current speeds for optical drives, I'd kind of guess this system
>>is not useful as a primary storage device.  Presuming WORM and not M-O,
>>it couldn't be used for frequent migration, due to the rapid rate at which
>>the platters would fill up with minor revisions.
>>--
>
>There are now available Read Write optical drives.  These are not WORM
>drives.  Somehow, the plater information is re-programmable.  Optical disks
>now are useful as a primary storage medium.  I do not know $$ or details.

I do... They are about $200 per 5.25" 600 meg platter.  A ten disk juke
box costs about $10,000 and is a scsi device.  The problem is lack of
software -- this 6 gigabyte thing acts like some sort of large dumb scsi
object without the proper software.

And now some engineering...
The way the read/write optical disk works is by taking advantage of two
nifty things.  First, some materials change color depending on the magnetic
field around them.  Second, the hysteresis curve of magnetic material
narrows when the material is heated.

The problem with high density conventional disks is that you need to get
a really tiny strong electomagnet really close to the disk to make a bit
small enough that it doesn't interfere with its neighbors, and at the
same time allows a lot of them on the disk.  This leads to lots and lots
of expensive engineering problems.

R/W optical gets around this in the following manner.  First, you take some
magentic material and coat it with some stuff that changes it's reflectivity
when in a magnetic field.  That's the "optical" disk.

Now, build a hard disk the usual way, but use a nice big head that is easier
to produce and is more stable than a high-tech floating head.  Install a
pointable laser that is powerful enough to heat a single spot on the disk
very very quickly.

Now, when you want to write a bit, you turn on the head with the proper
polarity to a level that is just below the amount needed to force the
cold media to the other side of the hysteresis curve.  Then, you zap the
tiny spot you want to flip with the laser.  It's threshold for flipping
drops because the curve narrows from the heat.  The data has been written.

To read the data, you shine a less powerful laser on the disk and measure
the reflected power.  1's will be one color, 0's the other.

and that's how these things work.  nifty, eh?

I hope you found today's lecture informative.  Remember, there is an
F test this Friday at 8 am, and none of the TA's speak English and I'll
be out of town for two weeks so there are no office hours.  Thank you.

((you realize that RPI turns out very large numbers of computer scientists
who understand electrical engineering))
(((now's the part where everyone tells me how wrong I am.  brace yourself)))
-- 
Rodney