new faster disk design idea

If you ever opened a hard drive, you will see a couple of platters and an
arm that has a head for each surface. So if 2 platters, then 4 surfaces and
4 heads. The arm moves the heads across the platters and the platters
rotate, so any spot on the disk can be seeked with both movement together.

So... why not have multiple heads on each surface? Imagine a row of heads
evenly spaced out on a bar at a right angle to the arm, the arm attaching at
the middle of the bar, forming a T shape. If there were 10 heads on each
bar, then you get 10x speed and 1/10 lateral seek time.

Good or bad idea? Feasible?

Clayton

In article k.net,
- C - wrote:
If you ever opened a hard drive, you will see a couple of platters and an
arm that has a head for each surface. So if 2 platters, then 4 surfaces and
4 heads. The arm moves the heads across the platters and the platters
rotate, so any spot on the disk can be seeked with both movement together.

So... why not have multiple heads on each surface? Imagine a row of heads
evenly spaced out on a bar at a right angle to the arm, the arm attaching at
the middle of the bar, forming a T shape. If there were 10 heads on each
bar, then you get 10x speed and 1/10 lateral seek time.

Good or bad idea? Feasible?


Sort of.

You've obviously never seen a Univac Fastrand drum (64 heads visible
under plexiglass). 1968. $170k for 90MB with performance specs
almost as fast a modern CD reader.

http://www.fourmilab.ch/documents/univac/fastrand.html

IBM 2305 disk had a head per track ( a rousing 11MB, with zero seek
time.) (late 70's into the 80's. cost as much as a house.) I think
it needed water cooling.

Multiple heads per surface was a maintenance (or reliability)
nightmare and was only used at the bleading edge of what's possible,
which is definatly not mass market retail.

If you think about it, the huge storage arrays that corporations are
buying are the current incarnation of your idea; hundreds of heads
over spinning data, but each head has it's own spindle. WIth proper
I/O subsystem design this gives you the access times and transfer
rates you are looking for, See

http://www.emc.com/products/systems/DMX_series.jsp?
openfolder=storage_systems

Disk drives have gotten lots better, trust me.

--
Al Dykes
-----------
adykes at p a n i x . c o m

On Tue, 15 Jun 2004 01:05:46 GMT in comp.arch.storage, "- C -"
wrote:

If you ever opened a hard drive, you will see a couple of platters and an
arm that has a head for each surface. So if 2 platters, then 4 surfaces and
4 heads. The arm moves the heads across the platters and the platters
rotate, so any spot on the disk can be seeked with both movement together.

So... why not have multiple heads on each surface? Imagine a row of heads
evenly spaced out on a bar at a right angle to the arm, the arm attaching at
the middle of the bar, forming a T shape. If there were 10 heads on each
bar, then you get 10x speed and 1/10 lateral seek time.

Good or bad idea? Feasible?

IIRC it was done on the IBM 3380E (double density -- two heads) and
IBM 3380K (triple density -- three heads), same rotational speed and
same track-track and max seek times AFAIK, with the heads dangling
below the arms, for less than double or triple the not inconsiderable
price ($10/MB).

So what typically happened was drives were consolidated 2(/3) to 1,
saving money, space, power etc., but that increased the number of
files and I/Os per drive by 2(/3), and also reduced the number of
simultaneous I/Os, seeks, and transfers by 2(/3), resulting in worse
overall performance.

If the drives had been addressed as if they had more tracks/cylinder
instead of more cylinders, and used for only sequentially accessed
files, users might have seen benefits.

More than three heads per were probably tried but may have had
problems with arm mass, flutter, head-track registration, flying
height, or various other physical considerations you can probably
think of.

--
Thanks. Take care, Brian Inglis Calgary, Alberta, Canada

(Brian dot Inglis at SystematicSw dot ab dot ca)
fake address use address above to reply

In article k.net,
- C - wrote:
So... why not have multiple heads on each surface?

Whoa, what's next, 8-track? Disco? The Nixon presidency over again?

This was common in the '70s, but the heavier the head is the slower it can
move and the more complicated *stuff* you have on the head the heavier it
is and the less reliable it is. It's an idea whose time has gone...

--
I've seen things you people can't imagine. Chimneysweeps on fire over the roofs
of London. I've watched kite-strings glitter in the sun at Hyde Park Gate. All
these things will be lost in time, like chalk-paintings in the rain. `-_-'
Time for your nap. | Peter da Silva | Har du kramat din varg, idag? 'U`

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Clinging to sanity, - C - mumbled in his beard:
[more heads on platters]

Yo!

Didn't HP have a prototype that went in that direction? Only they did not
have just more heads, but duplicated the whole electronics/heads/arms part,
essentially 2 independent drives accessing the same spindle. The aim was to
increase number of transactions/time, not bandwidth.

The name kittyhawk bounces around in my head, but I can't find anything in
google right now.

cheers
- -- vbi


- --
Baker's First Law of Federal Geometry:
A block grant is a solid mass of money surrounded on all sides by
governors.

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Such a SCSI drive was commercially shipped about 5 years ago....the Seagate
ST12450W.


"- C -" wrote in message
hlink.net...
If you ever opened a hard drive, you will see a couple of platters and an
arm that has a head for each surface. So if 2 platters, then 4 surfaces
and
4 heads. The arm moves the heads across the platters and the platters
rotate, so any spot on the disk can be seeked with both movement together.

So... why not have multiple heads on each surface? Imagine a row of heads
evenly spaced out on a bar at a right angle to the arm, the arm attaching
at
the middle of the bar, forming a T shape. If there were 10 heads on each
bar, then you get 10x speed and 1/10 lateral seek time.

Good or bad idea? Feasible?

Not feasible generally. Only a single head per physically actuator can be
active at any given time as the track servo following can only follow one
master at a time. The ST12450W had two complete head/actuator systems.

"- C -" wrote in message
hlink.net...
If you ever opened a hard drive, you will see a couple of platters and an
arm that has a head for each surface. So if 2 platters, then 4 surfaces
and
4 heads. The arm moves the heads across the platters and the platters
rotate, so any spot on the disk can be seeked with both movement together.

So... why not have multiple heads on each surface?

Because, as others have noted, it increases the mass and inertia of the
assembly. When you get twice the capacity as a result (as you do with a
head on each side of a platter, using a single arm), it's worth it;
otherwise, it isn't.

Imagine a row of heads
evenly spaced out on a bar at a right angle to the arm, the arm attaching
at
the middle of the bar, forming a T shape. If there were 10 heads on each
bar, then you get 10x speed and 1/10 lateral seek time.

No, you don't - not anything like it. In fact, performance may well get
worse.

The only way to significantly decrease the movement required for an average
seek is to make that bar nearly as wide as the disk surface (otherwise, you
still have to move it almost as far to get to a track 1/3 of the surface
away). That makes the bar *much* more massive, so it takes longer both to
move it a given distance and to stabilize it once it reaches its
destination.

The ultimate such 'bar' has already been described: it has a head for each
track on the surface, and doesn't move at all - and in fact was therefore a
performance win (though at significant cost). That was feasible 'way back
when, but at current track densities there's nowhere nearly enough room for
a head per track on such a bar.

Using multiple arms to access a single platter has also been mentioned.
However, you can get nearly the same effect simply by using two conventional
disks (plus get twice the capacity in the process) - and the production
efficiencies of using standard units make this a cheaper solution than a
single, low-volume dual-armed disk.

- bill

Ron Reaugh wrote:
Such a SCSI drive was commercially shipped about 5 years ago....the Seagate
ST12450W.


But the heads operating in parallel were very close to each other.

If that were different, thermal expansion would mean one of the heads
would mistrack.


Thomas

"Bill Todd" wrote in message ...
"- C -" wrote in message
hlink.net...
If you ever opened a hard drive, you will see a couple of platters and an
arm that has a head for each surface. So if 2 platters, then 4 surfaces
and
4 heads. The arm moves the heads across the platters and the platters
rotate, so any spot on the disk can be seeked with both movement together.

So... why not have multiple heads on each surface?

Because, as others have noted, it increases the mass and inertia of the
assembly. When you get twice the capacity as a result (as you do with a
head on each side of a platter, using a single arm), it's worth it;
otherwise, it isn't.

Imagine a row of heads
evenly spaced out on a bar at a right angle to the arm, the arm attaching
at
the middle of the bar, forming a T shape. If there were 10 heads on each
bar, then you get 10x speed and 1/10 lateral seek time.

No, you don't - not anything like it. In fact, performance may well get
worse.

The only way to significantly decrease the movement required for an average
seek is to make that bar nearly as wide as the disk surface (otherwise, you
still have to move it almost as far to get to a track 1/3 of the surface
away). That makes the bar *much* more massive, so it takes longer both to
move it a given distance and to stabilize it once it reaches its
destination.


Not to mention that you're only optimizing away the fastest part of
the seek. Head acceleration, deceleration and settling time and
whatnot are all still there for the nominally shorter seeks, and, as
you mentioned, will probably get slower due to increased arm mass.
And given that most seeks are quite short anyway, you'd need a lot of
heads to make this even marginally effective.

GREAT IDEA! WHY DON'T YOU GO INTO THE RAZOR BUSINESS AND MAKE A RAZOR
WITH 18 BLADES.


"- C -" wrote in message
hlink.net...
If you ever opened a hard drive, you will see a couple of platters and an
arm that has a head for each surface. So if 2 platters, then 4 surfaces
and
4 heads. The arm moves the heads across the platters and the platters
rotate, so any spot on the disk can be seeked with both movement together.

So... why not have multiple heads on each surface? Imagine a row of heads
evenly spaced out on a bar at a right angle to the arm, the arm attaching
at
the middle of the bar, forming a T shape. If there were 10 heads on each
bar, then you get 10x speed and 1/10 lateral seek time.

Good or bad idea? Feasible?

Clayton