Back to the top of the FAQ.

6. Disk

6.1. CAV, ZCAV and CLV

Many disks (hard, floppy and optical) run in CAV (Constant Angular Velocity) mode. In this case, the disk spins at a constant rate, and there are the same number of sectors per track on inner and outer tracks. This means that the bits are farther apart on the outer tracks, potentially wasting space. The transfer rate is constant, as the number of bits/track is same and the time/track doesn't vary.

CDs (and video laser disks, I believe) and early Macintosh floppies run at Constant Linear Velocity (CLV). That is, the bits are all roughly the same size, and the rotations per minute of the drive is adjust as the head moves in and out. This gives the best areal density of bits, at the sacrifice of seek speed, since every seek requires an adjustment of the rotation speed. The transfer rate is constant, as the size and spacing of bits is constant and the linear velocity is constant.

The current rage is ZCAV, Zoned Constant Angular Velocity. Most modern SCSI disks have this feature, and the newest MO drives do, as well. There are a number of zones defined on the disk. The number of sectors per track is different in each zone. Thus, the data is packed more densely than normal CAV, but seek speed is not sacrificed. Another effect of ZCAV is that the media transfer rate varies depending on the head position, because the time/track is constant and the bits/track vary; for example, the Seagate ST12450W Barracuda drive varies from 68 to 113 Mbits/sec, almost a factor of two different. has a couple of papers on this topic, and I (rdv) have a paper in consideration for a conference on the topic (6/96).

6.2. Optical {Brief}

See also Robotics section for library options. Slower than magnetic disks (in general)

6.2.1. CD-ROM

Historically produced off-site at significant first-copy cost but small cost for high volumes. Now on-site 'authoring' systems are available. Standard formats are available. Ads have been posted to the net offering services for as little as US$60 to convert a tape to CD. DVD (Digital Versatile Disk) (Next-Generation CD) {New}

Digital Versatile Disk (DVD), the new standard, is a two-layer single or double-sided CD, 8.5 or 17 GB, or double-sided single-layer CD, 9.4 GB. Transfer rate is 11 Mbits/sec (1.4 MB/s). contains a quick overview. First versions will be read only, later will come WORM, then rewritable.

First products are slated to be available by the end of 1996.

There were two new standards in the works, digital video disk (DVD) and high-density compact disk (HDCD). DVD was proposed by 8 major consumer electronics giants (including Toshiba, with Time-Warner on board) and would have featured a double sided disk capable of storing 5GB of data per side. HDCD, backed by Philips and Sony, would have held 3.7GB data, with the potential to double them up to hold 7.4GB by using a two-layer technology.

VHS/Beta wars all over again, along with issues such as backwards compatability to existing CDs, were avoided, thankfully. In December of 1995, everybody agreed on the new DVD format.

See the article by Alan Bell in the July 1996 Scientific American.

(John Wiest (, (Michael Gold) and others, 95/04/20, rdv, 96/7/1)

My (rdv) notes from the Goddard mass storage conference, 1996:

Mike Wingart, Sony, talked about DVD.

Their data rate is 11.08 Mbps, though video formats are generally used in a slower mode than that. Two sizes, 8 cm and 12 cm.

size	single layer,	double layer,
	single side	double side
8cm	1.4 GB		5.2 GB
12cm	4.7 GB		17 GB

track pitch is 0.74 mm, compared to 1.6 for CD (I wrote mm, but I'm sure that's microns).

Starts at the inner hub and moves out as it reads the first layer, when it switches to the second it reverses direction.

They are working for backward compatibility, but the CD-recordable format uses a dye polymer that's wavelength sensitive; CD-R is 750 nm, but they are using 650 nm laser.

Movie is only 4.8 Mbps (he didn't explain the discrepancy, but I presume they just don't use the extra bw). Video is 3.5 Mbps, the rest is audio (5.1 channels, 3 languages, 4 subtitles).

They run 130 to 472 minutes of video on 12 cm disks.

Using ISO 13346, the volume & file standard for write once and RW non-sequential media.

Using ISO 9660, the CD-ROM FS std, needs some modification to work?

DVD-ROM spec 1.0 released Sept. 6th, 1996; others coming soon.

Rewritable 2.6 GB single layer requires cartridge to protect disk. Cyclability of rewritable media is still an issue. GIANT CD-ROMS {New}

A bit of news that I came across recently:

GIANT CD-ROMs SLATED FOR 2000 Norsam Technologies Inc. in Albuquerque, N.M., is developing a CD-ROM technology that would enable users to store up to 165 gigabytes on a single disk -- almost 10 times as much data as can be stored on digital video disks. The additional capacity is made possible by replacing the prevailing 800- and 350 nanometer laser writing technology with a more powerful 50-nanometer particle beam. "The Norsam HD-ROM will ... be a major competitor in the high-availability data arena," says the company's president. The HD-ROM disks will be the same size as current CD-ROMs, but will require users to install high-density readers in their devices. (InternetWeek 14 Nov 97)

6.2.2. WORM {brief}

Write-Once-Read-Many Standards are less firm between vendors.

For info on file systems for WORM, see the reference to ISO/IEC 13346 in Also check out the OSTA (Optical Storage Technology Association) specs for UDF (Universal Disk Format). UDF is one of the file systems that will be used with DVD. (from and, 1996/3/20) Sony {brief}

In the July '95 Wired, p. 56, and seen at SIGGRAPH '95:

new Sony WORM drive, looks like 12", maybe bigger. 15GB on a $400 platter. drive is $21K. 76-disk autochanger is $112.5K. ZCAV. transfer rate 2.7 MB/sec. sustained.

call 1-800-222-7669.


6.2.3. Erasable

Better standards than WORM Magneto-Optical Physics

Magneto-optical disks are plastic or glass disks coated with a compound (often TbFeCo) that has special properties. The disk is read by shining a low-intensity laser (originally infrared, but experiments are being conducted all the way up to blue, I believe; the shorter the wavelength the higher the possible density, all things being equal (which they never are)) onto the media and examining the polarization of the reflected light. To write, a higher-intensity laser is used to heat the material up to its Curie point, where it becomes susceptible to a magnetic field. When the media cools again, its state is "frozen". The polarity of the reflected light during a read depends on the polarity of the magnetic field under which the media was last cooled. Once it has cooled it is no longer suceptible to magnetic fields. Thus, it can be compared in a sense to paleomagnetism. Sony MiniDisc {Brief,New}

Sony introduced a 2.5" removable cartridge disk drive in 1993 that will hold 140MB. The rewritable version is magneto-optical; there is also a read-only CD-like version. Also manufactured by others, including Sharp, who says it will have a 700 MB version in 1997. Great info available at or (rdv, 96/7/7) Magneto-optical, 5.25-inch

Same number of sectors on each track whether or not track is near center or outer edge. 640 MB. Made by IBM, HP, Sony, Ricoh, others? Magneto-optical ZCAV, 5.25-inch

        Zoned constant angular velocity - more sectors on outer tracks.
        GB: 1
        ECMA standard 183 going through ISO Fast Track.
        ADSTAR demonstrated (6/93), San Jose, CA, 408/256-7895. HP Corsair {Brief}

1.3 GB on a double-sided cartridge.

See also HP under MO autochangers in part 2. describes HP products. Maxoptix T4-1300

Does 1.3, 1.0 (read only) and 650 MB media. Max sustained read 2.0 MB/sec.

See also Maxoptix under MO autochangers in part 2 for contact info.

(rdv, 95/02/14) Pinnacle Micro {New}

Pinnacle Micro ( makes MP drives with capacities of 650 MB, 1.3 GB, and 2.6 GB, with transfer rates up to 6 MB/sec(!). Reportedly they're developing a 4.6 GB drive (Apex), too.

Pinnacle also makes 10x CD-ROM drives and CD-recordable drives.

(rdv, 96/7/8) Asaca HSMO {Brief}

My company (Asaca) makes a 12.24 MB/sec. MO drive that uses custom media and two four-beam heads in parallel to increase the transfer speed. Expensive.

Call our L.A. office, (310)827-7144 Other Multi-beam MO {None} 3.5-inch MO {Brief}

Generally looks like a slightly overweight floppy. All current ones are single-sided.

First generation 3.5 MO was 128 MB on a cartridge.

Second generation devices (available now) are 230 MB.

Third generation (due out this year?) will be 650 MB.

If they bring the drive price down, could displace floppies as the basic shirtpocket-transportable medium.

(rdv,95/1/20) Sony {Brief}

Sony now has 650 MB/side 3.5" MO with direct-overwrite capability (a major step forward in MO). See It's ZCAV, and nominally 1-2 MB/s. There's a reasonable discussion of MO physics and technology here, too. ( (Mark Holzbach), 10/95) Nikon 12-inch MO {Brief}

Holds 8 GB on a disk, with a transfer rate of ?>1MB/s. Sony 12-inch MO {Brief}

Sony also makes a 12" MO. 3.2 GB? (rdv,95/2/7) NEC 12-inch MO {Brief}

NEC also makes a 12" MO. (rdv,95/2/7)

6.2.4. Electron-Trapping {None}

6.2.5. Dual Function {Brief}

Capable of using both WORM and Erasable media. Some do the WORM in firmware -- the media is really rewritable. Others do true WORM. Some drives listed elsewhere, such as MiniDisc, support this. Panasonic/Toray {Brief}

The Panasonic/Toray phase-change drive **READS** CD's but **WRITES** phase-change discs which are not compatible with CD's and cost as much as M-O media. (Mike Schuster,, 8/95) IBM {None}

6.3. Magnetic

This area moves WAY too fast for me to keep up with all of the products and announcements; ideally, it should teach enough about principles and other information sources to allow users to find the information they need.

Quantum has a good educational web site, with info on the history and technology of disk drives, at

A lot of spec sheets for recent disk drives (when they say they also have "old drives", they mean early 90s, not 50s-80s) is available at Looks like a good site.

6.3.1. 5.25-inch Seagate

Seagate's Elite 9 is 9GB -- reports here of backordering, others of availability. Micropolis due out with an 8GB soon? (94/9/1)

The fastest (in sustained transfer rate) known 5.25" disk drive is the ST12450W2HP 1.78GB Barracuda drive from Seagate. The Barracuda family is large, so pay attention to the model number! It runs at 68-113 Mbits/sec., depending on head position (it's ZCAV). Assuming that data rate is pre-format, and subtracting 20% for the format overhead, that would be a sustained rate of 6.8-11 MB/sec. Of course, your mileage WILL vary according to transfer size, locality, etc. (rdv,95/2/7)

Seagate announced a couple of weeks ago a 23GB disk drive. Reportedly shipping in summer. (Brian A Berg , 1996/3/29)

6.3.2. 3.5-inch IBM

The IBM DCMS-310800 Ultrastar2 is 10.8 GB (1GB=10^9) after format, and its sustained rate is fast -- 8.4-14.2 MB/sec (presumably pre-format, so subtract 20%). Only 5400 rpm with an 8.9 msec seek time, so middle-of-the-pack on those numbers. Fast/wide SCSI-2 interface. (rdv,95/02/14)

6.3.3. Hard Disk Manufacturers {Brief}

Here's a partial list of web pages for manufacturers of disk drives. At many of these you can get reasonable tech info and support contacts. (Fujitsu) (now merged w/ Seagate)

NOTE: HP announced on about July 10th, 1996 that they're getting out of the hard disk business. The article I saw didn't give an exact time frame. (rdv, 96/7/13) has good info on older drives, and new info is available at

6.3.4. Bernouli {None}

6.3.5. Floptical {Brief}

I believe flopticals use an optical tracking mechanism to improve ordinary magnetic head positioning and therefore density.

The Compaq/3M/Matsushita floptical floppies actually hold 120MB formatted (according to the August 95 Byte), and can read and write standard 1.44MB floppies and read 720KB floppies. (John Brock,, 8/95)

Supposed to be available Dec. '95?

6.3.6. PC Removables {Brief}

SHMO -- I haven't followed this too closely. There's the Zip, Jaz, and the new SyQuest. SyQuest EZ135 {Brief}

As of August 1995, $199.00 for the drive, $20.00 for each cartridge (135 MB). Removable magnetic hard disk. SyQuest has been in the business for years; the 5.25" removables were popular with Macs.

SyQuest has a new 3.5 inch formfactor removable hard drive (due out june). 11msec seek time. Their rep tells me it will list for about $500 and cartridges will come in two flavors: a) 1.3 gig @ $94 (list) b) 650 mb @ $64 (list) (, 1996/3/29) Iomega Zip {Brief}

100 MB/disk, ~$200 for the drive, ~$15-20 for disks. SCSI or parallel interface, 1.5 MB/sec. transfer rate? Don't know anything about the technology, I assume it's simple magnetic hard drive. (rdv, 12/95)

I heard that Iomega has licensed the technology to Epson, Fujitsu and one other maker (rdv, 1996/3/29).

See Iomega Jaz {Brief}

1 GB, up to 6 MB/sec. xfer rate for a removable hard disk. Drives are $600-$700 U.S. and cartridges $150 for 1GB. See (rdv, 1996/4/10) SyQuest Removable Cartridge hard Drives

form factor: 2.5", 42MB form factor: 3.5", 105MB, 14msec ave seek, 3600 RPM, ave sustained transfer rate: 1.3MB/Sec, available in IDE and SCSI versions. Syquest Technology, Inc., 47071 Bayside Parkway, Frement CA 93438, Phone: 800/245- 2278. Kalok removable cartridge hard drives

3.5-inch form factor, 250 MB Phone: 408/747-1315 or 408-468-1800

6.3.7. Mainframe {Brief}

Mainframe disks are sometimes referred to as SLEDs (Single Large Expensive Disks). The term DASD (Direct Access Storage Device) usually refers to a mainframe disk, but is occassionally applied to any hard disk.

A description of CKD (Count, Key, Data) disks (as well as mainframe file systems) by Dick Wilmot is available.

6.4. Other

My Home Page at Caltech

email me at

Copyright 1996 Rod Van Meter