Optical Disk Info

[James Smart]

Regarding the latest summary of Erasable Opticals (and a few questions
that have been sent to me directly), I'd like to clarify a few things.
What follows is probably more than anyone really wanted to know, but I
found much of the information extremely useful.

NOTE: Keep in mind that all comparisons were done in 5/90, and may now be
out of sync with the vendors latest product offerings.

  ** Regarding Interchangability **

As long as the cartridges were formatted in an ISO compliant mode, and the
drives supported ISO formats (which most do now days), I no problems
interchanging cartridges between drives. The drives I tested were the
Ricoh, Tahiti, and Sony.

At the time that we performed our tests (5/90), virgin Sony cartridges
came preformatted in a "Sony Format Mode 1" which is non-ISO compliant.
The cartridges had to be reformatted for "Sony Format Mode 3" which is ISO
compliant. Many of the older Ricoh drives also used non-ISO compliant
cartridge formats. Perhaps this these 2 issues may explain much of the
incompatibilities that other users have indicated.

  ** Regarding Capacity **

NOTE: ALL CAPACITIES STATED ARE "PER SIDE".

NOTE: For the discussion below, I am only talking about 512 byte sectors.
(If a sector size of 1024-bytes are used, the capacity of the cartridge
increases somewhat. If you see ISO formats stating 650 MByte capacity, it
is referring to 1024-byte sector format, with ~325 Mbyte per side).

NOTE: The Ricoh erasable optical seemed to behave like the Sony drive.

                   ===================================

The ISO standard for Erasable Optical Disk standard provides for a maximum
of 18750 tracks of 31 sectors, a total of 581250 512-byte sectors (283.81
Mbytes) per side. The total number of available sectors (581250) includes
both data and alternates.

On the Sony Drive:

Of the 581250 sectors, 2048 are reserved by the drive as an area to slip
bad sectors into during formatting. This area is unavailable to be used by
the external system. Additionally, the format parameters that we (the
users) selected, reserved an additional 1024 alternate sectors for sector
slipping during normal use. This value is changeable. This leaves the
following:

         Maximum Capacity           :  581250 sectors
         Capacity reserved by Drive :    2048 sectors
         ---------------------------------------------
         Usable Capacity            :  579202 sectors   (282.81 Mbytes)
         Alternates                 :    1024 sectors
         ---------------------------------------------
         Available Data sectors     :  578178 sectors   (282.31 Mbytes)

      Maximim Data Capacity : 579202 sectors (282.81 Mbytes)

(ie by setting alternates = 0 -- A PERFECT DISK with no room for future
defects) Allow for a 10% overhead on the UNIX filesystem, with an
additional 10% (can be manipulated) for reserved free space on the
filesystem.

On the Tahiti Drive:

Of the 581250 sectors, 0 of the sectors are reserved by the drive as an
area to slip bad sectors into during formatting. Any defects found while
formatting are simply skipped and recorded in the ISO defect header.  The
format parameters that we (the users) selected, reserved 2048 alternate
sectors for slipping during normal use.  This value is changeable. This
leaves the following:

         Maximum Capacity           :  581250 sectors
         Capacity reserved by Drive :       0 sectors
         ---------------------------------------------
         Usable Capacity            :  581250 sectors   (283.81 Mbytes)
         Alternates                 :    2048 sectors
         ---------------------------------------------
         Available Data sectors     :  579202 sectors   (282.81 Mbytes)
      Maximim Data Capacity : 581250 sectors (283.81 Mbytes)

(ie by setting alternates = 0 -- A PERFECT DISK with no room for future
defects) Allow for a 10% overhead on the UNIX filesystem, with an
additional 10% (can be manipulated) for reserved free space on the
filesystem.

                   ===================================

The ZCAV format takes advantage of the fact that the tracks are stored on
a disk, with the inner tracks being shorter than the outer tracks.
Therefore, as tracks proceed to the outer edge of the disk, they must be
able to hold more and more data. Additionally, since the disk is spinning
at a constant speed, the outer edge of the disk must be revolving at a
higher speed than the inner edge of the disk.  Therefore, as data is
retrieved from the outer edge of the disk, transfer rates must increase
because more data is received quicker.

The Maxtor Tahiti drive IS THE ONLY DRIVE TO BE ABLE TO READ THE ZCAV
FORMAT AT THIS (5/90) TIME. To support the ZCAV format, special cartridges
must be purchased. According to MaxOptix, nearly all makers of the
erasable optical cartridges also make the ZCAV formatted disks.

When formating a ZCAV cartridge, the the Tahiti does a prescan on the
cartridge and determines the maximum number of bad sectors per band (a
band is group of tracks). It then formats the cartridge reserving this
number (plus some fudge) as alternate areas. The importance of this is
that the capacity of the cartridge will vary from side to side and
cartridge to cartridge depending on home many defects are found initially.
This contrasts with the ISO format where the format parameters determine
the amount of alternates, thereby allowing the capacity to be constant.

   The ZCAV format yields the following capacity per side:
         Maximum Capacity           :  907725 sectors
         Capacity reserved by Drive :       0 sectors
         ---------------------------------------------
         Usable Capacity            :  907725 sectors  (443.23 Mbytes)
         Alternates                 :    2048 sectors  (Estimated)
         ---------------------------------------------
         Available Data sectors     :  905677 sectors  (442.23 Mbytes - Est.)
      Maximim Data Capacity : 907725 sectors (443.23 Mbytes)

(ie A PERFECT DISK with no defects) Allow for a 10% overhead on the UNIX
filesystem, with an additional 10% (can be manipulated) for reserved free
space on the filesystem.

  ** Regarding Performance **

In 5/90 a colleague of mine performed some benchmarks of the Sony vs the
Tahiti. The specifications for the drives are as follows:

      Sony:
         Average Seek time      = 95msec
         Rotation speed         = 2400 rpm
         Drive transfer rate    = 7.4 Mbits/sec
         SCSI transfer rate     = 1.2 Mbytes/sec
      Tahiti I:
         Average Seek time      = 35msec
         Rotation speed         = 1800 rpm (C/C (ISO) format)
         Drive transfer rate    = 5.55 Mbits/sec (C/C (ISO) format)
         SCSI transfer rate     = 1.5 Mbytes/sec

For write tests (tar or dd to the optical) the Sony showed a 6-8%
performance advantage over the Tahiti. For read tests (tar from the
optical) the Sony was 1-2% faster than the Tahiti.

The faster rotational speed of the Sony, is the direct influence on its
performance advantage over the Tahiti.  However, since the time that these
tests were performed (5/90), the rotational speed of the Tahiti should
have been increased to 2400 rpm as well.  I would suspect that this would
result in a sizeable performance advantage for the Tahiti.

In seek tests, the Tahiti was about 15 percent faster than the Sony. With
the Tahiti's rotational speed is increased, this figure will increase
dramatically.

Note that the with the opticals, the speed in which data is
saved-to/retrieved-from the optical media starts to outweigh the seek time
issues. 

James Smart                    jws at sv001.sandiego.ncr.com
NCR/Teradata - JDO             uunet!ucsd!sv001!jws
San Diego, CA. 92121           uunet!ncrlnk!ncr-sd!iss-rb!sv001!jws
Disclaimer: My opinions are my own and not representative of my employer.