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From: mscritsm@isis.cs.du.edu (Milton Scritsmier)
Newsgroups: comp.sys.amiga.hardware
Subject: Re: Seagate Drives
Message-ID: <1991May24.052257.14539@mnemosyne.cs.du.edu>
Date: 24 May 91 05:22:57 GMT
References: <9105152355.AA05412@thunder.LakeheadU.Ca> <42421@cup.portal.com> <21748@cbmvax.commodore.com> <42601@cup.portal.com>
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>
>Second, after I posted my qustions I received the latest Seagate catalog.
>They have some 5400 RPM SCSI-2 drives they rate at 28MB/s internal
>transfer and 10MB/s external transfer.  Do you think they are saying
>the drive has the capability to transfer data+clock bits at 28MB/s
>and the SCSI protocol is holding it back to 10?  (I ask Seagate and
>their sales people won't speculate.)
>
Actually, that 28 MB/s figure is 28 megabits/sec.  Megabits/sec is the
standard measure these days for data transfer rates to the media.  It
does not take into account overhead such as the media format, but is
a raw transfer rate number.  Thus that 28 mbits/sec rate translates
to 3.5 megabytes/sec.  Thus even the current SCSI-1 synchronous transfer
rate of 5 megabytes/sec is sufficient to keep up with the drive.

>I know that current disks (40-100 MByte quantums for example) are the
>bottle neck in data transfer with the Amiga line of machines.  I am
>curious if disks are starting to come out that can transfer data
>faster than the SCSI interface.  I tried to calculate disk rotation
>speed versus bits per track but that information isn't included for
>SCSI disks.  If sustained transfer rates are increasing from 500KB/s
>to 2MB/s then there must be more bits per track since the fastest
>spinning drives are increasing RPM by only 50%.  
>
>What's the real story in sustained transfer rates?

There are more bits/track than before, especially since many drives now
divide the media into zones where the number of bits per inch along any
track is roughly constant.  This has the effect of putting more bits per
track on the outer part of the disk.  (Without zones, the number of bits
per track was constant across all tracks.  This meant that the size of
each bit cell was smaller nearer the inner part of the media than the
outer part of the media since the circumference of a track is smaller
near the inner part of the media than the outer part.  As a result, the
amount of data you could store in the drive was limited by how dense
you could pack the bits near the inner part of the media, and some capacity
was always wasted in the outer part of the media.)  Also, you are correct
in implying that the state of the art has improved in making each bit
smaller on the media.  But improving drive performance is like that old
saw that a chain is no stronger than its weakest link.  To go along with
improvements in media recording densities, drive manufacturers also had
to improve the DMA transfer rate of the drive interface by using buffers
and faster electronics.  Computer manufacturers had to improve their buses
to handle the faster rates.  There is a big difference between an XT bus
and an EISA or Zorro bus.  Currently, the raw performance of a single drive
subsystem is still limited by the media transfer rate because of the
mechanical nature of that process versus the electrical nature of everything
else involved.  JVC, however, supposedly has a 3 1/2 inch drive which
can transfer data from the media at very high transfer rates, on the order
of 15 megabytes/sec.  If this is so, then even SCSI-2 would be a bottleneck.