ESDI vs SCSI

[Rick Francis]

In article <98520 at sun.Eng.Sun.COM> williamt at sun.UUCP (William A. Turnbow) writes:
>
>	Don't know if anyone had followed this up, but my experience with
>ESDI/SCSI drives leads me to believe that ESDI drives are faster.
>
>	First off you have limiting factors in bus transfer speed.  On an
>AT bus I believe this is something like around 4-5 Meg/sec.  Neither of
>these drives comes close.  The limiting factor from what I have seen is
>interleave, # sectors/track and (rarely) spin rate.  The spin rate is
>almost always 3600 RPM or 60 RPS.  Using this one can quickly see the
>maximum sustained xfer rate for a drive with 17 or 34 sectors per track
>(512 bytes/sec * # secs/trac * 60 RPS)  = 522K/sec max for standard MFM
>and 1044K/sec max for standard ESDI.  I haven't seen any standards on how
>many secs/track the SCSI drives have (in the small form factors), but
>the transfer rates were about 3/4 to '=' the ESDI's we evaluated.  Also, SCSI
>controllers have *in the past* typically had about a 2-3 millisecond
>processing delay added on to commands because of the drives greater
>intelligence.
>
>	The synchronous transfer rate for SCSI is meaningless if it can't
>get it off the disk.  Of course if the SCSI controller has an on
>controller cache of some reasonable size, and perhaps a track read
>ahead then you might get better performance...
>
>-wat-

ESDI probably is faster than SCSI for a _single_ drive. But since
SCSI drives each have their own controller, a multiple drive SCSI
system can have better overall throughput than a multiple drive/one
controller ESDI system.  The SCSI bus is capable of "dynamic reconnect."
A host can issue a read request to one drive, disconnect from that
drive and issue a request to other drives.  When the first drive
has read the requested sector, it can reconnect to the host and
transfer the data at full SCSI-bus speed.  This transfer is not
limited by the rotational speed of the disk since the disk's controller
already has the data in its buffer.

The time required to process any given I/O request will be slower
for SCSI because of the buffering of data in the controller, but the
total number of requests processed per second can be much greater
for SCSI than for ESDI.  Of course it depends on how well the I/O
load is distributed across the disks, and how well the driver is written.

-- 
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Rick Francis                        rickf at uts.amdahl.com
Amdahl Corp.             {decwrl,sun,uunet}!amdahl!rickf
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  [Amdahl's opinions are expressed by Amdahl, not me.]