Mylex SCSI Controller

John Boteler bote at csense.UUCP
Thu Sep 28 03:46:44 AEST 1989


>From article <19245 at gatech.edu>, by ken at gatech.edu (Ken Seefried III):
> In article <369 at csense.UUCP> bote at csense.UUCP (John Boteler) writes:
>>Curious as to how a SCSI host controller can easily perform 
>>cache operations?
> Well...they have a microprocessor and a hoard of memory on board.
> Think about it...or at least go review the recent discussion of
> the DPT controller for some excellent commentary, pro and con, on
> the issues involved in caching disk controllers...

I don't need to.

I talked to one of the engineers from DPT at Comdex. He explained
to me the difficulty of caching a SCSI host controller: since
the controller handles up to seven devices on the SCSI bus,
putting the cache system on the controller makes for little
improvement, if any. The best solution, and one which has
been implemented by at least one drive manufacturer, is to
put the caching components on the SCSI drive. 

When a SCSI request is sent to the drive by the controller, the
controller is free to service one of the other six devices on
the bus and the drive can check its own cache for the presence
of the data requested and respond accordingly.

The previous discussion of caching controllers pertained primarily
to RLL and ESDI formats.

-- 
Bote
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