SCSI to IDE

[Charlie Carothers]

On 12/4/2010 2:31 PM, Chuck Guzis wrote:
> On 4 Dec 2010 at 19:33, Tony Duell wrote:
>
>> As far as I know, there is nothing in the SCSI spec that specifies a
>> minimum data rate or a maximum seek time. Soyou could even read 1 byte
>> out of an SD card and then send it over the SCSI interfce, and then do
>> the net byte, and so on. It would probably be better to buffer a
>> 'sector' in RAM on the microcontroller, and then transfer the buffer
>> to the SCSI interface.
>
> There's an excellent argument for heavily cache-ing SD card accesses
> when using it as a hard disk--to minimize repeated writes to a block
> (usually 16K) of SDRAM to extend longevity of the medium
That's a very good point.  I was rather naively thinking of a vintage 
system that is used only very occasionally.  That notion may be valid in 
some instances, but obviously not always.  If you are going to be doing 
much writing to the flash, I think you would need to do flash wear 
leveling in the bridge device.  If you are running an OS which (like 
some I could mention) seem to access the disk drive fairly frequently 
for no apparent reason (reading? writing? I'm never sure) then you 
should probably not consider using a flash drive at all unless you have 
enough RAM to make most of the writing unseen by the flash.  Then of 
course you must be sure the write from RAM to flash occurs when power is 
about to be lost.  This is starting to sound pretty complicated.  I'm 
sure most of the new SSD's take all of that into account.  Maybe what is 
really needed is a SCSI to SATA bridge. :-)
Later,
Charlie C.

--and to
> defer much slower writes as much as possible for performance reasons.
>
> If reading speed was an issue, one could employ the faster 4-bit
> access to SD rather than the simpler and much slower SPI mode that's
> usually used for slow embedded applications.
>
> --Chuck
>
>