[Info-vax] Question about IDE disks

[glen herrmannsfeldt]

H Vlems <hvlems at freenet.de> wrote:
(snip)

> from the way Seagate issues model numbers, I'd assume that the HDA and
> the drive electronics are two distinct products.
> Drive electronics (again, just my guess) are part drive specific and
> part depending on the connecting bus.
> A Seagate model looks like this:
>  ST <geometry><capacity><connection type>

Most of the time probably true, but I know of some cases where
matching disk size was a completely different HDA.  The cases
that I know of are from a long time ago, though.

> Where:
>  <geometry> is a single digit that tells you about the width and
> heigth of a drive. E.g. 1 is a 3.5" drive disk, approx. 1.5"high and 3
> is a 1"high drive
>  <capacity> is either in kB or in MB, depending on the age of the
> drive in four or five digits
(snip)

> Apparently that difference no longer mattered for the production of
> the HDA, hardware MTBF figures just went up, and an HDA was used for
> SCSI and IDE.
> Today SCSI lags behind, 300 GB is the maximum capacity for SCSI while
> IDE is available at 500 GB (perhaps even bigger).
> I do not know about the differences between serial SCSI and serial ATA
> (SAS and SATA) but I'm willing to propose that these drives share the
> same HDA's.

One reason is that SCSI drives often spin faster.  For a given number
of tracks and bit clock, a faster spinning disk has lower capacity.
Most important, it also has lower latency.  (Delay for the disk to
rotate to the desired block.)
 
> IDE drives (parallel ATA) have a facility called SMART which allowed
> the drive to inform the OS about the shape it was in. I don't know
> whether VMS can interpret the data but it surely is available. DEC
> provided utilities to extract data from DSSI and SCSI devices so
> perhaps it is possible to write a similar tool to inspect IDE drive
> logic?

Even more interesting, ATAPI used for CD-R and DVD-R sends SCSI
commands over the IDE bus.  SATA disks, it seems, also accept
SCSI commands and are often configure to the OS as SCSI disks.
(In linux they have sd names instead of hd names.)

-- glen