| In the case of the 8200/8400 systems you throw away a
perfectly good hose by putting an hose to XMI widget
on it. Then you put a CIXCD or KDM70 on the XMI bus.
The XMI isn't a total waste, since you can also use the
XMI FDDI and Ethernet adapters on it. For a DEC 7000
you already have the XMI and the KZMSA is arguably worse
then the CI adapters and HSC based storage.
The "RA" devices are disks. The largest capacity disk
of the series made was a 5.25" form factor device with
a capacity of 2 GB. Modern disks of compariable form-
factor are up 23 GB. The disks don't have any cache,
so getting the maximum data rate is pretty hard. On
the other hand the maximum is about 2.2 MB/sec so a
speed demon it isn't. The RA family does support command
queuing, so the random access performance isn't too bad,
for a 5 year old 5.25" disk.
The "TA" devices are tapes. The TA90/TA91 is an IBM 3480
class tape drive. These are fast tape drives, allowing
for the 2.2 MB/sec wire. The TA79 is your basic vacuum
column 9 track tape drive. Other than their age and
performance, the biggest draw back of these drives is the
foot-print. The TA79 uses as much floor space as an 8400,
the TA90 about three 8400s.
Of course your biggest problem is that you can't buy any
of this stuff new. If you've maxed out the SCSI controller
support on a 8400 class system and have a spare hose, then
you could put more disks and tapes on it; if you can find
any.
The KDM70 is an 8 port combination disk/tape controller that
plugs into the XMI. The HSC is a family of multi-port controllers
that connect via the CI. The most powerful member of the family
is the HSC95 which supports up to 48 MSCP disks and includes a
read cache (no read-ahead though). Some versions allow connecting
SCSI disks and tapes, but I don't think Digital UNIX supports
this feature. The modern follow-on to the HSC is HSJ, but it isn't
supported either.
To its credit, the HSC is a multi-host disk controller. Any
system on the CI has equal access to all devices on the HSC.
It would be unwise to share such disks among systems, but it
could make for very quick and direct failover, if the feature
were supported. The underlying storage architecture allows
for two connections to a device, though not at the same time.
This allows failover through multiple HSCs, but that isn't
supported either.
Beyond that, MSCP disks and TMSCP tapes are just more storage.
The problem is that compared to modern SCSI based storage they
are large, slow, expensive and proprietary.
|