Re: RAID with Disk ][

[dempson@actrix.gen.nz (David Empson)]

D Finnigan <dog_cow@macgui.com> wrote:

> Imagine this: One controller card, 2 Disk ][ units, and ProDOS device
> drivers to handle the following types of RAID configurations. I think that
> it can be done.
> 
> Mirroring- write data to Drive 1, then write it to Drive 2. Reads could
> choose either drive 1 or 2.

If you want simultaneous writes, then it won't work with a standard
controller card: it is only capable of accessing one drive at a time.

Even with two controller cards, you can't write to two 5.25" drives at
once on a 1 MHz 6502 due to lack of CPU time.

If you are doing sequential writes, I don't see what this achieves apart
from an instant backup.

It would be horribly slow if the writes alternated for each block
written, since you have to allow for the drives to get up to speed and
for rotational latency. If you do a larger continuous write, it can't be
done with the ProDOS API unless you have a big RAM buffer somewhere not
needed by any application (good luck with that).

Custom controller hardware would partly solve the problem, but
simultaneous writes are still unlikely to be feasible due to
insufficient current from the power supply to run two 5.25" drive motors
at the same time.

> Concatenation- Drive 1 and Drive 2 combine to yield a whopping 280KB
> (without needing to flip). The device driver would need to scan both
> catalogs, or make a combined in-memory catalog for a speed-up.

So you want to replace one somewhat unreliable 5.25" disk with two of
them?

Apart from that, this combination is at least achievable given the
hardware and software constraints. Performance would suffer somewhat due
to directories and files being fragmented between the two drives.

> Striping - Can this be done in any meaningful way??? I don't think so. Not
> with the given hardware, I don't suppose.

No. Same problem as sequential access mirroring.

> Out of the possibilities, I think that concatenation would be most useful.
> Of course, not being able to access more than one drive at a time is a big
> hinderance. But then again, this is the Apple: if we didn't mind in-built
> hinderances and hardware limitations, we'd have upgraded to the Mac years
> ago. :-)

-- 
David Empson
dempson@actrix.gen.nz