15 Nov
2007
15 Nov
'07
3:32 p.m.
Roy J. Tellason wrote:
On Thursday 15 November 2007 00:36, Doc Shipley
wrote:
Possible, and has been done fairly often in storage arrays.
Google "multi-initiator scsi" and rock on. :)
Hehe. Did that and the first two hits say "not recommended" and "not
supported" in the text that google is showing me there...
I've got a feeling there's not even the code there for it Linux - it just
doesn't know how to handle commands sent to the HBA at all. Few years ago when
I looked though; maybe it's different now...
It's certainly technically possible - I used to look after a pair of SGI O200
machines which have a bunch of disk storage shared between them on the same
SCSI bus.
Both USB and SCSI have the device-to-device capability
written into
some specs, but it's rarely implemented. ("USB on the go").
Which
reminds me of a comment I saw somewhere about the possibility of
having a SCSI bus with more than one host adapter on it. Is that even
possible? Another thought, though, is that might make for some
interesting
possibilities with that Bigboard II, which has that SASI port that seems (as
far as I can tell from just looking at signal names) to be fairly compatible
with SCSI, maybe hooking _that_ into a SCSI bus might have some serious
uses.
Hmm. I think SASI's sufficiently different that you'd be hacking your own SCSI
drivers to do it - from memory there were some extra phases in the SCSI
protocol which weren't in SASI. That's assuming whatever HBA you use doesn't
try and do the work for you and then barf because the device isn't SCSI. Plus
I don't think SASI had any concept of parity, so you'd probably have to hack a
bit of hardware there to generate the parity bit (assuming you couldn't
disable it via software).
Note that Linux chokes on devices that don't respond to the identify command -
which is every SASI device and some early SCSI ones too. I'm not sure how
deeply set that is into the 'core' SCSI layer, but I don't think it's down
to
driver behaviour (i.e. you can't get around it by hacking your own HBA
driver). No idea what other OSes are like...
I'm sure it *could* be done though, but it's a reasonable amount of work. It'd
be simpler to build your own SASI interface and drive it from a suitable I/O
board (a PC parallel port doesn't quite have enough lines unfortunately) from
user-land. After all, the sort of data transfer rates for SASI stuff isn't
exactly blinding, so some sort of polled-I/O bit-twiddling should be good enough.
Sure beats trying to transfer any nontrivial amount
of data over a
serial link, and has the potential for going one heck of a lot faster!
Been there done that with a few ST-412 drives (via Adaptec SCSI bridge
boards). Think it was about 8 hours per 20MB or thereabouts.
So many things to look at, and never enough time...
Yep... I got a PC SASI interface all designed and then halfway wired up ages
ago, then just haven't had the time to do anything more with it for 6 months
or so :-(
cheers
Jules