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regading scsi cable

 
Hoff
Honored Contributor

Re: regading scsi cable

>>> Can we use single ended SCSI cable to make a SCSI Cable cluster system.<<<<

Within a cluster, SCSI is and can only be a "cluster storage interconnect". A storage-only interconnect.

There is no host-to-host communications over SCSI. Only host-to-disk.

Hosts can share storage access on a SCSI bus, assuming appropriate controllers and cabling.

For clustering, you need a network, and/or CI, and/or DSSI, and/or Memory Channel (MC) or other such "cluster communications interconnect". You can then add multi-host SCSI -- shared SCSI -- into the cluster configuration.

Fibre Channel is similar to SCSI here, in that it too is a storage-only interconnect.

To help troubleshoot a shared-storage (storage-only) interconnect and to determine if it is even feasible here, please indicate which SCSI controllers are present. Only a subset of the SCSI controllers are supported for multihost operations within an OpenVMS Cluster.

Stephen Hoffman
HoffmanLabs LLC
Steven Schweda
Honored Contributor

Re: regading scsi cable

There seems to be some info in Appendix A at:

http://h71000.www7.hp.com/doc/82FINAL/6318/6318pro_021.html#scsi_techsum

KZPBA-CB (or -CY), the differential card, is
mentioned, not -CA/-CX, the single-ended
card. Also, in reference to a wide bus,
there's this:

Only the KZPBA-CB is supported in a
multihost SCSI OpenVMS Cluster
configuration.

Table A-4 Maximum SCSI Interconnect Distances
(on the next page) makes sense, except for
this entry:

Single ended Ultra 20.5 m

I assume that the author (or proofreader) was
drunk at the time. For a conflicting (and
more reasonable) summary, see, for example,
"Table 1. The SCSI Family Tree" in:

http://www.noncombatant.org/trove/benway-storage-200005.pdf
Hoff
Honored Contributor

Re: regading scsi cable

The manuals tend to be less current than other materials, such as the SPD or the QuickSpecs, or the support matrix. The manuals don't tend to get opened and updated quite as often as do these other documents, and can thus trail reality when it comes to supported hardware. FWIW.

http://h18000.www1.hp.com/products/quickspecs/12551_div/12551_div.HTML
http://h18002.www1.hp.com/alphaserver/options/asds20/asds20_options.html
Colin Butcher
Esteemed Contributor

Re: regading scsi cable

Hello,

First - I do not recommend this for a production system. It's a simple, crude and unsupported cheap way to kludge up a disc subsystem. Your call - it's cheap, but not pretty and not rock-solid reliable.

I have something similar running here in my lab. It's a AS4100 Galaxy box with 2x FWD (note the differential) SCSI adapters with internal termination (1 per partition), different SCSI IDs (7 and 6) and a "Y" cable between them with the single point of the "Y" going into the front of a DWZZB-VA (FWD to Wide SE SCSI adapter) which is in a BA356 shelf with 2 pairs of 9GB discs.

It's very crude, but in the absence of a fibre or SCSI based disc array it's good enough for my limited purposes. I then use HBVS to create a shadowed system disc. I also had to fool around with SCSI port allocation classes to get it to behave sensibly.

So, it can be made to work, but for a production system I'd do it a different way by using an array controller on a shared SCSI bus, or maybe move to using fibrechannel storage if you can - used HSG80s are cheap enough these days.

Cheers, Colin (http://www.xdelta.co.uk).
Entia non sunt multiplicanda praeter necessitatem (Occam's razor).
Paul Jerrom
Valued Contributor

Re: regading scsi cable

Howdy,
You can have up to three nodes sharing a common SCSI bus in a 'SCSI cluster' (been there, won't be returning!), but they do take some configuring. You can use a KZPSA, KZPBA or other FWD card, but you do need the correct cabling (with Y cables and external terminators) plus you have to take the internal termination jumpers off the card. You can't 'just' stick a SCSI cable between servers and expect it to work. As others have mentionned too, you need (depending on the VMS version) to either set the SCSI bus ID's, or put in placer SCSI cards so the device is seen as identical from both nodes, or use allocation classes on your SCSI buses. So it's not a trivial task. BTW, I seem to remember also that you can use single ended SCSI as long as you use a DWZZA (or DWZZB?), and also you could use a DWZZH (DOC hub) - in theory at least for up to 4 nodes on the same bus.
FWIW I would set up the cluster first, then add the shared storage in afterwards.
Have fun!
PJ
Have fun,

Peejay
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