[comp.os.vms] Answers on dual-porting DSA disks and BACKUP /NOCRC on tapes

nagy%warner.hepnet@LBL.ARPA (07/08/87)

>Rob Malouf
>Marine Sciences Research Center
>State University of New York
>Stony Brook, NY  11794-5000
     
>First, do I really need software checksums in my VAX backups?
>Does anyone have any experience with not using them?  Any comments?
>I am running a VAX 11/730 with VMS 4.5 and a Cipher 990 GCR
>CacheTape 9-track tape drive for backups.

	The GCR standard defines the on-tape format of the data.  This
	includes a hardware-written CRC (actually a bunch of them) for
	each tape record.  Thus you can, and seemingly safely, do backups
	with /NOCRC on GCR (6250 bpi) and TK50 tapes.  I would not advise
	anyone to leave off the /NOCRC on 1600 bpi tapes where the tape
	format standard does NOT provide for a hardware-generated CRC.
     
>...
>However, I noticed in the documentation that an RA81 disk drive can
>be driven by two UDA50 controllers.  If this is really true,
>then I could connect the 11/730's RA81 to controllers in both machines.
>This kind of dual-porting would create a shared disk drive and a "poor-man's"
>VAXcluster!  All my instincts say that it could never work, but why not?  Has
>anyone actually tried this?

	Wont't work: DSA disks (RA60, RA80 and RA81) are all dual-ported
	but only support static dual-porting.  This means that the drive
	can be MOUNTed from only one port at a time; mounting the volume
	causes the microprocessor in the controller to load some information
	(Replace Cache Table) from the disk into memory (of the controller).
	No provision is made (nor means exists) for the two controllers
	on two systems (or even on one system) to communicate and insure
	that this information is maintained in synchrony.  Only MassBus
	disks (750, 78x and 86xx systems can interface these) support
	dynamic dual-porting.

	BTW: dynamic dual-porting of MassBus disks in a VAXCluster is
	supported and works very well.

= Frank J. Nagy
= Fermilab Research Division EED/Controls
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