ITEM: I8078L

What should be purchased to setup twin-tailing on a 520?


Customer has a 520 system with 3 internal 670 drives and a 2.3GB tape drive.
He has two volume groups: rootvg on one 670 MB drive, and data VG on the
other two drives.  Datavg takes about 800MB.  Customer would like some 
redundancy so that if he loses a disk, SCSI adapter, or the 520 he can
be back up an running quickly (within an hour or two).  Ascii terminals
are attached via a 64 port controller.

What hardware needs to be purchase to setup this configuration?


First of all, terminal access can be handled via a switch or purchasing 
another 64-port controller and putting it in a 220 (and moving the cables 
in the event the 520 fails).

We considered putting the 670 drives go into a 9334 (but can we twin tail 
with it?). But two external 1GB drives are less expensive.  

Question - can we use the current SCSI adapter on the 520 in twin
tailing?  Has adaptor, 1 internal drive (disconnect internal 2
drives), tape drive and then two external 1GB, then on other side
adaptor and drive (at least) - this is 7 devices, so he could if cable
length is within 6m limit.

A better option is to buy a SCSI adapter for 520, and a 1GB external drive,
a 220 with an additional SCSI adapter and twin tail the two additional SCSI 
 adapters with the 1 GB drives in the chain.  Then mirror datavg from the 520
with the internal drives to the external drive.  Even  though the 220
has a connection to only 1 copy of the datavg mirror, it will be able to 
import datavg and vary it on if quorums are turned off for datavg.  If this
occurs the VGDA's timestamp will be updated and the "missing PPs" will be
marked stale on the VGDA on the external 1 GB drive.  When the 520 is fixed, 
the 220 should be shut down, then power up the 520 with the external drive up 
too.  When datavg is varied on, the most recent copy of the VGDA will be 
read and syncvg will run in the background to bring the old copy up to date.

Support Line: What should be purchased to setup twin-tailing on a 520? ITEM: I8078L
Dated: April 1994 Category: N/A
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