[Info-vax] Unpleasant Disk Shadowing Surprise

[Bob Koehler]

In article <c8f1c0db-56f1-482d-8e60-e7ab42aa9bd9 at z8g2000yqb.googlegroups.com>, tadamsmar <tadamsmar at yahoo.com> writes:
> 
> According to the console log, more than 30 seconds after the watchdog
> sounded, the shadow set changed state, the offending disk went
> offline, a mount verification started and completed.
> 
> Immediately after the mount verification completed, VMS started
> working again.

[...] 

> Is this to be expected?  We have had VMS and disk shadowing running
> the application for 20 years or so, but I don't know that we have ever
> had a disk error while the watchdog was configured to sound, so we
> might not have noticed the halting and recovery.

   While 3 minutes will kill almost any real-time application (depending
   on the definition of "real-time"), every other system I've tried
   would have crashed during such an event (many aren't supposed to).

   Even VMS can't reach out and patch up broken hardware, but the only
   reason you'ld see this behaviour would be if you were actually
   accessing the disk with the problem.  Most of the time for such
   issues I never actually saw a problem until my overnight backup 
   accessed an otherwise rarely visited file.  (And my real-time
   application, which couldn't tolerate 10ms, wasn't running.)

   If your definition of real-time can't handle 3 minutes of
   interruption, then you probably need to engineer a different solution
   than the kind of shadowing approach you're using now.

   But 3 minutes over 20 years is better than 99.9999%.  I can remember
   when "5 nines" was all the rage, and you got 6.