[Info-vax] SYS$TIMEZONE.DAT

[Paul Sture]

On 2015-03-16, <lists at openmailbox.org> <lists at openmailbox.org> wrote:
> On Sun, 15 Mar 2015 13:37:21 -0400
> David Froble via Info-vax <info-vax at rbnsn.com> wrote:
>
>> Yes, but maybe the pertinent question is, "how many users need
>> accurate time?"
>
> VMS clustering can work without accurate time across the cluster?

Yes.  The place where problems become evident is the batch system, but
VMS clustering can work quite happily when times across members are not
in sync.  The hardware clocks on different systems usually go out of
sync in a gentle fashion, which isn't a big deal for clustering itself.

It's important to remember here that clustering came long before
connectivity to an external time source was common.  A former colleague
did rig up a radio source to implement a home grown time synchronisation
service, but that was later rendered ineffective by relocating the
systems in a deep cellar :-)

The batch system problem with skewed times is particularly evident when
submitting jobs queued via a command such as SUBMIT /AFTER=TOMORROW

Here the submitting node calculates the absolute date and stores it in
the queue database, but if the executing node is already at "tomorrow"
it will execute immediately.  Vice versa when the times are the other
way around.  For this reason the general recommendation is to use
SUBMIT /AFTER="TOMORROW+00:10:00" or similar.

>> If not at all, then any changes is an imposition for no gain.  If
>> "it's nice, but not critical", pretty much the same.
>> 
>> Not too many things in this world are perfect, maybe none at all.
>
> Time is something most clustering and most interconnected computers
> need to agree on.

For VMS the driving factor is whether your applications need accurate
time, not clustering itself (cue someone coming along with a real
life example that contradicts that statement).

Hoff alluded to this the other day in his mention of how VMS handles
time zones and what happens at a daylight saving time change.  This
is a much deeper problem for VMS.

-- 
Real Daleks don't climb the stairs - real Daleks level the building.