HP 3000 Manuals HP 3000 Manuals
Introducing SETCLOCK and SHOWCLOCK, Two New CI Commands
by Doug Perry
Commercial Systems Division
Overview
SETCLOCK and SHOWCLOCK are two new CI commands that help you manage and
show information about the system clock. Now you can change the system
time or time zone without rebooting the system and can obtain information
about the system time and time zone.
Background
On the HP 3000, the system time traditionally was considered a protected
system resource, only to be changed during a system boot. The following
information describes some of the reasons:
* Operating System Issues:
System accounting (the reporting of user CPU seconds and connect
minutes) depends on the system clock, and this information is used
to bill users on some systems for computer services.
Some database logging and recovery systems rely on transaction log
files that are circular files of timestamped records. The end of
data is detected when the recovery program observes an
out-of-sequence timestamp. Thus, if the time were to be set
backwards while logging was in progress, some transactions might
not be recoverable with such a system.
* User Application Issues:
Any application relying on the forward progression of time may
give inconsistent results if the system time were to be set
backwards. An automated teller application, for instance, may
timestamp a customer making a withdrawal before a deposit, when in
fact the customer made the deposit first.
A file backup system that relies on the access time of files may
not back up the expected files if the system time is changed.
Any application that times external events (such as the times
employees clock in or out) may give inaccurate data if the system
time is set forward or backward.
The above list is certainly not exhaustive, but it does hint at the
importance of protecting the system time as a valuable resource. Still,
many user requests indicated that the ability to change the system time
in a supported way without having to reboot the system was needed. The
SETCLOCK and SHOWCLOCK commands satisfy these requests. SETCLOCK
provides a safe way to change the system clock while the system is
running, and SHOWCLOCK provides information about system time, current
time correction (described below), and time zone.
How SETCLOCK Changes System Time
SETCLOCK changes the system time by introducing a time correction into
the system. This correction is gradually consumed. If the correction is
positive, the system clock appears to speed up. If the correction is
negative, the system clock appears to slow down. At no time does the
system time actually appear to go backwards. When the correction is
completely consumed, the system clock appears to resume its normal pace.
This correction is consumed gradually. Up to two hours may be necessary
to achieve a correction of one hour.
To establish this correction, SETCLOCK accepts either a new date and time
or a time correction (in seconds) as a parameter.
SETCLOCK also enables the user to set a new time zone, which is the
difference between local time and Universal Time (Greenwich Mean Time).
For instance, such a change occurs when going between Standard and
Daylight Savings time. The purpose in changing time zones is to achieve
a change in local time (to the new time zone) without changing Universal
Time.
When a new time zone is requested, the behavior of local and Universal
Time on the system is complex and is different for a forward or backward
change in local time. The difference is necessary to assure that no
out-of-sequence timestamps occur in either local or Universal Time
regardless of the direction of the time zone change. Please see the
article "Using the SETCLOCK Command" in this Communicator for specific
information on the behavior of local and Universal Time when the time
zone is changed.
SETCLOCK requires that the user have both diagnostician (DI) and either
operator (OP) or system manager (SM) capabilities. In addition, if the
user has SM capability, an option to SETCLOCK gives you immediate
permission to change the system time either forwards or backwards. For
the reasons stated above, such an immediate change can have very serious
consequences, but the capability is there for situations that require it.
How SHOWCLOCK Reports System Time
SHOWCLOCK is similar to SHOWTIME in that it reports on the system time
and requires no parameters. An example of SHOWCLOCK is given below:
:SHOWCLOCK
SYSTEM TIME: FRI, JUL 24, 1987, 8:47:35 AM
CURRENT TIME CORRECTION: -3600 seconds
TIME ZONE: 7 HOURS 0 MINUTES WESTERN HEMISPHERE
You can see that SHOWCLOCK displays system time including seconds,
whereas SHOWTIME only shows hours and minutes.
The current time correction in this example indicates that a SETCLOCK
command was issued and the system clock appears to run slightly slow for
awhile until the 3600-second correction is consumed. When the clock
returns to its normal pace, the system time will be an hour slower than
if the correction had not been established.
The time zone of 7 hours in the Western hemisphere places the system
either at Pacific Daylight Savings Time or Mountain Standard Time,
depending on the time of year.
For more information about these two commands, please refer to the
article "Using the SETCLOCK Command" in the "Technical Articles" chapter
in this Communicator.