MPE/iX Commands Reference Manual (32650-90877)

472 Chapter12
Command List X
Commands SAVE thru SHUTQ
there will never be a case where two consecutive timestamps appear to be out of sequence
and where system time appears to run backwards.
This change in clock speed is accomplished by establishing a system time correction which
is gradually consumed. During this time the system clock speeds up or slows down as
necessary. When the correction reaches zero, the system clock resumes its normal pace.
The rate of the correction depends on the load on the system. The correction rate will be
slowed down by frequent timestamp requests, file accesses and frequent operating system
activity such as context switches. In general, the correction will take no longer than twice
the requested time difference. For example, a request to slow down the clock by one hour
will take a maximum of two hours to complete.
Results of the ;CANCEL Parameter
Any time during an on-going correction, issuing this command with the ;CANCEL
parameter will immediately set the correction to zero and cause the system clock to
resume its normal pace. Any previous correction will remain. When this option is used, the
system will report the amount of correction which was cancelled.
How a System Time Change Affects Accounting Information
Changing the system time, even gradually, may cause accounting CONNECT-MINUTES
to be distorted. Anyone logging on before the change and then logging off after the change
is completed will have their accounting CONNECT-MINUTES data distorted; if the time
change is forward, CONNECT-MINUTES will be increased by the amount of the time
change, and if the time change is backward, CONNECT-MINUTES will be decreased by
the amount of the time change.
Dangers in Using the ;NOW Parameter
The ;NOW parameter permits immediate forward or backward time changes. However,
several dangerous situations can occur:
• Any applications which rely on the forward progression of time may give inconsistent
results if the time is immediately set backwards. Such applications include the
processing of timestamped transactions in which the sequence of those transactions is
important.
• In order to recover data in case of an unexpected hardware or software failure, some
applications require that the system time must never seem to go backwards. For
instance, some applications log transactions to a circular file. These transactions are
timestamped, and if the transactions must be recovered, the recovery program
determines the end of data by looking for timestamps which are out of sequence. If the
system time is set backwards immediately, transactions which occur after the time
change may not be recovered. Therefore, do not set the time backwards using the ;NOW
option if there are applications which log their transactions using timestamps.
• Accounting CPU-SECONDS data may be distorted. The user whose process was active
during an immediate forward or backward change might seem to have a
CPU-SECONDS time which is an extremely large positive or negative number.
• STORE/RESTORE, TurboSTORE/XL, or any other file archive system based on dates
or times may not store or restore the files in the expected manner, since some files may
have creation or access times in the future or may even have access times which precede
their creation times.