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Changing unix system clock when Oracle database is running.

Posted on 01-Aug-2005 By Admin No Comments on Changing unix system clock when Oracle database is running.

Q: How does changing the system clock effect Oracle?? specifically, point

in time recovery?? Is it different with V6 and Oracle7??

A: Changing the system clock should really have no effect on the Oracle kernel

until it comes time to do time-based media recovery. Both in V6 and V7 there

may be problems if you use RECOVER DATABASE UNTIL TIME if time has changed

between the time you recover and the time the backup is taken.

Consider:

3pm 4pm 4.30 5pm–>4pm 4.16pm 4.30 5pm

|———|——-|——-|—–|——-|——-|——–|

cold/hot T1 T2 T3 clock T4 T5 T6

backup change

If you wish to recover to the T5, you will not be able to since Oracle will

recover to the first occurance of 4:30 which is T2. In Oracle7 you can recover

to a specific SCN (RECOVER … UNTIL CHANGE) which is systemclock independent,

but when using RECOVER … UNTIL TIME will not give the desired results.

So the lesson to be learned, is when the clock is set back, you should take

full database backup immediately after the system clock change.

NOTE: this only applies in the fall when we set the clocks back.

Oracle, SQL scripts

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