Databases Reference
In-Depth Information
Studying the output, you can see the demand for flashback logs went up at 21:57 to 287,563,456, or about 287MB.
If you estimate the total size of flashback logs as 190MB, then the older logs will be deleted to make room for the new
ones at 21:57. This information helps you when deciding the optimal value of the flashback logs.
How It Works
This view V$FLASHBACK_DATABASE_STAT shows the estimated flashback data within hour-long intervals. Table
13-2
describes the columns of the view.
Table 13-2.
Columns of V$FLASHBACK_DATABASE_STAT
Column Name
Description
BEGIN_TIME
The beginning of the interval
END_TIME
The end time of the interval
FLASHBACK_DATA
The amount of flashback data generated in bytes in this time interval
DB_DATA
The amount of database change data generated in bytes in this time interval
REDO_DATA
The amount of redo generated in bytes in this time interval
ESTIMATED_FLASHBACK_SIZE
The estimated size of the total flashback logs retained to satisfy the retention
target at the end of this time interval, shown in the column END_TIME
CON_ID
The container ID (in case of Oracle Database 12.1 and above)
Estimating the Space Occupied by Flashback Logs in the Fast
Recovery Area
Problem
You want to estimate how much space will be needed for the flashback logs to be retained enough to flash back by a
time period specified by the retention target.
Solution
To estimate the total size of all flashback logs required for the retention target, follow these steps:
1.
Check the dynamic performance view V$FLASHBACK_DATABASE_LOG:
SQL> select * from v$flashback_database_log;
OLDEST_FLASHBACK_SCN OLDEST_FL RETENTION_TARGET FLASHBACK_SIZE
-------------------- --------- ---------------- --------------
ESTIMATED_FLASHBACK_SIZE CON_ID
------------------------ ----------
2193903 21-JUL-12 1440 367001600
21479424 0
Note the value of
ESTIMATED_FLASHBACK_SIZE
, which is 21,479,424, or about 20MB in this
case. This should ideally be your size of the flashback logs.
2.
