Related knowledge of fallback

Undo_management=manual

When a new database is created, Oracle establishes a default system fallback segment in the system tablespace. The system fallback segment should only be used for fallback records that hold Oracle system transactions, and no user transaction should use the system fallback segment.

#在创建数据库之后, you should immediately establish a non-system fallback segment in the system tablespace, because after the database is created, users need to create additional tablespaces and fallback segments first. At this point, the fallback records for these operations are saved in this non-system fallback segment and are not saved in the system fallback segment. This non-system fallback segment in the system table space can be deleted after other tablespaces and fallback segments have been established.

The number of fallback segments obtained when an instance is started is determined by several methods:
• The number of public fallback segments acquired by the instance is specified by transaction,transaction_per_rollback_segments
• The private fallback segment of the instance acquired by Rollback_segments is determined by the name of the account.
#此外, the maximum number of fallback segments allowed for an instance is Max_rollback_segments=ceil (transaction/transaction_per_rollback_segments).

In one instance, if both a public fallback segment and a private fallback segment are used, the instance obtains the private fallback segment specified by the rollback_segments parameter, and then passes the Ceil (transactions/transactions_per_rollback_ segments) The difference between the calculated value and the number of private fallback segments that have been acquired needs to be compensated by obtaining the public fallback section.

• Create a private fallback segment
Create rollback segments XXX
Tablespace XXX
Storage (initial XXX
Next XXX
Optimal XXX
Pctincrease 0
Minextents XXX
Maxextents XXX)
• Create a public fallback segment
Create public rollback segments XXX
......

#online offline
alter ROLLBACK segment XXX Online/offline
• Use parameter files (private fallback only)
Rollback_segments
Add a private fallback segment to the parameter, and after the reboot, the fallback segment's status will be permanently onlne
Fallback segment without write parameter file after the database restarts, reboot again, change to offline
The public fallback segment will not be restricted by the parameter file


In the fallback segment, the header of the segment stores information about the fallback segment transaction in use, a transaction can only use a fallback segment to hold its fallback information, and a fallback segment can hold multiple transaction fallback information.