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Rollback segments are the undo record, which contain the picture before the database changes. These segments are a critical part of the recovery process and are applied during the rollback phase of the recovery process. The OraRollSegTest measures the load and the efficiency of the rollback segments.

Measurement	Description	Measurement Unit	Interpretation
Writes	Indicates the total number of bytes written into this rollback segment	Number	This gives you an indication of the load on this rollback segment.
Waits	Indicates the number of header waits	Number	This indicates the number of times the Oracle Server had to wait to acquire a rollback segment.
Gets	Indicates the number of header gets	Number	This indicates the number of gets that have happened in this rollback segment.
Current_hit_ratio	Indicates the waits to gets ratio for the time between the last and the current measurement interval	Percentage	This indicates the efficiency at which the rollback segments have performed during the last monitored interval. Ideally, the Hit Ratio should be >= 99%.
Overall_hit_ratio	Indicates the the waits to gets ratio from the time the instance was started	Percentage	Ideally, the Hit Ratio should be >= 99%. If not, consider adding additional rollback segments. Also, check the system undo header, system undo block, undo header, undo block statistics under "Wait Statistics", for additional information on rollback contention.