Easy. Just look at the ratio of writes to gets. On the segments with 39 wraps each transaction has only generated between 70 and 80 bytes of undo on average. If you have multiple concurrent transactions in each rollback segment (as you appear to have) then each new transaction starts a new undo block. Therefore, you are only using 70 to 80 bytes in each undo block. If your database block size is 2K, as it appears to be, then the number are exactly as one would expect.

Incidentally, it is worth noting that the low utilization of undo blocks places additional load on DBWn. Therefore, when tuning DBWn, one might check for this scenario and if it is found increase the number of rollback segments to reduce rollback segment sharing and thus reduce the load on DBWn.

Thanks for the reply but, If you look at my figures, total writes to RBS are approx 2M and extent size is 1M then how could that generate 39 wraps according to your definition of wrap. In your opinion what could be the solution here?

A wrap is when undo generation moves into the next extent in the cycle, not when it moves back to the first extent.

Here is a qurestion regarding wraps. (I am trying to solve a snapshot too old problem). As I understand that oracle writes from 1st extent to the last allocated extent of the RBS and then it decides whether to wrap to the 1st extent or allocate a new extent at the tail end. If 1st extent is not active then it wraps back. In that scenario all the extents of a new RBS will have to be filled once before it wraps. Therefore I an unable to find explanation for following statistics

    RSSIZE     WRITES       GETS    HWMSIZE      WRAPS    EXTENDS
---------- ---------- ---------- ---------- ---------- ----------
  21282816    1382592      19270   21282816         39          0
  21282816    5105196      20931   21282816          6          0
  21282816    1657848      20419   21282816         39          0
  21282816    2808142      17134   21282816          4          0
  21282816    1436596      19263   21282816         39          0
  21282816    2552714      17121   21282816          4          0