Why snapshot is too old?

I have 10 rollback seg. optimal 8M ( all rollbacks are same ). now I got this
snapshot too old messages. should I make optimal to 16M?

if I plan to add more rollback segments. should I enlarge tablespace also? I
calculate my current rollback seg: max_extents * db_lock * 10 >
rollback_seg_tablespace.

Ans1:
Reset your rollback segment to have a larger minimal set too..

A rollback segment will not allocate a new segment if a particular segment
is marked stale. So if a second transaction is using the same rollback
segment it might end up using a stale segment from a large select that you
might call later.

Ans2:
Try to enlarge yout rollback segments' tablespace.

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