never mind., I found it.
there is another question: how do I know which rollback seg is which? It doesn't have a ROLLBACK_NAME column. Another words: how do I link this view with dba_rollback_segs?
desc v$rollstat;
------------------------
Name Null? Type
------------------------------- -------- ----
USN NUMBER
EXTENTS NUMBER
RSSIZE NUMBER
WRITES NUMBER
XACTS NUMBER
GETS NUMBER
WAITS NUMBER
OPTSIZE NUMBER
HWMSIZE NUMBER
SHRINKS NUMBER
WRAPS NUMBER
EXTENDS NUMBER
AVESHRINK NUMBER
AVEACTIVE NUMBER
STATUS VARCHAR2(15)
CUREXT NUMBER
CURBLK NUMBER