Undo Tablespace Reuse

Question

Hi,
I am using 10.2.0.3.0 on Oracle Solaris 5.10.
I have an undo tablespace 'UNDOTBS1' which has 5 data files of sizes 60GB. Here are my undo parameters:
SQL> show parameter undo
NAME                                 TYPE        VALUE
undo_management                      string      AUTO
undo_retention                       integer     900
undo_tablespace                      string      UNDOTBS1I have a long running batch process, which is inserting billions of records in the table and generating lots of undo. The batch is running for the last two days, and generating lots of undo and developer says that he is committing after 3 million records, though he was told to commit after 10000 records. Now all the data files of undo tablespace have become full except one, which is also getting filled up.
I see from dba_rollback_segs that there are some 25 segments which are showing status offline.
does it mean that the oracle will reuse those segments and I dont need to add another data file to the undo tablespace?
How would I know during this running process, that oracle would reuse space in undo tablespace, or will just throw an error, after exhausting all the data files.
Thanks and regards

Answer 1

Hi,
As the undo segments are used in a curcular fashion, dba_free_space may not give correct picture. use following sql.
col PCT_USED head '% Used'
select a.used, b.allocated, round((a.used/b.allocated)*100,2) PCT_USED, c.unexpired, d.active from
(select sum(BYTES/1024/1024) used from dba_undo_extents where status in ('UNEXPIRED','ACTIVE')) a,
(select bytes/1024/1024 allocated from dba_data_files where tablespace_name = 'UNDOTBS1') b ,
(select sum(bytes/1024/1024) unexpired from dba_undo_extents where status = 'UNEXPIRED') c,
(select sum(bytes/1024/1024) active from dba_undo_extents where status = 'ACTIVE') d
Here ACTIVE - they are being used
UNEXPIRED - they will not be released until the undo_retention time. But if all other extents are used, then oracle may reallocate unexpired blocks and any session which is trying to read from unexpired block may received "snapshot too old error".
EXPIRED - here these blocks will be reused, you can consider them as free.
Thanks,
Siva