I was wondering if anyone knew an explanation to this question.

When I have a tablespace that's is OFFLINE... as in:
alter tablespace tools offline;

I get an error when I try to put that tablespace in backup mode... as in:

SQL> alter tablespace TOOLS begin backup
*
ERROR at line 1:
ORA-01128: cannot start online backup - file 7 is offline
ORA-01110: data file 7: '/u02/oracle/oradata/PMGR/tools01.dbf'

And when I query "status" from v$backup, all the data files show "not active"... as in:

SQL> select status from v$backup;

STATUS
------------------
NOT ACTIVE
NOT ACTIVE
NOT ACTIVE
NOT ACTIVE
NOT ACTIVE
NOT ACTIVE
NOT ACTIVE
NOT ACTIVE

That makes sense, BUT!!!!
When you put all the tablespaces in backup mode, including SYSTEM... the command for TOOLS still fails, but when you query v$backup... you find that the "status" for all data files are "ACTIVE"... as in:

SQL> alter tablespace SYSTEM begin backup;
alter tablespace UNDOTBS1 begin backup;
alter tablespace CWMLITE begin backup;
alter tablespace DRSYS begin backup;
alter tablespace INDX begin backup;
alter tablespace ODM begin backup;
alter tablespace TOOLS begin backup;
alter tablespace USERS begin backup;
alter tablespace XDB begin backup;

Tablespace altered.

SQL>
Tablespace altered.

SQL>
Tablespace altered.

SQL>
Tablespace altered.

SQL>
Tablespace altered.

SQL>
Tablespace altered.

SQL> alter tablespace TOOLS begin backup
*
ERROR at line 1:
ORA-01128: cannot start online backup - file 7 is offline
ORA-01110: data file 7: '/u02/oracle/oradata/PMGR/tools01.dbf'


SQL>
Tablespace altered.

SQL>
Tablespace altered.

SQL> select status from v$backup;

STATUS
------------------
ACTIVE
ACTIVE
ACTIVE
ACTIVE
ACTIVE
ACTIVE
ACTIVE
ACTIVE


I would think that the data files associated with TOOLS would be "NOT ACTIVE".

How is it does it show up as "ACTIVE"... as if the "begin backup" command for TOOLS executed, when in fact it did NOT?