Oracle: Can a tablespace stay in backup mode forever?
When we put the tablespace in backup mode Oracle copies whole changed data blocks into the redo stream in order to make the backup safe from fractured block. Copying whole data block instead of only the change may degrade the performance. Oracle strongly recommends to put the tablespace in backup mode only when required.

What Happens When A Tablespace/Database Is Kept In Begin Backup Mode (Doc ID 469950.1) (MOS Account needed)

The following is the excerpt from the Oracle Documentation.


  Caution:
  
  If you fail to take the tablespace out of backup mode, then Oracle Database continues to write copies of data blocks in this tablespace to the online redo logs, causing performance problems. Also, you receive an ORA-01149 error if you try to shut down the database with the tablespaces still in backup mode.


Making User-Managed Backups of Online Tablespaces and Data Files