Redo Byte Address (RBA)

Recent entries in the redo thread of an Oracle  instance are addressed using a 3-part redo byte address, or RBA. An RBA is  comprised of

• the log file sequence number (4 bytes)
• the log file block number (4 bytes)
• the byte offset into the block at which the redo record starts (2 bytes)

RBAs are not necessarily unique within their thread, because the log  file sequence number may be reset to 1 in all threads if a database is opened  with the RESETLOGS option.

RBAs are used in the following important ways.

With respect to a dirty block in  the buffer cache, the
low RBA is the address of the redo for the  first change that was applied to the block since it was last clean, and the
high RBA is the address of the redo for the most recent change to  have been applied to the block.

Dirty buffers are maintained on the buffer cache  checkpoint queues in low RBA order. The
checkpoint RBA is the  point up to which DBWn has written buffers from the checkpoint queues if  incremental checkpointing is enabled -- otherwise it is the RBA of last full  thread checkpoint. The checkpoint RBA is copied into the
checkpoint progress  record of the controlfile by the checkpoint heartbeat once every 3 seconds.  Instance recovery, when needed, begins from the checkpoint RBA recorded in the  controlfile.
The target RBA is the point up to  which DBWn should seek to advance the checkpoint RBA to satisfy instance  recovery objectives.

The on-disk RBA is the point up to which  LGWR has flushed the redo thread to the online log files. DBWn may not write a  block for which the high RBA is beyond the on-disk RBA. Otherwise transaction  recovery
(rollback) would not be possible, because the redo needed to undo a  change is always in the same redo record as the redo for the change itself.

The term sync RBA is sometimes used to refer to the point up to  which LGWR is required to sync the thread. However, this is not a full RBA --  only a redo block number is used at this point.

The low and  high RBAs for dirty buffers can be seen in X$BH. (There is also a
recovery  RBA which is used to record the progress of partial block recovery by PMON.)  The incremental checkpoint RBA, the target RBA and the on-disk RBA can all be  seen in X$TARGETRBA. The incremental checkpoint RBA and the on-disk RBA can also  be
seen in X$KCCCP. The full thread checkpoint RBA can be seen in X$KCCRT.