Friday, April 29, 2011

What is RBA ?

An  RBA (Redo Block Address) points  to  a  specific phyical  location  within a redo logfile . The "tail of the log" is the RBA of the most recent redo entry written to the redo log file . It is ten bytes long and has  three  components .

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)

For Example :  RBA [0x775.2.10]  maps to Log squence , Block number with byte offset .

There are different types of RBA available in SGA , the following are :

Low RBA : Dirty buffer contains first redo change address called Low RBA. From x$bh we can check low RBA.

High RBA : Dirty buffer contains last and most recent redo changes address called High RBA. From x$bh we can check High RBA.

Checkpoint  RBA : DBWR  has written  buffers from  checkpoint queue  are pointing  to  checkpoint  RBA while  incremental checkpoint  is  enabled. This  RBA copies  in  to  control  file’s checkpoint  progress record. When instance recovery occurs that time it starts from checkpointing  RBA from control  file. We  can check this RBA from x$targetrba (sometimes from x$kccrt).

On-disk RBA : That RBA which was flushed in to online Redo Log File on disk. This RBA recorded in to control file record  section. We can check from x$kcccp for on-disk RBA (sometimes from x$targetrba).

How RBA comes in Pictures :
CKPT records checkpoint information to controlfile for maintaining book keeping information like checkpoint  progress . Each instance checkpoint  refers  to some  RBA (called checkpoint RBA) whose  redo prior to this RBA have been written to disk. Hence recovery time is difference between checkpoint RBA and end of the redo log  .

Given a  checkpoint RBA, DBWR writes  buffers  from  the  head  of  the queue  until  low RBA of the buffer at  the head of the checkpoint queue  is greater  than  the checkpoint  RBA . At  this  point ,CKPT can  record  this checkpoint  progress  record  in  control file  (phase 3).
PHASE(1)  process  initiating  the checkpoint (checkpoiting  RBA or current RBA is marked) (The RBA of the last change made to a buffer) at the time reuqest is initiated.
PHASE (2)  DBWR  writes all  required  buffers  i.e  all  buffers  that  have  been modified at RBAs less than or equal to the checkpoint RBA. After all required buffers have been written, in
PHASE (3)  CKPT process records the completion of the checkpoint in control file.

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 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.


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