CHECKPOINTS CONFUSIONS CLEARED FROM KARAN

CHECKPOINTS CONFUSIONS CLEARED FROM KARAN

Well, lot of DBA's in this world are still unaware about checkpoint facts, after having a lovely breakfast i decided to post on this....

  An RBA indicates position in the redo log. 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 (checkpoitn 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 3rd phase , CKPT process records the completion of the checkpoint in control file.

# BCQ (buffer checkpoint queue) which contains modified buffers linked in ascending oreder of their low RBAs. In response to a checkpoint request, buffers are written from the head of the checkpoint queue, i.e in ascending order of low RBA values. after the buffer is written it is unlinked from checkpoint queue.

# ACQ (active checkpoint queue) is a queue that contains active checkpoint requests. Whenever a checkpint request is made , a new active checkpoint entry describing the request is added to the ACQ. Note that multiple checkpoint requests can be there in ACq like datafiles has to checkpointed before they are backed up, alter system checkpoint, or at log switch.

# Periodically CKPT can record this low RBA to controlfile (using a very light weight controlfile update protocol). This periodically recorded lowest low RBA is current position of the incremental checkpoint for this instance.

# so incremental checkpointing greatly improves crash recovery times with negligible impact on normal activity.

# IF DBWR continiually writes from the head of the checkpoint queue, the instance checkpoint (lowest low -RBA of modified buffers) will keep on advancing.

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.

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.

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