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Oracle incremental checkpoint

Oracle Database Tips by Donald BurlesonMay 2, 2015

Question:  Regarding incremental checkpoints, the Oracle database concepts 11gr2 manual states that an incremental checkpoint is used to prevent a large flush of the data buffers are redo log switch time:

"An incremental checkpoint is a type of thread checkpoint partly intended to avoid writing large numbers of blocks at online redo log switches. DBWR checks at least every three seconds to determine whether it has work to do. When DBWn writes dirty buffers, it advances the checkpoint position, causing CKPT to write the checkpoint position to the control file, but not to the data file headers. . . .

During instance recovery, the database must apply the changes that occur between the checkpoint position and the end of the redo thread. Some changes may already have been written to the data files. However, only changes with SCNs lower than the checkpoint position are guaranteed to be on disk."

Can you explain incremental checkpoints in plain English?

Answer: An incremental checkpoint is sort of like when you are sitting on the toilet taking a large dump and you flush multiple times to prevent clogging the toilet.

The "fast start" recovery (and the fast_start_mttr_target) is directly related to the incremental checkpoint.  By reducing the checkpoint time to be more frequent than a log switch, Oracle will recover and re-start faster in case of an instance crash. 

The docs note that a DBWR writes buffers to disk in advance the checkpoint position, writing the "oldest" blocks first to preserve integrity.

A "checkpoint" is the event that triggers writing of dirty blocks to the disks and a "normal" checkpoint only occurs with every redo log file switch. 

In a nutshell, an "incremental" directs the CKPT process to search for "dirty" blocks that need to be written by the DBWR process. thereby advancing the SCN to the control file.

The DBWR wakes up every 3 seconds, seeking dirty blocks and sleeps if he finds no blocks.  This prevents a "burst" of writing when a redo log switches.

 
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