Overview of Checkpoints

Question 1

Overview of Checkpoints

Answer 1

+ Checkpoint is a crucial mechanism in instance db shutdowns, instance recovery and Oracle db operations generally
+ Datastructures that indicate the checkpoint position, which is the SCN in the redo stream where instance recovery must begin
+ The checkpoint position is determined by the oldest dirty buffer in the db buffer cache.
+ The checkpoint position acts as a pointer to the redo stream and is stored in the control file and in each data file header.
+ The writing of modified db buffers in the db buffer cache to disk

Question 2

Purpose of Checkpoints

Answer 2

+ Reduce the time required for recovery in case of an instance or media failure
+ Ensured regular db writes of dirty buffers to disk
+ Ensures the db writes all committed data to disk during a consistent shutdown

Question 3

When Oracle Database Initiates Checkpoints

Answer 3

The CKPT is responsible for writing checkpoints to the data file headers and control file.
Checkpoints occur in the situations:
\+ Thread checkpoints
\+ Tablespace and data file checkpoints
\+ Incremental checkpoints

Question 4

Thread checkpoints

Answer 4

Thread checkpoints - db writes to disk all buffers modified by redo in a specific thread before a certain target.
\+ Consistent db shutdown
\+ ALTER SYSTEM CHECKPOINT statement
\+ Online red log switch
\+ ALTER DATABASE BEGIN BACKUP statement

Question 5

Tablespace and data file checkpoints

Answer 5

Tablespace and data file checkpoints
+ The db writes to disk all buffers modified by redo before a specific target.
+ A tablespace checkpoint is a set of datafile checkpoints, one for each data file in the tablespace.
+ These chekpoints occur in a variety of situations, including making a tablespace read-only or taking it offline normal, shrinking a data file, or executing ALTER TABLESPACE BEGIN BACKUP

Question 6

Incremental checkpoints

Answer 6

Incremental checkpoints

+ Is a thread checkpoint partly intented to avoid writing large number of blocks at the online redo log switch.