Checkpoints

Checkpoints

When System failure occurs

o We must consult log to determine those transaction that need to be redone and those transaction that need to be undone.

o We need to search entire log to determine this information.

There are two major difficulties with this

o Search process is time consuming

o Most of transaction that need to be redone have already written their update into the database. Although redoing them will cause no harm.

• To reduce this type of overhead, we introduced checkpoints.

• During execution system maintains the log, using one of the two techniques Deferred update or Immediate update.

• In addition, System periodically performs checkpoints, which requires following sequence of action to take place

• Output onto stable storage all log records currently residing in main memory.

• Output to the disk all modified buffer blocks

• Output onto stable storage a log record.

• Transactions are not allowed to perform any update actions such as

• Writing to a buffer block or

• Writing to log record

While a checkpoint is in progress.

• Checkpoint can be occur automatically and manually.

• Checkpoints continue to occur whenever the database is shut down (normal or immediate) or when a redo log switch occurs.

• To force a checkpoint, issue the following SQL command:

o alter system switch logfile;

o Alter system checkpoint;

Example :

• T1 can be ignored (updates already output to disk due to checkpoint)

• T2 and T3 redone.

• T4 undone