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<firstterm>Checkpoints</firstterm><indexterm><primary>checkpoint</></> are points in the sequence of

transactions at which it is guaranteed that the data files have

been updated with all information logged before the checkpoint. At

checkpoint time, all dirty data pages are flushed to disk and a

special checkpoint record is written to the log file. As result, in

the event of a crash, the recoverer knows from what point in the

log (known as the redo record) it should start the REDO operation,

since any changes made to data files before that record are already

on disk. After a checkpoint has been made, any log segments written

before the redo record are no longer needed and can be recycled or

removed. (When <acronym>WAL</acronym> archiving is being done, the