Checkpoints
To avoid rescanning the whole logfile during the analysis phase it is advisable to save the DPT and the TT regularly to the logfile, forming a checkpoint. Instead of having to run through the whole file it is just necessary to run backwards until a checkpoint is found. From that point it is possible to restore the DPT and the TT as they were at the time of the crash by reading the logfile forward again. Then it is possible to proceed as usual with Redo and Undo.
The naive way for checkpointing involves locking the whole database to avoid changes to the DPT and the TT during the creation of the checkpoint. Fuzzy logging circumvents that by writing two log records. One Fuzzy Log Starts Here record and, after preparing the checkpoint data, the actual checkpoint. Between the two records other logrecords can be created. During recovery it is necessary to find both records to obtain a valid checkpoint.
Read more about this topic: Algorithms For Recovery And Isolation Exploiting Semantics