Crash Recovery

Why Recovery is Needed?

• Recovery algorithms ensure database consistency, atomicity, and durability even after failures.
• Recovery consists of two steps:
  1. Actions during normal execution to prepare for possible failures.
  2. Actions after failure to restore the database.

Log Sequence Numbers (LSN)

Definition:

• Each log entry has a unique Log Sequence Number (LSN).
• The log is a sequential, append-only file.
• Various components track LSNs to ensure recovery.

Types of LSNs

Writing Log Records

1. Each data page contains a pageLSN (most recent update).
2. The system tracks flushedLSN (max LSN flushed to disk).
3. Before writing a page to disk, WAL ensures:

pageLSN <= flushedLSN

(Ensuring the log is written before the page is modified.)

Rule: The log record must be written before the actual page update.