Recap

Log Sequence Numbers (LSN)

• LSNs identify log records and are linked in a backward chain per transaction using prevLSN.
• pageLSN allows comparison of data pages and log records to determine the last written entry.

Main Ideas of ARIES

• WAL with STEAL/NO-FORCE
• Fuzzy Checkpoints (snapshot of dirty page IDs)
• Compensation Log Records (CLRs) ensure undo actions persist
• ATT (Active Transaction Table) tracks transactions at the crash moment
• DPT (Dirty Page Table) identifies which pages might not be on disk

Phases of ARIES

Analysis Phase

Steps:

1. Start scanning from the last checkpoint.
2. If a <TXN-END> record is found, remove the transaction from ATT.
3. For other records:

• Add the transaction to ATT (status = UNDO).