WAL — Write-Ahead Log
Definition
A database technique where changes are first written to a sequential log file before being applied to the main database files, ensuring durability and crash recovery.
Context & Usage
WAL is fundamental to ACID-compliant databases. Examples: PostgreSQL WAL, SQL Server transaction log, ZFS ZIL, ext4 journal. Hardware impact: WAL files demand low-latency sequential writes — typically placed on dedicated NVMe (Intel Optane historically, Samsung PM1735 / Micron 7450 MAX modern).
Examples
- PostgreSQL WAL files on dedicated NVMe
- SQL Server log file (.ldf) on Optane
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