WAL

The principle of WAL (Write-Ahead Logging) is that all modification operations must be sequentially appended to the WAL log file before being written to the actual data file, used to achieve transaction durability and crash recovery.

Notable implementations of append-only logs include Apache Kafka's commit log and database write-ahead logs (WAL).

WAL (Write-Ahead Logging) ensures that even if the system crashes, a consistent state can be recovered from the log.

Every Postgres update must go through the WAL flush to disk and acquire a row lock, and the old row is not deleted immediately but left for the vacuum process to clean up.

In older versions of Qdrant, proxy segments did not persist intermediate state, so the WAL could not be truncated, and recovery time on node restart degraded from seconds to tens of minutes.

Slot lag is a typical operational problem in PostgreSQL logical replication: a logical replication slot retains WAL logs on the primary until the subscriber confirms consumption, and lagging or disconnected subscribers cause WAL to keep accumulating on the primary, bloating the disk and even filling up the primary.

During recovery one should read the tail rather than the full text, because everything needed to run is in the state files and the last WAL line.

The 1983 disk checkpoint scheme shares the same origin as this article's WAL state scheme: replacing wholehearted trust in mutable state with a small number of ordered, append-only records.

Common technical implementations of an append-only ledger include blockchain underlying data structures, WAL (Write-Ahead Log), and event sourcing (Event Sourcing).

The concept of durable execution was first popularized by workflow engines such as Temporal and Cadence, and its theoretical roots can be traced back to database transaction logs and WAL (Write-Ahead Logging).

Temporarily disabling WAL synchronization during imports (e.g., by using UNLOGGED TABLEs or wal_level=minimal) can trade higher throughput for a loss of data durability guarantees.

Postgres's native COPY command can deliver several times to tens of times the speed of row-by-row INSERT, but is still constrained by bottlenecks such as single-process execution, row-format parsing, and full WAL writes.

WAL (Write-Ahead Log) is the core mechanism by which Postgres guarantees crash recovery and primary-replica replication; every data change must be written to the WAL before being flushed to disk, making it a significant source of I/O amplification during large bulk writes.

Every time SQLite commits a transaction, it needs to flush the WAL to disk to ensure durability, which involves the fsync system call and can take several milliseconds on mechanical hard drives.

PostgreSQL internally contains multiple highly coupled subsystems, including a query parser, planner/optimizer, executor, storage manager (including MVCC multi-version concurrency control), and WAL write-ahead logging.

Traditional Postgres PITR relies on a base full backup plus WAL archiving, requiring restoration of the full backup followed by serial replay of all WAL logs during recovery.

PostgreSQL supports backup via pg_basebackup combined with WAL archiving.

The underlying principle of PITR relies on Postgres's Write-Ahead Log (WAL), where all data changes are sequentially appended to the WAL before being written to the data files.

WAL files are often easier to interpret than main data files because they record every change operation sequentially in append-write order.

SQLite's WAL mode supports concurrent reads and writes without mutual blocking by first recording write operations to a separate log file.