What is Delayed Durability in SQL Server

 Delayed Durability is a SQL Server feature where a transaction can be considered committed before its log records are physically flushed to disk. Normally, SQL Server follows Write-Ahead Logging (WAL) and waits for the transaction log to be written to disk before returning success to the client. With Delayed Durability enabled, SQL Server returns COMMIT success immediately and postpones the log flush. This improves transaction throughput and reduces log I/O waits, but introduces a risk: if SQL Server crashes before the log is flushed, recently committed transactions can be lost.

Normal Durability vs Delayed Durability

Feature	Normal Commit	Delayed Durability
Log Flushed Before Success	Yes	No
Data Loss Possible	No	Yes
Throughput	Lower	Higher
WRITELOG Waits	More	Less
Suitable for Banking	Yes	No
Suitable for Logging/Audit Systems	Sometimes	Yes

 

Delayed Durability is used in high-throughput systems where transaction log flushes become a bottleneck and occasional loss of recently committed transactions is acceptable. Typical use cases include telemetry systems, logging frameworks, audit data collection, IoT sensor ingestion, clickstream tracking, caching systems, and staging databases. By delaying log flushes, SQL Server reduces WRITELOG waits and increases transaction throughput significantly.