Monday, 6 July 2015

SQL Server 2014 New Features - Delayed Durability


Writing the transaction log records to the disk is required for a transaction to be durable. This is one of the ACID properties.

SQL Server transaction commit process can be fully durable or delayed durable(lazy commit). In fully durable, transaction is synchronous between application\client and database in which SQL Server sends successful acknowledgement only after the log records are written to the disk. 

In delayed durable, transaction is asynchronous in which SQL Server sends commits as successful before the log records are written to the disk. Transaction commit process does not wait for the log I/O to finish. Log records are kept in memory which will automatically flushes to disk once the log buffer fills (or) user can manually executes the system stored procedure sp_flush_log to flush the content to disk. As log records are flushed to disk once the log buffer fills by default, single instance of flushing capacity will be in larger chunk which reduces I/O contention and increases throughput during concurrent transactions.

We can set the transaction durability in database level or in {atomic block\Transaction\Commit} level.