SQL SERVER – Basics of Delayed Durability in SQL Server 2014

SQL SERVER - Basics of Delayed Durability in SQL Server 2014 durability SQL Server 2014 has introduced many new features and one of performance is Delayed Transaction Durability. This concept is indeed very interesting. To explain this feature in detail, we will require to understand what is Full Transaction Durability. The current default of SQL Server is Full Transaction Durability. A common question I often received is why would we go away from to delayed durability. The answer is – Performance prioritation over Durability.

I try to stay as brief as possible to explain various concepts over here. I will try to convert this blog post in questions, and answer format so it is easy to understand.

What is Full Transaction Durability?
In Full Transaction Durability transactions write the transaction log to disk before returning control to the client / user. This means your data is safe on disk before it is committed. SQL Server uses Write Ahead Log logic for this default behavior of Full Transaction Durability.

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What is Delayed Transaction Durability?
In Delayed Transaction Durability transactions are written asynchronously and in batches to the disk before returning control to the client / user. This means your transaction log records are kept buffer and written to disk either when it is filled up or buffer flush happens.

When to use Full Transaction Durability?
You should full transaction durability when you can’t afford any data loss.

When to use Delayed Transaction Durability?
You should use Delayed Transaction Durability when performance is priority over data loss.

What particular sign I should observe in my SQL Server, which suggests that if I implement Delayed Durability it will improve my performance?
Well, if your performance issue is due to latency in transaction log writes, or in another word, if you notice the bottleneck on transaction log writes, you can implement delayed transaction durability.

When are transaction written to the disk in case of delayed transaction durability?
There are three different scenarios, when in-memory transaction logs are flushed to the disk (as per MSDN).

  • A fully durable transaction in the same database makes a change in the database and successfully commits.
  • The user executes the system stored procedure sp_flush_log successfully.
  • The in-memory transaction log buffer fills up and automatically flushes to disk. (Update: One of my SQL Expert friend suggest this statement requires amendment and it is on the way, I will update once I have more details).

Well, that’s it for today. In upcoming blog posts we will see a working demo for Delayed Transaction Durability.

Reference: Pinal Dave (https://blog.sqlauthority.com)

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