Synchronous Commit

Synchronous Commit

While I was at PGConf.EU a couple of weeks ago in Madrid, I attended a talk by Grant McAlister discussing Amazon RDS for PostgreSQL.  While it was interesting to see how Amazon had made it very simple for developers to get a production PostgreSQL instance quickly, the thing that really caught my eye was the performance benchmarks comparing the fsync and synchronous commit parameters.

sync_commitFrighteningly, it is not that uncommon for people to turn off fsync to get a performance gain out of their PostgreSQL database. While the performance gain is dramatic, it carries the risk that your database could become corrupt. In some cases, this may be OK, but these cases are really rather rare. A more common case is a database where it is OK to lose a little data in the event of a crash. This is where synchronous commit comes in. When synchronous commit is off, the server returns back success immediately to the client, but waits to flush the data to disk for a short period of time. When the data is ultimately flushed it is still properly sync to disk so there is no chance of data corruption. The only risk if the event of a crash is that you may lose some transactions. The default setting for this window is 200ms.

In Grant’s talk, he performed a benchmark that showed turning off synchronous commit gave a bigger performance gain than turning off fsync. He performed an insert only test so I wanted to try a standard pgbench test. I didn’t come up with the same results, but the I still saw a compelling case for leaving fsync on while turning off synchronous commit.

I ran a pgbench test with 4 clients and a scaling factor of 100 on a small EC2 instance running 9.3.5. What I saw was turning off fsync resulted in a 150% performance. Turning off synchronous commit resulted in a 128% performance gain. Both are dramatic performance gains, but the synchronous commit option has a lot less risk.


Speaking of conferences, the call for papers is open for PGConf US 2015. If there is a topic you’d like to present in New York in March, submit it here.