Disabling commitlog in Cassandra

Question

When restarting a Cassandra node a lot of time is spend on replaying the commitlog to achieve consistency. In our application, it is more important to bring the node back up and running fast, than to achieve consistency. Therefore we have set “durable_writes = false” on all our manually created keyspaces to disable the commitlog. (We have not touched the system keyspaces). Nevertheless, when we restart a note it still uses about one hour on replaying the commitlog.

• What is left in my commitlog?

• Can I in any way investigate the content of the commitlog?

• How can the commitlog be turned off (if not durable_writes = false)?

Answer 1

durable_writes is set per keyspace, so if there are any keyspaces with it still enabled there will still be mutations in the commitlogs to replay on startup. You may want to walk output of describe schema.

There are some tables (ie system) that you want to keep durable, but it shouldn't have that much to cause an impact to startup. When starting up it logs out which keyspace/tables its reading so you can check which ones its replaying.

One hour is a very long time and has a certain smell to it, there may be something else going on here and probably warrants additional investigation. Some ideas is to check the logs and make sure it is the commitlog replay thats taking time (not rebuilding index summaries or something). Also check that there are not old commit logs that C* doesn't have permissions to delete or something that would stick around.