Is redis a durable datastore?

Question

By "durable" I mean, the server can crash at any time, and as long as the disk remains in tact, no data is lost (see ACID). Seems like that's what journaling mode is for, but if you enable journaling, doesn't that defeat the purpose of operating on in-memory data? Read operations might not be affected by journaling, but it seems like journaling would kill your write performance.

Answer 1

Redis is not usually deployed as a "durable" datastore (in the sense of the "D" in ACID.), even with journaling. Most use cases intentionally sacrifice a little durability in return for speed.

However, the "append only file" storage mode can optionally be configured to operate in a durable manner, at the cost of performance. It will have to pay for an fsync() on every modification. To configure this, set these two options in your .conf file:

 appendonly yes
 appendfsync always

From the docs: How durable is the append only file?

Check redis.conf, you can configure how many times Redis will fsync() data on disk. There are three options:

• Fsync() every time a new command is appended to the append log file. Very very slow, very safe.
• Fsync() one time every second. Fast enough, and you can lose 1 second of data if there is a disaster.
• Never fsync(), just put your data in the hands of the Operating System. The faster and unsafer method.

(Note that the default for appendfsync in the configuration file shipping with Redis post-2.0.0 is everysec, and not always.)