Is Redis a memory only store like memcached or does it write the data to the disk

Question

Is Redis memory only store like memcached or does it write the data to the disk? If it does write to the disk, how often is the disk written to?

Answer 1

Redis persistence is described in detail here:

http://redis.io/topics/persistence

By default, redis performs snapshotting:

By default Redis saves snapshots of the dataset on disk, in a binary file called dump.rdb. You can configure Redis to have it save the dataset every N seconds if there are at least M changes in the dataset, or you can manually call the SAVE or BGSAVE commands. For example, this configuration will make Redis automatically dump the dataset to disk every 60 seconds if at least 1000 keys changed: save 60 1000

Another good reference is this link to the author's blog where he tries to explain how redis persistance works:

http://antirez.com/post/redis-persistence-demystified.html

Answer 2

Redis holds all data in memory. If the size of an application's data is too large for that, then Redis is not an appropriate solution.

However, Redis also offers two ways to make the data persistent:

1) Snapshots at predefined intervals, which may also depend on the number of changes. Any changes between these intervals will be lost at a power failure or crash.

2) Writing a kind of change log at every data change. You can fine-tune how often this is physically written to the disk, but if you chose to always write immediately (which will cost you some performance), then there will be no data loss caused by the in-memory nature of Redis.