Safety of Replication when Master Has Persistence Turned off
In setups where Redis replication is used, it is strongly advised to have persistence turned on in the master and in the replicas. When this is not possible, for example because of latency concerns due to very slow disks, instances should be configured to avoid restarting automatically after a reboot.
- We have a setup with node A acting as master, with persistence turned down, and nodes B and C replicating from node A.
- Node A crashes, however it has some auto-restart system, that restarts the process. However since persistence is turned off, the node restarts with an empty data set.
- Nodes B and C will replicate from node A, which is empty, so they’ll effectively destroy their copy of the data.
Diskless Replication
Normally a full resynchronization requires creating an RDB file on disk, then reloading the same RDB from disk to feed the replicas with the data.
With slow disks this can be a very stressing operation for the master. Redis version 2.8.18 is the first version to have support for diskless replication. In this setup the child process directly sends the RDB over the wire to replicas, without using the disk as intermediate storage.