how does “Disk-backed” replication work in redis cluster

Question

the redis.conf says:

1) Disk-backed: The Redis master creates a new process that writes the RDB file on disk. Later the file is transferred by the parent process to the slaves incrementally

I just dont know what does "transferred by the parent process to the slaves" mean?

thank you

Answer 1

It is simple. First read the RDB file into a buffer, and use socket.write to send this to salve's port which is listenning this.

The implemention is more complex than what I said. But this is what redis do. You can refer the replication.c in redis/src for more details.

EDITED:

Yes, the disk-less mechanism just use the child process directly sends the RDB over the wire to slaves, without using the disk as intermediate storage.
Actually, if you use disk to save the RDB and redis master can serve many slaves at the same time without queuing. Once the disk-less replication serve on slave, and if another slave comes and want do a full sync, it need to be queued to wait for the first slave to finish. So there are another settings repl-diskless-sync-delay to wait more slave to do this parallel.

And these two method only occur after something wrong happens. In the normal case, the redis master and salve through a well connected wire to replicate the redis command the slave to keep the same between the master and slave. And if the wire is break or the slave fall down, then need do a partial resync action to obtain the part slave missed. If the psync is not possible to achieve, it will try do full resync. The full resync is what we talked about.

This is how a full synchronization works in more details:

The master starts a background saving process in order to produce an RDB file. At the same time it starts to buffer all new write commands received from the clients. When the background saving is complete, the master transfers the database file to the slave, which saves it on disk, and then loads it into memory. The master will then send all buffered commands to the slave. This is done as a stream of commands and is in the same format of the Redis protocol itself.

And the disk-less replication is just a new feature which supports the full-resync in that case to deal with the slow disk stress. More about it refer to https://redis.io/topics/replication . such as how do psync and why psync will fail, you can find answer from this article.