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What is the difference between Redis's diskless replication and disk-based replication?

When a new replica connects (or an existing one falls too far behind to catch up incrementally), the master needs to send it a full copy of the dataset to bootstrap from, and Redis supports two different mechanisms for producing that initial transfer.

Disk-based ReplicationDiskless Replication
Master writes a full RDB snapshot to a local disk file first, then transfers that file to the replica.Master streams the RDB snapshot data directly over the network socket to the replica, without writing it to local disk first.
Extra disk I/O and disk space needed for the temporary snapshot file.Avoids that disk I/O and space entirely, generating the snapshot data on the fly during transfer.
Can be beneficial if disk I/O is fast and network is comparatively slow, since the file is ready before transfer starts.Beneficial when disk I/O or space is the constrained resource, e.g. on cloud instances with limited or slow local disk.
repl-diskless-sync norepl-diskless-sync yes

Diskless replication is generally the better default on modern cloud infrastructure where local disk can be a scarcer or slower resource than network bandwidth, which is why it's commonly enabled in production. repl-diskless-sync-delay adds a small configurable wait before starting a diskless sync, giving multiple replicas that connect around the same time a chance to be served by a single generated snapshot stream instead of the master redundantly generating one per replica.

What does diskless replication avoid, compared to disk-based replication?
What does repl-diskless-sync-delay help with?

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