A backlog that is too small
High write rate against the 1 MB default, which covers under a second
writes-per-second: 50000 bytes-per-write: 200 repl-backlog-size: 1mb dataset: 30gb
Redis Replication Bandwidth Calculator
Give the write rate and the replica count. The important output is how long the backlog covers, because past that a reconnecting replica transfers the entire dataset again.
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Examples
Real input you can load into the tool above. Each one shows a different thing going wrong, because that is what the tool is for.
A sized backlog
The same write rate with a backlog covering several minutes
writes-per-second: 50000 bytes-per-write: 200 repl-backlog-size: 3gb dataset: 30gb
Common mistakes
These are the ones that fail silently. The config is accepted, nothing raises an error, and the consequence arrives later.
Leaving repl-backlog-size at the default
1 MB covers well under a second on a busy instance, so any brief disconnection forces a full resync of the whole dataset.
Instead:Size it for the longest expected disconnection, including a replica restart.
Forgetting each replica multiplies outbound traffic
Every replica receives the full write stream. Three replicas is three times the outbound bandwidth, and across zones it is billed.
Instead:Include the replica count in the network budget.
Restarting replicas together
Simultaneous full resyncs saturate the primary's network and disk at once.
Instead:Stagger them, and watch for the resync completing before starting the next.
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