A large dataset
400 GB across 64 GB nodes with one replica each
dataset: 400gb node-memory: 64gb replicas: 1
Redis Cluster Sizing
Give the dataset size and the memory per node. The result assumes only part of each node is usable for data, because replication buffers, client buffers and the fork a save performs all live outside maxmemory.
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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.
No replicas
The same cluster with no failover, which is a cluster that cannot survive a node loss
dataset: 400gb node-memory: 64gb replicas: 0
Common mistakes
These are the ones that fail silently. The config is accepted, nothing raises an error, and the consequence arrives later.
Setting maxmemory to the machine's memory
Replication buffers, client output buffers and the copy-on-write during a BGSAVE all sit outside it. The process is then OOM killed during a save.
Instead:60 to 70% of the machine is the usual starting point.
Running a cluster with fewer than three primaries
Failover needs a majority of primaries to agree a node is down, which is impossible below three.
Instead:Three primaries minimum, even when one would hold the data.
Assuming an even slot split means even load
Slots are the unit of assignment, not of work. A hash tag concentrating a large tenant puts all of it on one node.
Instead:Check per-node memory, not slot counts.
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