How the Redis calculator works

01 maxmemory and the eviction policy

• maxmemory is the most memory Redis uses for data; past it Redis applies the eviction policy. 0 means no limit, the default on 64-bit systems. Redis docs
• allkeys-lru evicts the least recently used keys, and Redis calls it a good default when you have no reason to prefer another. noeviction instead returns errors for writes at the limit. Redis docs The calculator writes allkeys-lru, taking this Redis to be a cache. Assumption
• Buffers for replication and the AOF don't count towards maxmemory, so Redis can use somewhat more. Redis docs

02 From used_memory to maxmemory

maxmemory           = max(peak × (1 + limit margin), average × (1 + request margin))   rounded up to a whole mb
memory to provision = 2 × maxmemory

The margins come from the sensitivity, workload type and environment you choose; the sizing model explains them, and they are assumptions too. In Redis's configuration, mb is 1024 × 1024 bytes, so MiB pass through unconverted. Redis docs

The samples are used_memory, which includes Redis's own overhead and not only the data, so a maxmemory sized from them is on the generous side. Assumption

Worked example

A cache peaking at 630 MiB of used_memory, medium sensitivity, production:

A steady cache, averaging 1000 MiB with a 1010 MiB peak at high sensitivity, where the average decides:

03 Memory for the process

• In a write-heavy application, Redis can use up to twice its normal memory while it saves an RDB file or rewrites the AOF, because the forked child shares pages that writes then copy. Redis docs
• With replication, Redis makes RDB saves even with persistence off, unless replication is diskless. Redis docs
• Redis also advises setting maxmemory below the free memory, to leave room for its overhead and fragmentation: with 10 GB free, set it to 8 or 9 GB. Redis docs

So the calculator provisions the worst case, 2 × maxmemory. Assumption Without persistence or replication, the note suggests about 1.25 × maxmemory, which is Redis's 10-to-8 ratio turned around. Assumption

04 What the calculator writes

maxmemory 819mb
maxmemory-policy allkeys-lru

• Both lines go in redis.conf, or at runtime with CONFIG SET maxmemory 819mb, as the eviction guide shows. Redis docs
• CONFIG SET takes effect from the next command, and CONFIG REWRITE writes the running configuration back to the redis.conf the server started with. Redis docs

05 Every warning, and why

06 What to paste

redis_memory_used_bytes{instance="redis:6379"}

• redis_exporter publishes INFO's used_memory as redis_memory_used_bytes. redis_exporter source
• Without it, sample redis-cli INFO memory and paste the used_memory: lines; the calculator reads them as bytes.

07 All assumptions in one place

Besides the margins of the shared sizing model:

Our default: a value we chose. Our reading: how we read documentation that isn't explicit. About your setup: something we take for granted about your data or cluster.

Values you can change

The calculator’s Advanced: margins and defaults section, closed by default, lets you replace these values when your setup differs. Each field’s “?” says how it moves the result; a value outside the range is used at its nearest end, and “Reset to defaults” puts them all back.

08 References

• Key eviction — maxmemory, the policies, and buffers not counted.
• Redis administration — up to 2× during saves, setting maxmemory below free memory, replication.
• redis.conf — memory units.
• CONFIG SET and CONFIG REWRITE.
• redis_exporter source — redis_memory_used_bytes.

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