A write-heavy instance
Where the AOF grows quickly and rewrites are frequent
dataset: 20gb writes-per-second: 20000 bytes-per-write: 200
Redis RDB and AOF Size Calculator
Give the dataset size and the write rate. The output covers RDB size, AOF growth, and the peak disk a rewrite needs, plus the memory the fork requires.
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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 read-heavy instance
The same dataset with light writes, where the AOF barely grows
dataset: 20gb writes-per-second: 200 bytes-per-write: 150
Common mistakes
These are the ones that fail silently. The config is accepted, nothing raises an error, and the consequence arrives later.
Sizing the machine for the dataset only
BGSAVE and AOF rewrite fork. Copy-on-write means the extra memory needed is proportional to what is WRITTEN during the save, and on a busy instance approaches a full second copy.
Instead:Size for the fork. This is the most common cause of a Redis instance being OOM killed.
Provisioning disk for the dataset size
A rewrite needs the old and the new AOF at the same time, plus the RDB.
Instead:Provision for the peak. A full disk during a rewrite stops writes with MISCONF.
Leaving stop-writes-on-bgsave-error off
Its default of yes makes a failed save loud. Turning it off means a full disk produces no symptom until you need the snapshot.
Instead:Leave it on, and alert on rdb_last_bgsave_status.
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