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Etcd is designed to be able to cache it's entire working set in memory.

https://etcd.io/docs/v3.3.12/dev-guide/limit/ https://github.com/etcd-io/etcd/blob/master/Documentation/op...

TiKV, a distributed kv store that can store many terabytes of data, actually uses etcd internally for metadata storage.

Most databases fit in memory though.

Many do, and even large DBs can be accomodated by throwing terabytes of RAM at the problem, but etcd is fundamentally targeted at a different problem domain than a general-purpose database. Everything in etcd is represented by a single-threaded state machine, which is great for simplicity and correctness, but that comes with performance limitations.

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