使用 Citus PostgresQL 需要注意哪些设置

Question

We are looking into using CitusDB. After reading all the documentation we are not clear on some fundamentals. Hoping somebody can give some directions.

In Citus you specify a shard_count and a shard_max_size , these settings are set on the coordinator according to the docs (but weirdly can also be set on a node).

What happens when you specify 1000 shards and distribute 10 tables with 100 clients?

1. Does it create a shard for every table (users_1, users_2, shops_1, etc.) (so effectively using all 1000 shards.

2. If you would grow with another 100 clients, we already hit the 1000 limit, how are these tables partitioned?

3. The shard_max_size defaults to 1Gb. If a shard is > 1Gb a new shard is created, but what happens when the shard_count is already hit?

4. Lastly, is it advisible to go for 3000 shards? We read in the docs 128 is adviced for a saas. But this seams low if you have 100 clients * 10 tables. (I know it depends.. but..)

Answer 1

Former Citus/current Microsoft employee here, chiming in with some advice.

Citus shards are based on integer hash ranges of the distribution key. When a row is inserted, the value of the distribution key is hashed, the planner looks up what shard was assigned the range of hash values that that key falls into, then looks up what worker the shard lives on, and then runs the insert on that worker. This means that the customers are divided up across shards in a roughly even fashion, and when you add a new customer it'll just go into an existing shard.

It is critically important that all distributed tables that you wish to join to each other have the same number of shards and that their distribution columns have the same type. This lets us perform joins entirely on workers, which is awesome for performance.

If you've got a super big customer (100x as much data as your average customer is a decent heuristic), I'd use the tenant isolation features in advance to give them their own shard. This will make moving them to dedicated hardware much easier if you decide to do so down the road.

The shard_max_size setting has no effect on hash distributed tables. Shards will grow without limit as you keep inserting data, and hash-distributed tables will never increase their shard count under normal operations. This setting only applies to append distribution, which is pretty rarely used these days (I can think of one or two companies using it, but that's about it).

I'd strongly advise against changing the citus.shard_count to 3000 for the use case you've described. 64 or 128 is probably correct, and I'd consider 256 if you're looking at >100TB of data. It's perfectly fine if you end up having thousands of distributed tables and each one has 128 shards, but it's better to keep the number of shards per table reasonable.