What is the best size for cache tier in Ceph?

Question

I have a Ceph Cluster with 500 TB of capacity. I want to create cache tier for it.

I have a 20TB SSD. Is that sufficient for use 20 TB for cache 500TB? what is the best way to calculate it?

Answer 1

Also please consider the following:

It is important to understand the type or workload (I/Os) will you receive. Usually when you have many customers on the cluster the cache fills up quickly and it will not help as much.

Answer 2

Traditionally, we recommend one SSD cache drive for 5 to 7 HDD. properly, today, SSDs are not used as a cache tier, they cache at the Bluestore layer, as a WAL device.

Depending on the use case, capacity of the Bluestore Block.db can be 4% of the total capacity (Block, CephFS) or less (Object store).

Especially for a small Ceph cluster (less than 1 PB raw storage), it makes sense to consider all-SSD storage, particularly for the Block use case, as prices of solid state and HDD are not that distant for medium capacity drives.