Currently, a tiered storage pool must have two tiers:
- Tier 0 is used for metadata, and the best-performing storage should be designated as Tier 0.
- Tier 1 is used for user data.
When creating a tiered storage pool, at least four unused SDs must be available for each tier. When you create a tiered storage pool, you first create the user data tier (Tier 1), then you create the metadata tier (Tier 0).
During normal operation, one tier of a tiered storage pool might become filled before the other tier. In such a case, you can expand one tier of the storage pool without expanding the other tier. When expanding a tier, you must:
- Make sure that the SDs being added to the tier have the same performance characteristics as the SDs already in the tier. For example, do not add NL SAS (near line SAS) based SDs to a tier already made up of flash drives.
- Add SDs to the tier. See the span-create man page for more information about minimum SD counts and creating storage pools.