You must have identified a valid aggregate composed of HDDs to convert to a Flash Pool aggregate.
You must have determined write-caching eligibility of the volumes associated with the aggregate, and completed any required steps to resolve eligibility issues.
You must have created an SSD storage pool to provide the SSD cache to this Flash Pool aggregate.

Any allocation unit from the storage pool that you want to use must be owned by the same node that owns the Flash Pool aggregate.
You must have determined how much cache you want to add to the aggregate.

You add cache to the aggregate by allocation units. You can increase the size of the allocation units later by adding SSDs to the storage pool if there is room.
You must have determined the RAID type you want to use for the SSD cache.

After you add a cache to the aggregate from SSD storage pools, you cannot change the RAID type of the cache RAID groups.
You must have determined the maximum cache size for your system and determined that adding SSD cache to your aggregate will not cause you to exceed it.

You can see the amount of cache that will be added to the total cache size by using the storage pool show command.
You must have familiarized yourself with the configuration requirements for Flash Pool local tier.

About this task

If you want the RAID type of the cache to different from that of the HDD RAID groups, you must specify the cache RAID type when you add the SSD capacity. After you add the SSD capacity to the aggregate, you can no longer change the RAID type of the cache.

After you add an SSD cache to an aggregate to create a Flash Pool aggregate, you cannot remove the SSD cache to convert the aggregate back to its original configuration.

ONTAP System Manager

Beginning with ONTAP 9.12.1, you can use ONTAP System Manager to add SSDs to an SSD storage pool.

Steps

Click Storage > Tiers and select an existing local HDD storage tier.
Click kabob icon and select Add Flash Pool Cache.
Select Use Storage Pools.
Select a storage pool.
Select a cache size and RAID configuration.
Click Save.
Locate the storage tier again and click kabob icon.
Select More Details and verify that the Flash Pool shows as Enabled.

CLI

Steps

Mark the aggregate as eligible to become a Flash Pool aggregate:

storage aggregate modify -aggregate aggr_name -hybrid-enabled true

If this step does not succeed, determine write-caching eligibility for the target aggregate.
Show the available SSD storage pool allocation units:

storage pool show-available-capacity
Add the SSD capacity to the aggregate:

storage aggregate add aggr_name -storage-pool sp_name -allocation-units number_of_units

If you want the RAID type of the cache to be different from that of the HDD RAID groups, you must change it when you enter this command by using the raidtype parameter.

You do not need to specify a new RAID group; ONTAP automatically puts the SSD cache into separate RAID groups from the HDD RAID groups.

You cannot set the RAID group size of the cache; it is determined by the number of SSDs in the storage pool.

The cache is added to the aggregate and the aggregate is now a Flash Pool aggregate. Each allocation unit added to the aggregate becomes its own RAID group.
Confirm the presence and size of the SSD cache:

storage aggregate show aggr_name

The size of the cache is listed under Total Hybrid Cache Size.