If you use external volumes as pool-VOLs, you can put the external volumes in tiers by setting the External LDEV Tier Rank for the external volumes. The External LDEV Tier Rank consists of the following three types: High, Middle, and Low. The following examples describe how tiers may be configured:

Example 1: Configuring tiers by using external volumes only

Tier 1: External volumes (High)

Tier 2: External volumes (Middle)

Tier 3: External volumes (Low)

Example 2: Configuring tiers by combining internal volumes and external volumes

Tier 1: Internal volumes (SSD)

Tier 2: External volumes (High)

Tier 3: External volumes (Low)

You can set the External LDEV Tier Rank when creating the pool, changing the pool capacity, or setting the Edit External LDEV Tier Rank window. The following table explains the performance priority (from the top) of data drives.

Priority	Data drive type
1	SSD
2	SAS 15K rpm
3	SAS 10K rpm
4	SAS 7.2K rpm
5	External volume* (High)
6	External volume* (Middle)
7	External volume* (Low)
*Displays as External Storage in the Drive Type/RPM.

Reserved pages for relocation operation: A small percentage of pages, normally 2, are reserved per tier to allow relocation to operate. These are the buffer spaces for tier relocation.

Tier relocation workflow: Tier relocation is performed taking advantage of the buffer space allocated for tier relocation, as mentioned previously. Tier relocation is also performed to secure the space reserved in each tier for new page assignment. The area is called the buffer space for new page assignments. When tier relocation is performed, Dynamic Tiering reserves buffer spaces for relocation and new page assignment.

During relocation, a tier may temporarily be assigned over 98% of capacity, or well under the allowance for the buffer areas.