Chapter 6. Storage pools 233
When you add MDisks to pools, you must assign them to the correct storage tiers. It is 
important to set the tiers correctly if you plan to use the Easy Tier feature. Otherwise, the 
wrong tier can corrupt Easy Tier algorithm logic and might affect system performance. For 
more information about storage tiers, see Chapter 10, “Advanced features for storage 
efficiency” on page 403. 
The storage tier setting can be also changed after the MDisk is assigned to the pool.
Check the Externally encrypted checkbox if back-end storage currently performs data 
encryption. For details on IBM Storwize V7000 encryption, see Chapter 12, “Encryption” on 
page 603.
After the task is completed, click Close.
The external MDisks that are assigned to a pool within IBM Storwize V7000 are displayed in 
Pools → MDisks by Pools. 
When a new MDisk is added to the pool that already has other MDisks and volumes, the 
system does not immediately redistribute extents that are used by volumes between new and 
existing MDisks. This task is performed by the Easy Tier feature. It automatically balances 
volume extents between the MDisks to provide the best performance to the volumes.
The system decision to turn on Easy Tier for pool movement of extents—between tiers of 
storage (inter-tier) or between MDisks within a single tier (intra-tier)—is based on the activity 
that is monitored. Therefore, when you add an MDisk to a pool, extent migration is not 
performed immediately. No migration of extents occurs until there is sufficient activity to 
trigger it. 
If Easy Tier is turned off, no extent migration is performed. Only newly allocated extents can 
be written to a new MDisk.
For more information about IBM Easy Tier feature, see Chapter 10, “Advanced features for 
storage efficiency” on page 403.
To assign an external MDisk to a storage pool with CLI, use the addmdisk command. You 
need to specify the MDisk name or ID, MDisk tier, and target storage pool, as shown in 
Example 6-25. The command returns no feedback.
Example 6-25   addmdisk example
IBM_Storwize:ITSOV7K:superuser>addmdisk -mdisk mdisk3 -tier enterprise Pool0
IBM_Storwize:ITSOV7K:superuser>
6.3.4  Actions on external MDisks
External MDisks support specific actions that are not supported on arrays. Some actions are 
supported only on unmanaged external MDisks, and some are supported only on managed 
external MDisks.
Note: If the external storage LUs to virtualize behind the Storwize V7000 contain data and 
this data needs to be retained, do not use the “Assign to pool” option to manage the 
MDisks. This option destroys the data on the LU. Instead, use the Import option. For more 
information, see Chapter 9, “Storage migration” on page 385.