Extent pools

An extent pool is a logical construct that is used to manage a set of extents of the same type that are associated with a given rank group.

When an extent pool is defined, it must be assigned a rank group and an extent type. One or more ranks with the same extent type can be assigned to an extent pool, but a rank can be assigned to only one extent pool. There can be as many extent pools as there are ranks. There are no constraints on the number or size of ranks assigned to the extent pool. All extents of the ranks assigned to an extent pool are independently available for allocation to logical volumes that are associated with the extent pool.

Typically, all the ranks within an extent pool have the same characteristics relative to RAID type, disk-drive module (DDM) RPM, and DDM interface rate. This allows for a differentiation of logical volumes by assigning them to the appropriate extent pool for the desired characteristics. Different extent sizes for the same device type (for example, count-key-data or fixed block) can be supported on the same storage unit, but these different extent types must be in different extent pools.

You can use the same RAID types in an extent pool. When an extent pool is defined, you must assign the following attributes:

• Server affinity
• Extent type
• RAID type

When you create extent pools, the following rules apply:

• You must configure a minimum of two extent pools to use server 0 and server 1.
• More than one rank can reside in an extent pool, but two extent pools cannot be created from only one rank. You can create one extent pool from one rank, unless the LUN capacity is greater than the capacity of one rank in the extent pool.

Review the following general considerations:

• One rank per pool will not constrain addresses.
• Ranks can be added to an extent pool at any time.
• The logical volumes defined in one extent pool can be different LSSs.
• The logical volumes in different extent pools can be in the same LSS and are only limited by the odd and even server affinity.
• Ranks can be removed from an extent pool if no extents on the rank are currently assigned to the logical volumes.
• Deleting an extent pool releases the ranks that are assigned to it and makes those ranks available for assignment to another extent pool.
• Any extent can be used to make a logical volume.
• Thresholds warn you when you are nearing the end of usable space on the extent pool.
• A Reserve Space option prevents the logical volume from being created in reserved space until the space is explicitly released.