IBM DS8000 manual Logical volumes, Volume groups

￿There are thresholds that warn you when you are nearing the end of space utilization on the extent pool.

￿There is a Reserve space option that will prevent the logical volume from being created in reserved space until the space is explicitly released.

Note: A user can't control or specify which ranks in an extent pool are used when allocating extents to a volume.

Logical volumes

Logical volumes, also know as LUNs when configured for open systems or CKD volumes when configured for zSeries, can only be made from one or more extents residing in the same extent pool. This means that a logical volume cannot span multiple extent pools. Ranks must be added to extent pools to make larger LUNs. We use the terms volume and LUN interchangeably throughout the rest of this chapter when referring to logical volumes.

Some limitations are:

￿A specific volume is in one LSS.

￿Multiple volumes in one extent pool or one rank can be in the same or different LSSs, as shown in Figure 10-6 on page 198.

￿Multiple volumes in different extent pools and on different ranks can be in the same LSS, also as shown in Figure 10-6.

￿The minimum volume/LUN size is one extent.

–For CKD the minimum size is a 3390 Mod1.

–For FB the minimum size is 1GB.

Note: The user can specify volume sizes in binary, decimal, or block sizes. When you specify binary form, 1 GB is equal to 1073741924 bytes, instead of the decimal size, as in the ESS, where 1GB is 1000000000 bytes.

￿The maximum volume/LUN size is equal to the size of the extent pool, with the following limitations: 56 GB for CKD, with appropriate zSeries software support, and 2 TB for FB. For example, if only one rank was residing in the extent pool, then the maximum LUN size would be equal to the capacity of that one rank.

￿The maximum number of logical volumes at GA for the DS8000 is 64K; of the 64K LUNs, a maximum of 32K can be for FB and 32K can be for CKD.

￿Volumes can be deleted and the extents reused without having to format the ranks or arrays they reside in.

Volume groups

A volume group is a collection of logical volumes. Volume groups are created to provide FB LUN masking by assigning logical volumes and host attachments to the same volume group. For CKD volumes, one volume group for ESCON and FICON attachment with an anonymous host attachment is automatically created.

Volume groups can be thought of as LUN groups. Do not confuse the term volume group here with that of volume groups on pSeries. The DS Storage Manager volume groups have the following properties:

￿Volume groups enable FB LUN masking.

194DS8000 Series: Concepts and Architecture