IBM DS8000 manual Consolidate logical volumes, Considerations for new logical volume size, 295

“To avoid a single point of failure in a sysplex, IBM recommends that, for all couple data sets, you create an alternate couple datasets on a different device, control unit, and channel from the primary.”

You may interpret this to mean not just having separate LCUs, but rather separate physical control units, if possible.

Besides careful planning and depending on the configuration complexity, it might take weeks or months to complete the planning and to perform the actual migration.

14.1.2 Consolidate logical volumes

Another aspect of consolidation within a data migration effort might be to plan for larger volume sizes and consolidate or fold more than one source volume to a new and bigger target volume. Up until now, most customers relied on a standard size volume, which is a 3390-3 with 3,339 cylinders per 3390-3. With the affinity of volumes to RAID arrays and the underlying 8-packs, the increasing DDM size in the ESS 2105 made it impossible to utilize the full RAID array capacity of a RAID rank with 146 GB and even larger DDMs, when choosing 3390-3 volumes. The full RAID array capacity could only be utilized when changing to bigger volume sizes to reduce the number of volumes per RAID array or LSS.

This is not necessary any more with the DS8000 models due to their flexibility to configure back-end storage independently from the DDM sizes. There is no affinity any longer between an LSS and physical disk sets. The allocation of a volume happens in extents or increments of the size of a 3390-1 volume or 1113 cylinders, and a 3390-3 consists of exactly three extents from an extent pool. A 3390-9 with 10017 cylinders is comprised of exactly 9 extents out of an extent pool. There is no affinity any longer to a disk set for a logical volume like a 3390-3 or any other sized 3390 volume. Despite the fact that it is not required any longer to move from a 3390-3 volume type to some bigger volume type with the DS8000, you might still want to plan in the course of a data migration to reduce the number of volumes.

Considerations for new logical volume size

Although the newly announced DS8000 storage servers now support logical 3390 volumes of up to 65520 cylinders or 982800 tracks, you might still want to plan for a standard sized logical volume which is smaller than 65520 cylinders. Consider that full volume operations will take longer to copy or dump when the volume size is increased. This also applies to the first full initial volume replication for Metro Mirror, when creating the pairs, as well as for XRC. A compromise has to be planned for, which might be different from configuration to configuration.

In a pure system-managed storage environment with no full volume operations any more, except for migration perhaps, a volume size of 30051 cylinders might be fine for most of the data. This is the space of nine 3390-3 volumes or exactly 27 extents out of an extent pool. This would guarantee that all space is fully utilized when staying with increments of the standard extent size, which is 1113 cylinders or the equivalent of a 3390-1 model. When you plan for a larger volume size consider a multiple of a 3390-3 model, if possible.

With installations still performing full volume operations to a significant extent, you might want to plan for smaller volumes. For example, to full dump nine volumes of 3390-3 in parallel will most likely have a shorter elapsed time than dumping a single volume with the capacity of nine 3390-3 volumes. In such an environment a standard 3390-9 volume might still be appropriate. Although a 3390-9 volume has three times the capacity of a 3390-3, when changing from ESCON to FICON the throughput increases roughly by a factor of 10 and shortens the elapsed time, especially for highly sequential I/O.

Chapter 14. Data migration in zSeries environments 295