Frequently Asked Questions About ISP

• What is the relationship between a storage pool and a volume template?

Storage pools contain disks, VxVM objects such as volumes, and a set of volume templates. A storage pool is defined by the volume templates that it contains. The ISP Configuration Database contains a number of storage pool definitions that you can use to create a storage pool object in VxVM. Each definition contains a list of volume templates and the default policy settings for the pool definition. When you create a storage pool object in a disk group from a storage pool definition, these volume templates and policies also get installed.

• When do I need to specify a template set?

A storage pool contains volume templates that define its characteristics. A template set is simply a collection of related volume templates that you can associate with a storage pool.

• When do I need to specify a storage pool set?

A disk group that you want to use with ISP must be configured to contain one data storage pool, and optionally one or more clone storage pools. You can use a storage pool set definition to organize a disk group so that it contains data and clone storage pools with well-defined characteristics.

• If I create a storage pool using a storage pool definition, can I later create a volume in that pool using a volume template other than those that are associated with the pool?

The answer can be illustrated by an example. Suppose you create a storage pool using the mirrored_volumes pool definition. This installs volume templates that allow you to create mirrored volumes and volumes with similar characteristics in the pool. If you attempted to create a striped volume in the pool, the resulting volume is mirrored as well as striped. However, you are not constrained from creating volumes with other characteristics. You can use rules to bypass restrictions that are imposed by higher-level abstractions like volume templates and capabilities.

Bear in mind that creating a storage pool from a storage pool definition does not uniquely determine the capabilities of volumes that you create in the pool. Only when you create a volume can you specify its capabilities and ensure consistency between the volumes in a pool.

• What does "prefabricated" mean as used in volume templates?

The term prefabricated implies that the characteristics of a volume are implemented using hardware rather than software. For example, PrefabricatedRaid5 implies the use of RAID-5 LUNs that have been set up in an array's hardware, rather than being implemented as a VxVM RAID-5 volume.

• Can I use both hardware and software RAID volumes in a storage pool?

A storage pool is usually configured either for prefabricated RAID devices or for VxVM volumes that are created in software. This provides consistency in performance and failure tolerance within the pool. If you add both hardware and software RAID templates to a pool, ISP allocates templates that are appropriate to the capabilities of the volumes that you specify.

• How does ISP discover LUN hardware characteristics?

ISP relies on the VERITAS Array Integration Layer (VAIL) to provide detailed information on LUN characteristics via array-specific modules.

• How do I restrict allocation to storage from certain vendors, for example EMC BCV or Hitachi?

By default, ISP use LUNs with similar characteristics for allocating storage. If insufficient storage is available, it relaxes this constraint. You can make the constraint mandatory by specifying the capability ArrayProductId when creating a volume. This forces ISP to allocate storage on LUNS that share the same product identifier. Alternatively, you can use the capability ConfineToSimilarStorage. This makes ISP use LUNs from the same vendor, but allows the product identifiers of these LUNs to differ.

• When I create a volume, ISP uses space on the same disks unless I choose other disks from the storage pool manually. Why doesn't ISP automatically spread the volumes across the other disks in the storage pool to enhance I/O performance?

ISP is tuned to use as few disks as possible. In any case, I/O performance depends on many factors: the way storage is connected to the system, the inherent capabilities of the storage, how volumes are configured and how they share storage, the type of I/O requests made by applications, and the amount of I/O from/to each volume. For example, allocating two volumes to a single disk array that has a large cache would probably provide better overall I/O performance than placing one volume on the disk array and the other on a JBOD. If necessary, you can configure smaller storage pools in separate disk groups to restrict the allocation of storage. Alternatively, you can explicitly specify the storage that can or cannot be allocated to a volume.