Silicon Graphics InfiniteStorage 4000 Series, Configuring And Maintaining

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Chapter 3: Configuration Concepts 43
Disk Pools A disk pool is a set of drives that is logically grouped together in the storage array.
The drives in each disk pool must be of the same drive type and drive media type, and
they must be similar in size. As with a volume group, you can create one or more
volumes in the disk pool. However, the disk pool is different from the volume group
in the way that the data is distributed across the drives that comprise the disk pool.
In a volume group, the data is distributed across the drives based on a RAID level.
You can specify the RAID level when you create the volume group. The data for each
volume is written sequentially across the set of drives in the volume group.
In a disk pool, the storage management software distributes the data for each volume
randomly across a set the drives in the disk pool. Each disk pool must have a
minimum of eleven drives. Although there is no limit on the maximum number of
drives that can comprise a disk pool, the disk pool cannot contain more drives than the
maximum limit for each storage array. The storage management software
automatically configures the RAID level when you create the disk pool. You cannot
set or change the RAID level of disk pools or the volumes in the disk pools.
NOTE
Because disk pools can co-exist with volume groups, a storage array can
contain both disk pools and volume groups.
Disk Pool Benefits

Better use of drives
– When you add drives to a storage array, the storage
management software automatically detects the drives and prompts you to create
a single disk pool or multiple disk pools based on the drive type and the current
configuration. If disk pools were previously defined, the storage management
software provides the option of adding the compatible drives to an existing disk
pool. When new drives are added to an existing disk pool, the storage
management software automatically redistributes the data across the new
capacity, which now includes the new drives that you added. The data in the
volumes remain accessible when you add the drives to the disk pool. When you
delete disk pool volumes, the capacity of those volumes is added to the total
usable capacity of the disk pool and, therefore, can be reused.
NOTE
You have the option to manually create a disk pool, if you prefer not to proceed
with the automatic disk pool creation process.

Reduced hot spots
– A host might access some drives in the volume group for
data more frequently than other drives because of the sequential manner in which
the data is written to the drives. This frequency of access to drives creates hot
spots in the volume group. In a disk pool, the hot spots are significantly reduced
because of the random manner in which the data is spread across a large number
of drives. The reduction of hot spots in the disk pool improves performance of the
storage array.