HP XP P9500 Storage - Manual (Page 15)

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Disadvantages
Some disadvantages to using fixed-sized provisioning are:
•
If you use only part of the entire capacity specified by an emulation type, the rest of the
capacity is wasted.
•
You cannot use the capacity efficiently. The size of the drive is fixed. When you create a
fixed-sized volume, a small amount of capacity remains permanently inaccessible.
•
In a fixed-sized environment, manual intervention can become a costly and tedious exercise
as workloads grow over time or as new ones are added to the mix. The problem becomes
more complex as new servers are added to the scenario.
When to use fixed-sized provisioning
Fixed-sized provisioning is a best fit in the following scenarios:
•
When custom-sized provisioning is not supported.
Custom-sized provisioning
Custom-sized (or variable-sized) provisioning has more flexibility than fixed-sized provisioning and
is the traditional storage-based volume management strategy typically used to organize storage
space on a server. It requires fewer logical devices, but larger ones.
To create custom-sized volumes on a storage system, an administrator first creates array groups
of any RAID level from parity groups. Then, volumes of the desired size are created from these
individual array groups. These volumes are then individually mapped to one or more host ports
as a logical unit.
Following are two scenarios where custom-sized provisioning is an advantage:
•
In fixed-sized provisioning, when several frequently accessed files are located on the same
volume and one file is being accessed, users cannot access the other files because of logical
device contention. If the custom-sized feature is used to divide the volume into several small
volumes and I/O workload is balanced (each file is allocated to each volume), then access
contention is reduced and access performance is improved.
•
In fixed-sized provisioning, not all of the capacity may be used. Some capacity on the volume
may remain inaccessible to other users. If the custom-sized feature is used, smaller volumes
can be created that do not waste capacity.
The following illustrates custom-sized provisioning in an open-systems environment using standard
volumes of independent array groups:
Basic provisioning 15