HP XP P9500 Storage - Manual (Page 16)

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.
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 an address composed of
a control unit and logical device within the control unit.
Following are three 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 smaller
volumes and I/O workload is balanced (each file is allocated to different volumes), then access
contention is reduced and access performance is improved.
•
In fixed-sized provisioning, not all of the capacity may be used. Unused capacity on the volume
will remain inaccessible to other users. If the custom-sized feature is used, smaller volumes
can be created that do not waste capacity.
•
Applications that require the capacity of many fixed-sized volumes can instead be given fewer
large volumes to relieve device addressing constraints.
The following illustrates custom-sized provisioning in an open-systems environment using standard
volumes of independent array groups:
To change the size of a volume already in use, you first create a new volume larger (if possible)
than the old one, and then move the contents of the old volume to the new one. The new volume
would be remapped on the server to take the mount point of the old one, which is retired.
A disadvantage is that this manual intervention can become costly and tedious and this provisioning
strategy is appropriate only in certain scenarios.
Expanded LU provisioning
If a volume larger than the largest volume is needed in a custom-size volume, the traditional storage
system-based solution is to use the logical unit size expansion (LUSE) feature to configure an
expanded logical unit (LU). This method is merely a simple concatenation of LDEVs, which is a
capacity rather than a performance configuration.
16 Introduction to provisioning