Hardware manual

10–1
10 Advanced volume operations
Only knowledgeable group administrators should perform advanced volume operations, including creating thin-
provisioned volumes, using template volumes and thin clones, changing a volume size, using volume collections,
and scheduling volume operations.
About thin provisioning
You can use thin provisioning technology to more efficiently allocate storage space, while still meeting application
and user storage needs. With a thin-provisioned volume, the group allocates space based on volume usage,
enabling you to “over-allocate” or “over-provision” group storage space.
Recommendation: Dell recommends that you fully understand the benefits and risks of using thin
provisioning
before implementing it in your environment. Environments that use thin provisioning should
have around-the-clock support to handle any space allocation issues and prevent service level
disruption.
Thin provisioning volumes is beneficial in a number of env
iron
ments. For example, if your environment does not
easily allow you to expand file systems or raw disks, you can give thin-provisioned volumes excessively large
reported sizes to account for future growth. The group automatically allocates space to volumes only if usage
patterns warrant the space.
Thin provisioning also helps you plan for future group expansion. For example, you can size volumes according to
the
i
r maximum possible space requirements, even if the group currently cannot provide all the required space (that
is, you can “over-provision” group space). As volume usage increases, you can expand group capacity, with no
user impact. You do not need to change drive letters, expand volume sizes, or add volumes.
However, if your environment requires guaranteed space for volume, thin provisioning might
be inappropriate.
Thin
provisioning is most effective if you can accurately predic
t how volume usage increases over time.
When you create a volume, you specify the reported size for the volume. The
reported size is seen by iSCSI
initiators. The actual amount of pool space that the group allocates to a volume is called the volume reserve. The
value of the volume reserve depends on whether you enable thin provisioning on a volume:
• No thin provisioning – The volume reserve is equal to the reported size
.
For example, even if only 10% of a volume is in use
, the group allocates the full reported size.
• Thin provisioning – The volume reserve is equal to or less than the reporte
d size, depending on volume usage
and the thin provisioning settings.
Initially, the group allocates the minimum amount of volume reserve for a
thin-provisioned volume. The
minimum is 10% of the reported volume size or the user-specified percentage.
As initiators write to the volume, free volume reserve decreases. When free volume reserve falls belo
w a
threshold, the group increases volume reserve, up to a user-defined maximum (assuming available free pool
space):
– For a volume with a reported size of 100GB or greater,
when free volume reserve is less than 6 GB, the
group allocates an additional 10 GB.