Volume Features; Thin Provisioning - Dell PS4000 Configuration Manual

3.3.2

Volume Features

Each volume supports a set of features and capabilities that include the following:

Ability to define a volume as thin-provisioned

Support for snapshots

Support for replication

Support for creating clones

Support for multiple host shared access
3.3.3

Thin Provisioning

Thin provisioned volumes provide administrators with an additional option for managing data growth
within the SAN. Thin provisioning allows volumes to present a logical capacity to the host(s)
connecting to the volume that is different (larger) than the actual physical storage resource allocation
used by the volume at any given time.
A volume can be provisioned as a thin provisioned volume at creation or after creation. The following
rules apply to thin provisioned volumes:

A minimum physical allocation of 10% of the logical allocation is required

If a volume is converted to a thin provisioned volume, physical allocation cannot be less than
the amount of physical space already used within the volume

Any pool free space allocated to a thin provisioned volume is not returned to the free pool if
the host's file system capacity usage of that volume is reduced (due to file system
defragmentation, data removal, etc)

If a thin provisioned volume's allocated space exceeds the ―maximum in-use space‖ setting,
the volume will go into an offline state.

Converting a volume to a thin provisioned volume may not reduce the physical allocation of
space the volume is using. Actual physical space recovered will depend on previous writing
patterns for the OS file system hosted on the volume.

All initial settings for ―minimum volume reserve‖, ―In-use volume reserve warning level‖ and
―maximum in-use space‖ are default recommendations and can be changed by the
administrator within the constraints defined above.
The following best practices should be considered when using thin provisioned volumes:

Use Pool Free Space, not Group Free Space when making all determinations of thin provisioned
volume physical capacity allocation.

Create regular volumes before creating thin provisioned volumes. This provides the
administrator with a better view of the remaining available free space in the pool.

Set each thin provisioned volume's ―In use Volume Reserve Warning Level‖ to a level that
allows the administrator to ensure that additional physical storage can be deployed before the
volume uses the remainder of current pool free space.

Ensure that the Sum of all ―In use Volume Reserve Warning Levels‖ for all thin provisioned
volumes does not exceed the current pool free space capacity minus 10%.

The storage administrator and application owner should reach an agreement on storage use and
procedures in determining the best automatic growth policy, reducing the frequency of
monitoring and intervention.
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