HPP Enterprises P4000 SAN - Manual (Page 12)

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Figure 4. Configuring 2-Way Replication and Thin Provisioning
You can change volume properties at any time. However, if you change volume size, you may also
need to update the XenServer configuration as well as the VM’s OS in order for the new size to be
recognized.
Comparing full and thin provisioning
You have two options for provisioning volumes on the SAN:
 Full Provisioning
With Full Provisioning, you reserve the same amount of space in the storage cluster as that
presented to the XenServer host. Thus, when you create a fully-provisioned 10GB volume, 10GB of
space is reserved for this volume in the cluster; if you also select 2-Way Replication, 20 GB of
space (10 GB x 2) would be reserved. The Full Provisioning option ensures that the full space
requirement is reserved for a volume within the storage cluster.
 Thin Provisioning
With Thin Provisioning, you reserve less space in the storage cluster than that presented to
XenServer hosts. Thus, when a thinly-provisioned 10GB volume is created, only 1GB of space is
initially reserved for this volume; however, a 10GB volume is presented to the host. If you were also
to select 2-Way Replication, 2GB of space (1 GB x 2) would initially be reserved for this volume.
As the initial 1GB reservation becomes almost consumed by writes, additional space is reserved from
available space on the storage cluster. As more and more writes occur, the full 10GB of space will
eventually be reserved.
Benefits of thin provisioning
The key advantage of using thin provisioning is that it minimizes the initial storage footprint during
deployment. As your needs change, you can increase the size of the storage cluster by adding
storage nodes to increase the amount of space available, creating a cost-effective, pay-as-you-grow
architecture.