Chapter 9. Advanced features for storage efficiency 467
In this example, the storage administrator makes 7500 GiB of storage space visible to the
hosts, even though the storage system has only 5000 GiB of usable space, as shown in
Figure 9-23. In this case, all 15 hosts cannot immediately use all 500 GiB that is provisioned
to them. The storage administrator must monitor the system and add storage as needed.
Figure 9-23 Thin provisioning concept
9.3.1 Configuring a thin provisioned volume
Volumes can be configured as thin-provisioned or fully allocated. Thin-provisioned volumes
are created with real and virtual capacities. You can still create volumes by using a striped,
sequential, or image mode virtualization policy, as you can do with any other volume.
Real capacity defines how much disk space is allocated to a volume. Virtual capacity is the
capacity of the volume that is reported to other IBM Storwize V5000 Gen2 components (such
as FlashCopy or remote copy) and to the hosts. For example, you can create a volume with
real capacity of only 100 GiB, but virtual capacity of 1 tebibyte (TiB). The actual space used
by the volume on IBM Storwize V5000 Gen2 is 100 GiB, but hosts see a 1 TiB volume.
A directory maps the virtual address space to the real address space. The directory and the
user data share the real capacity.
Thin-provisioned volumes are available in two operating modes:
Autoexpand
Non-autoexpand
You can switch the mode at any time. If you select the autoexpand feature, the IBM Storwize
V5000 Gen2 automatically adds a fixed amount of more real capacity to the thin volume as
required. Therefore, the autoexpand feature attempts to maintain a fixed amount of unused
real capacity for the volume.
This amount is known as the
contingency capacity. The contingency capacity is initially set to
the real capacity that is assigned when the volume is created. If the user modifies the real
capacity, the contingency capacity is reset to be the difference between the used capacity and
real capacity.
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