Thin-provisioned volumes

A thin-provisioned volume presents a different capacity to mapped hosts than the capacity that the volume consumes in the storage pool.

The system supports thin-provisioned volumes in standard pools and in data reduction pools. In standard pools, thin-provisioned volumes are created as a specific volume type, that is based on capacity savings criteria. These properties are managed at the volume level. With data reduction pools, all the benefits of thin-provisioning are available to all the volumes that are assigned to the pool. Only fully allocated volumes do not gain these benefits. Data reduction pools enhance capacity efficiency for thin-provisioned volumes by monitoring the hosts use of capacity. When the host indicates that the capacity is no longer needed, the space is released and can be reclaimed by the data reduction pool to be redistributed automatically. Standard pools do not support reclaiming capacity. For added capacity savings, you can specify compression and deduplication on volumes in data reduction pools.

Thin-provisioned volumes can also help simplify server administration. Instead of assigning a volume with some capacity to an application and increasing that capacity as the needs of the application change, you can configure a volume with a large virtual capacity for the application. You can then increase or shrink the real capacity as the application needs change, without disrupting the application or server.

Virtual capacity is the volume storage capacity that is available to a host. Real capacity is the storage capacity that is allocated to a volume copy from a pool. In a fully allocated volume, the virtual capacity and real capacity are the same. However, in a thin-provisioned volume, the virtual capacity can be much larger than the real capacity.

The virtual capacity of a thin-provisioned volume is typically significantly larger than its real capacity. Each system uses the real capacity to store data that is written to the volume, and metadata that describes the thin-provisioned configuration of the volume. As more information is written to the volume, more of the real capacity is used. The system identifies read operations to unwritten parts of the virtual capacity and returns zeros to the server without using any real capacity.

The system must maintain extra metadata that describes the contents of thin-provisioned volumes. As a result, the I/O rates that are obtained from thin-provisioned volumes can be lower than the rates obtained from fully allocated volumes that are allocated on the same MDisks.

When you configure a thin-provisioned volume in a standard pool, you can use the warning level attribute to generate a warning event when the used real capacity exceeds a specified amount or percentage of the total virtual capacity. You can also use the warning event to trigger other actions, such as taking low-priority applications offline or migrating data into other storage pools. For thin-provisioned volumes in data reduction pools, warning level value cannot be set, since capacity reporting is handled at the pool level.

If a thin-provisioned volume in a standard pool does not have enough real capacity for a write operation, the volume is taken offline and an error is logged (error code 1865, event ID 060001). Access to the thin-provisioned volume is restored by either increasing the real capacity of the volume or increasing the size of the storage pool that it is allocated on.

When you create a thin-provisioned volume in standard pools, you can choose the grain size for allocating space in 32 KB, 64 KB, 128 KB, or 256 KB chunks. The grain size that you select affects the maximum virtual capacity for the thin-provisioned volume in standard pools. The default grain size is 256 KB. If you select 32 KB for the grain size, the volume size cannot exceed 260,000 GB. The grain size cannot be changed after the thin-provisioned volume is created in a standard pool. Generally, smaller grain sizes save space but require more metadata access, which can adversely impact performance. If you are not going to use the thin-provisioned volume as a FlashCopy® source or target volume, use 256 KB to maximize performance. If you are going to use the thin-provisioned volume as a FlashCopy source or target volume, specify the same grain size for the volume and for the FlashCopy function. Grain size cannot be set on thin-provisioned volume copies in data reduction pools. The grain size of 8 KB is the default size for thin-provisioned volume copies in data reduction pools.

When you create a thin-provisioned volume, set the cache mode to readwrite to maximize performance. If the cache mode is set to none, the system cannot cache the thin-provisioned metadata, which decreases performance. A volume or volume copy created from a data reduction pool must have a cache mode of readwrite. If you try to create a thin provisioned or compressed volume copy from a data reduction pool and the volume cache mode is not readwrite, the operation fails.

The autoexpand feature prevents a thin-provisioned volume from using up its capacity and going offline. As a thin-provisioned volume uses capacity, the autoexpand feature maintains a fixed amount of unused real capacity, called the contingency capacity. For thin-provisioned volumes in data reduction pools, the autoexpand feature is always enabled to maintain contingency capacity. For thin-provisioned volumes in standard pools, the autoexpand feature is optional. However without this feature enabled, the contingency capacity can get used up, causing the volume to go offline. If you are using standard pools and want to determine whether an application requires the autoexpand feature, you can create a test thin-provisioned volume with the autoexpand feature turned off. If the application causes the volume to run out of capacity and go offline, you can then create a with the autoexpand feature turned on.