For some PS Series SAN environments, you can use thin provisioning technology to more efficiently provision storage, while still meeting application and user storage needs. With thin provisioning, businesses no longer have to over-buy storage while large amounts of space remain unused.
Normally, storage administrators will fully provision a volume. That is, space equal to the volume size will be allocated from free space. Because not all operating systems support expanding file systems or raw disks, an administrator may need to allocate more storage space than is currently required in order to accommodate possible future growth.
For example, an administrator may create a 100 GB volume for a organization that will use only a small fraction of the volume. Because the full 100 GB has been allocated to the volume, any unused volume space cannot be used for other purposes. If the space is never used, vital storage resources are wasted.
In contrast, a thin-provisioned volume is initially allocated only a portion of the volume size. As data is written to the volume, more space is automatically allocated (if available), and the volume reserve increases up to a user-defined limit. Regular event messages are generated as space is used, giving the administrator the opportunity to make adjustments, as needed.
A volume reserve for a thin-provisioned volume can automatically grow only if free pool space is available. If space is not available, the auto-grow operation will fail. If in-use space consumes all the volume reserve, the volume will be set offline.
Because thin-provisioned volumes are allocated only the storage space that is required, you can more efficiently utilize storage resources, while eliminating the need to perform difficult resize operations on the host.
Note: Thin provisioning is not always appropriate or desirable in an IT environment. It is most effectively used when you know how a volume will grow over time, the growth is predictable, and users do not need immediate, guaranteed access to the full volume size. Administrators must monitor thin-provisioned volumes and must be prepared to increase storage space by adding or moving arrays to the pool where the volume resides.
If you must guarantee the full volume size, thin provisioning is not recommended for the volume.
In a PS Series group, there are four user-settable parameters that control space utilization in a thin-provisioned volume:
As data is written to a thin-provisioned volume, volume reserve is consumed. When free volume reserve is less than 6 GB, the group will attempt to allocate additional pool space to the volume reserve, which can increase in size up to the maximum in-use space value described below. Space is allocated in increments of 10 GB, depending on various factors, including the amount of free pool space.
If there is not enough free pool space to increase the volume reserve, the volume reserve will not increase. If in-use space consumes all the volume reserve, the volume will be set offline. If free pool space becomes available, allowing the volume reserve to increase automatically, the volume will be automatically set online.
Note: Snapshot reserve, local replication reserve, and replica reserve for a volume are all based on the current volume reserve, not the reported volume size.
After the in-use warning value is reached, additional warning messages are generated according to the following policy:
If the in-use space reaches the maximum allowed, the volume will be set offline. If pool space is available, and you increase the volume size or the maximum in-use space setting, the volume will be automatically set online.
For information about creating a thin-provisioned volume, see Creating Volumes. For information about enabling and disabling thin provisioning on an existing volume, see Enabling and Disabling Thin Provisioning.