Thin provisioning file systems

Thin provisioning is a method of controlling how a file system's free space is calculated and reported. Administrators use thin provisioning to optimize the utilization of storage and to plan resource acquisition in a way that helps minimize expenses, while ensuring that there is enough storage for all the system needs.

Thin provisioning allows you to oversubscribe the storage connected to the storage server. As long as the available storage is not completely allocated to file systems, the oversubscription cannot be noticed by storage system users.

When thin provisioning is enabled and storage is oversubscribed, if a client attempts a write operation and there is insufficient storage space, the client will receive an insufficient space error, even though a query for the amount of free space will show that space is still available. When storage is oversubscribed, the storage server's administrator must ensure that this situation does not occur; the storage server does not prevent this situation from occurring. To resolve this situation, the storage server's administrator must either disable thin provisioning or add storage.

When thin provisioning is enabled, the storage server reports the amount of free space for a file system based on the file system's expansion limit (its maximum configured capacity), rather than on the amount of free space based on the amount of storage actually allocated to the file system. Because file systems can be allowed to automatically expand up to a specified limit (the expansion limit), additional storage is allocated to the file system as needed, instead of all the storage being allocated to the file system when it is created.

For example, a file system has an expansion limit of 20 TB, with 6 TB already used and 8 TB currently allocated. If thin provisioning is enabled, the server will report that the file system has 14 TB of free space, regardless of how much free space is actually available in the storage pool. For more information about storage pools, refer to the Storage Subsystem Administration Guide. If thin provisioning is disabled, the server will report that the file system has 2 TB of free space.

By default, thin provisioning is disabled for existing file systems and for newly created file systems. Enable and disable thin provisioning using the filesystem-thin command (currently there is no way to enable or disable thin provisioning using NAS Manager).

Thin provisioning works on a per file system basis, and does not affect the capacity reported by the span-list --filesystems and filesystem-list commands. Also, NAS Manager displays the actual file system size. As a result, the administrator can perform proper capacity planning.

For more information about CLI commands, refer to the Command Line Reference.

If thin provisioning is enabled and you disable file system auto-expansion for a storage pool, the free space reported for each of the file systems in that storage pool is the same as if thin provisioning were not enabled. This means that the free space reported becomes equal to the difference between the file system's current usage and the amount of space in all storage pool chunks currently allocated to that file system. If you re-enable file system auto-expansion for file systems in the storage pool, free space is again reported as the difference between the file system's current usage and its expansion limit, if an expansion limit has been specified.

When thin provisioning is enabled, and the aggregated file system expansion limits of all file systems exceeds the amount of storage connected to the server/cluster, warnings are issued to indicate that storage is oversubscribed. These warnings are issued because there is an insufficient amount of actual storage space for all file systems to grow to their expansion limit.