EMC Symmetrix thin devices are logical devices that can be used in many of the same ways that Symmetrix devices have traditionally been used. Unlike traditional Symmetrix devices, thin devices do not need to have physical storage completely allocated at the time the device is created and presented to a host.

A thin device is not usable until it has been bound to a shared storage pool known as a thin pool. Multiple thin devices may be bound to any given thin pool. The thin pool is comprised of devices called data devices that provide the actual physical storage to support the thin device allocations.

When a write is performed to a part of any thin device for which physical storage has not yet been allocated, the EMC Symmetrix allocates physical storage from the thin pool for that portion of the thin device only. 

The minimum amount of physical storage that can be reserved at a time for the dedicated use of a thin device is referred as a data device extent. An entire thin device extent is physically allocated to the thin device at the time the thin storage allocation is made as a result of a host write operation.  The data device extent is allocated from any one of the data devices in the associated thin pool. Allocations across the data devices are balanced to ensure that an even distribution of allocations occurs from all available data devices in the thin pool.

The minimum amount of physical storage that must be mapped to a thin device at a time is called a thin device extent. For EMC Symmetrix, the thin device extent size is the same as the data device extent size.

When a read is performed on a thin device, the data being read is retrieve from the appropriate data device in the thin pool to which the thin device is associated. If for some reason a read s performed against an unallocated portion of the thin device, zeroes are returned to the reading process.

When more physical data storage is required to services existing or future thin devices, for example, when a thin pool is approaching full storage allocations, data devices can be added to existing thin pools dynamically without needing a system outage. New thin devices can also be created and associated with existing thin pools.

When data devices are added to a thin pool they can be in an enabled or disabled state. In order for the data device to be used for thin extent allocation it needs to be in the enable state. For it to be removed from the thin pool, it needs to be in a disabled state. A data device can be disabled only if it does not have any thin extent allocations.

symmetrix thin device

Figure 1: Thin devices and thin pools containing data devices.

The above figure depicts the relationships between thin devices and their associated thin pools. There are nine devices associated with thin Pool A and three thin devices associated with thin pool B.