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PowerMax thin devices are logical devices that can be used in many of the same ways that thick devices have traditionally been used. Unlike standard thick 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 PowerMax thin device is bound immediately upon creation to a thin pool. The thin pool is comprised of special devices known as data devices that provide the actual physical storage to support the thin device allocations.
When a write is performed to part of any thin device for which physical storage has not yet been allocated, the PowerMax allocates physical storage from the thin pool for that portion of the thin device only. The PowerMaxOS satisfies the requirement by providing a unit of physical storage from the thin pool called a thin device extent. This approach reduces the amount of storage that is actually consumed.
The thin device extent is the minimum amount of physical storage that can be reserved at a time for the dedicated use of a thin device. 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. A round-robin mechanism is used to balance the allocation of data device extents across all of the data devices in the thin pool that are enabled and that have remaining unused capacity. This provides the benefit of striping the data over many physical devices, thus improving performance. The thin device extent size on the PowerMax is a single 128 KB track.
When a read is performed on a thin device, the data being read is retrieved from the appropriate data device in the thin pool (assuming the data is not in cache). If a read is performed against an unallocated portion of the thin device, zeros are returned.
The maximum size of a thin device on the PowerMax is 64 TB. This is also the maximum size that VMware permits for a number of storage objects, including physical RDMs and VMFS.