Dell Unity: Pool space issues due to large levels of preallocation space

The "Preallocated" space is large or has increased over short amount of time.

When large amounts of writes occur over a short amount of time, the preallocated space in a pool can increase. 

Preallocated space:

• Drive capacity reserved for storage resources, but not yet used or reclaimed. For example, space reserved for new, incoming data, or space to reclaim after a snapshot expires.

If the issue occurring is regarding space on a LUN, reference Knowledge Article 000019209 Dell EMC Unity: Difference between LUN allocated space and total pool space used for a LUN (User Correctable)

• A storage resource has several free windows reserved for new writes; these windows are 512 KB in size.
• New writes regardless of size are sent to the available free windows. 
  For example, an 8 KB write occupies a 512 KB window which is then referred to as a partial window.
• Uncommitted consumption includes both free windows and partial windows. Space in partial windows is not usable. There is background processing that consolidates the data to allow for the conversion of partial window space to free windows.
• The background processes can free up the unused partial windows to the free windows. Once the free windows reach a certain point, it returns those windows to the pool's "Free space".
• Several cases have been seen where the rate of I/O writes to a storage resource is greater than the rate of partial free space to free window conversion which results in the growth of pre-allocated space for that storage resource.
• The tricklewindow parameter, for the array's background process, can be modified to increase the rate of consolidation of partial windows. 

To Monitor pool space allocation and usage in Unisphere:

1. Under STORAGE, select Pools
2. Select the relevant pool, and then select the Edit icon.
3. Select Usage tab.
4. Under the Capacity option, you will see the Current Pool Capacity.

Note the following:

• Non-base Space: The space used by all snapshots and thin clones in the pool.
• Used: The percentage of the pool's total space that has been reserved by its associated storage resources. This total includes the space used by thin clones and snapshots. When this value is over 100%, the pool is oversubscribed.
• Preallocated Space: The amount of remaining space in the pool that is reserved for, but not actively being used by, a storage resource.
• Free: Specifies the amount of space that is free (available to be used)

Contact Dell Unity Support for assistance in investigating any Pool space issues and adjusting the tricklewindow parameter.  Reference this KB number 00051695.