The following types of thin-provisioned volumes are available:  The main risk that is associated with using thin-provisioned volumes is running out of real  capacity in the storage volumes, pool, or both, and the resulting unplanned outage. Therefore,  strict monitoring of the used capacity on all non-autoexpand volumes and monitoring of the  free space in the storage pool is required. When you configure a thin-provisioned volume, you can define a warning level attribute to  generate a warning event when the used real capacity exceeds a specified amount or  percentage of the total virtual capacity. You can also use the warning event to trigger other  actions, such as taking low-priority applications offline or migrating data into other storage  pools. If a thin-provisioned volume does not have enough real capacity for a write operation, the  volume is taken offline and an error is logged (error code 1865, event ID 060001). Access to  the thin-provisioned volume is restored by increasing the real capacity of the volume, which  might require increasing the size of the storage pool from which it is allocated. Until this time,  the data is held in the Storwize V7000 cache. Although in principle this situation is not a data  integrity or data loss issue, you must not rely on the Storwize V7000 cache as a backup  storage mechanism. Space is not allocated on a thin-provisioned volume if an incoming host write operation  contains all zeros. Important:  Set and monitor a warning level on the used capacity so that you have  adequate time to respond and provision more physical capacity.  Warnings must not be ignored by an administrator. Consider the use of the autoexpand feature of the thin-provisioned volumes to reduce  human intervention required to maintain access to thin-provisioned volumes. When you create a thin-provisioned volume, you can choose the grain size for allocating  space in 32 kibibytes (KiB), 64 KiB, 128 KiB, or 256 KiB chunks. The grain size that you select  affects the maximum virtual capacity for the thin-provisioned volume. The default grain size is  256 KiB, which is the preferred option. If you select 32 KiB for the grain size, the volume size  cannot exceed 260,000 gibibytes (GiB). The grain size cannot be changed after the  thin-provisioned volume is created.  Generally, smaller grain sizes save space, but require more metadata access, which can  adversely affect performance. If you are not going to use the thin-provisioned volume as a  FlashCopy source or target volume, use 256 KiB to maximize performance. If you are going to  use the thin-provisioned volume as a FlashCopy source or target volume, specify the same  grain size for the volume and for the FlashCopy function. In this situation, ideally grain size is  equal to the typical I/O size from the host. Chapter 3. Planning  69 GLYPH<SM590000> Non-autoexpand volumes  have a fixed amount of assigned real capacity. In this case, the  user must monitor the volume and assign more capacity when required. Although it  prevents starving other thin provisioned volumes, it introduces a risk of an unplanned  outage. A thin-provisioned volume goes offline if host tries to write more data than what  can fit into the allocated real capacity. GLYPH<SM590000> Autoexpand volumes  allocate real capacity from a storage pool on demand, which  minimizes required user intervention. However, a malfunctioning application can cause a  volume to expand until its real capacity is equal to the virtual capacity, which potentially  can starve other thin provisioned volumes in the pool. 