When you create a volume, you can designate that it is thin-provisioned to save capacity for the volume. A thin-provisioned volume has different virtual capacity and a real capacity.
The virtual capacity of a thin-provisioned volume is typically significantly larger than its real capacity. Each system uses the real capacity to store data that is written to the volume, and metadata that describes the thin-provisioned configuration of the volume. As more information is written to the volume, more of the real capacity is used.The Lenovo Storage V7000clustered system identifies read operations to unwritten parts of the virtual capacity and returns zeros to the server without using any of the real capacity.
Lenovo Storage V7000 must maintain extra metadata that describes the contents of thin-provisioned volumes. This means the I/O rates that are obtained from thin-provisioned volumes are slower than those obtained from fully allocated volumes that are allocated on the same MDisks.
Thin-provisioned volumes can also help simplify server administration. Instead of assigning a volume with some capacity to an application and increasing that capacity as the needs of the application change, you can configure a volume with a large virtual capacity for the application, and then increase or shrink the real capacity as the application needs change, without disrupting the application or server.
When you configure a thin-provisioned volume, you can use the 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 either increasing the real capacity of the volume or increasing the size of the storage pool that it is allocated on.
When you create a thin-provisioned volume, you can choose the grain size for allocating space in 32 KB, 64 KB, 128 KB, or 256 KB chunks. The grain size that you select affects the maximum virtual capacity for the thin-provisioned volume. The default grain size is 256 KB, and is the strongly recommended option. If you select 32 KB for the grain size, the volume size cannot exceed 260,000 GB. The grain size cannot be changed after the thin-provisioned volume has been created. Generally, smaller grain sizes save space but require more metadata access, which can adversely impact performance. If you are not going to use the thin-provisioned volume as a FlashCopy source or target volume, use 256 KB 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.
When you create a thin-provisioned volume, set the cache mode to readwrite to maximize performance. If the cache mode is set to none, the Lenovo Storage V7000 system cannot cache the thin-provisioned metadata, which decreases performance.
The autoexpand feature prevents a thin-provisioned volume from using up its capacity and going offline. As a thin-provisioned volume uses capacity, the autoexpand feature maintains a fixed amount of unused real capacity, called the contingency capacity. For thin-provisionedvolumes that are not configured with the autoexpand feature, the contingency capacity can get used up, causing the volume to go offline. To determine if an application requires a thin-provisioned volume with the autoexpand feature, create a thin-provisioned volume with the autoexpand feature turned off. If the application causes the volume to run out of capacity and go offline, you can then create a thin-provisioned volume with the autoexpand feature turned on.