252 Implementing the IBM Storwize V7000 with IBM Spectrum Virtualize V8.2.1
The grain size is defined when the volume is created. The grain size can be 32 KiB, 64 KiB, 
128 KiB, or 256 KiB. The grain size cannot be changed after the thin-provisioned volume is 
created. The default grain size is 256 KiB, which is the preferred option; however, the 
following factors must be considered when deciding on the grain size:
/SM590000Smaller grain size helps to save space. If a 16 KiB write I/O requires a new physical grain 
to be allocated, the used space is 50% of a 32 KiB grain, but just over 6% of 256 KiB grain. 
If no subsequent writes to other blocks of the grain occur, the volume provisioning is less 
efficient for volumes with larger grain.
/SM590000Smaller grain size requires more metadata I/O to be performed, which increases the load 
on the physical back-end storage systems.
/SM590000When a thin-provisioned volume is a FlashCopy source or target volume, specify the same 
grain size for FlashCopy and the thin-provisioned volume configuration. Use 256 KiB grain 
to maximize performance.
/SM590000Grain size affects maximum size of the thin-provisioned volume. For 32 KiB size, the 
volume size cannot exceed 260 TiB.
Figure 7-7 shows the thin-provisioning concept.
Figure 7-7   Conceptual diagram of thin-provisioned volume
Thin-provisioned volumes store user data and metadata, and each grain of user data requires 
metadata to be stored. Therefore, the I/O rates that are obtained from thin-provisioned 
volumes are lower than the I/O rates that are obtained from fully allocated volumes.
The metadata storage used is never greater than 0.1% of the user data. The resource usage 
is independent of the virtual capacity of the volume. 
Thin-provisioned volume format: Thin-provisioned volumes do not need formatting. A 
read I/O, which requests data from not allocated data space, returns zeros. When a write 
I/O causes space to be allocated, the grain is “zeroed” before use.