IBM Storwize V5010 Manual page 318

Thin-provisioned volumes can be used as volumes that are assigned to the host, by
FlashCopy to implement thin-provisioned FlashCopy targets, and with the mirrored volumes
feature.
When a thin-provisioned volume is initially created, a small amount of the real capacity is
used for initial metadata. I/Os are written to grains of the thin volume that were not previously
written, which causes grains of the real capacity to be used to store metadata and the actual
user data. I/Os are written to grains that were previously written, which updates the grain
where data was previously written.
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 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 TiB. The grain size cannot
be changed after the thin-provisioned volume is created. Generally, smaller grain sizes save
space, but they require more metadata access, which can adversely affect performance.
When using thin-provisioned volume as a FlashCopy source or target volume, use 256 KiB to
maximize performance. When using thin-provisioned volume as a FlashCopy source or target
volume, specify the same grain size for the volume and for the FlashCopy function.
Figure 6-6 shows the thin-provisioning concept.
Figure 6-6 Conceptual diagram of thin-provisioned volume
Thin-provisioned volumes store user data and metadata. Each grain of data requires
metadata to be stored. Therefore, the I/O rates that are obtained from thin-provisioned
volumes are less 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. If you are using the thin-provisioned
volume directly with a host system, use a small grain size.
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Implementing the IBM Storwize V5000 Gen2 with IBM Spectrum Virtualize V8.1