Chapter 6. Volume configuration 297
The real capacity of a thin volume can be changed if the volume is not in image mode.
Increasing the real capacity enables a larger amount of data and metadata to be stored on
the volume. Thin-provisioned volumes use the real capacity that is provided in ascending
order as new data is written to the volume. If the user initially assigns too much real capacity
to the volume, the real capacity can be reduced to free storage for other uses.
A thin-provisioned volume can be configured to
autoexpand. This feature causes the IBM
Spectrum Virtualize to automatically add a fixed amount of more real capacity to the thin
volume as required. Therefore, autoexpand attempts to maintain a fixed amount of unused
real capacity for the volume, which is known as the
contingency capacity.
The contingency capacity is initially set to the real capacity that is assigned when the volume
is created. If the user modifies the real capacity, the contingency capacity is reset to be the
difference between the used capacity and real capacity.
A volume that is created without the autoexpand feature, and therefore has a zero
contingency capacity, goes offline when the real capacity is used and it must expand.
Autoexpand does not cause the real capacity to grow much beyond the virtual capacity. The
real capacity can be manually expanded to more than the maximum that is required by the
current virtual capacity, and the contingency capacity is recalculated.
To support the auto expansion of thin-provisioned volumes, the storage pools from which they
are allocated have a configurable capacity warning. When the used capacity of the pool
exceeds the warning capacity, a warning event is logged. For example, if a warning of 80% is
specified, the event is logged when 20% of the free capacity remains.
A thin-provisioned volume can be converted nondisruptively to a fully allocated volume (or
vice versa) by using the volume mirroring function. For example, the system allows a user to
add a thin-provisioned copy to a fully allocated primary volume, and then remove the fully
allocated copy from the volume after they are synchronized.
The fully allocated-to-thin-provisioned migration procedure uses a zero-detection algorithm
so that grains that contain all zeros do not cause any real capacity to be used.
6.1.6 Compressed volumes
This is a custom type of volume where data is compressed as it is written to disk, saving
additional space. Compression is a separately orderable license that is set on a per enclosure
basis. One license is required for each control or expansion enclosure and each enclosure in
any external storage systems that use virtualization.To use the compression function, you
must obtain the IBM Real-time Compression license.
Thin-provisioned volume format: Thin-provisioned volumes do not need formatting. A
read I/O that requests data from deallocated data space returns zeros. When a write I/O
causes space to be allocated, the grain is “zeroed” before use.
Note: For IBM Storwize V5000 Gen2, only Storwize V5030 and Storwize V5030F models
support compression.
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