About ISP allocation behavior


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About ISP allocation behavior

The behavior that ISP exhibits when allocating storage can be controlled by setting the value of the allocation_priority_order attribute. For example, this attribute determines if growing the storage pool is preferred to dropping the desired rules when there is insufficient storage to create a volume. The default behavior can be changed by editing the definition of the attribute in the /etc/default/allocator file, or, for a single invocation of the vxassist or vxvoladm command, by specifying the -o allocation_priority=value option as a command-line argument.

Values of the precedence order shows that the value argument can take an integer value from 1 to 12.

Values of the precedence order

Value	Precedence order
1	S < T < A < D
2	S < T < D < A
3	S < A < T < D
4	S < D < T < A
5	S < A < D < T
6	S < D < A < T
7	A < S < T < D (default behavior)
8	D < S < T < A
9	A < S < D < T
10	D < S < A < T
11	A < D < S < T
12	D < A < S < T

Precedence order symbols shows the meaning of the precedence order symbols.

Precedence order symbols

Symbol	Description
A	Autogrow: If permitted by the value of the `AutoGrow` setting, the storage pool may be grown to increase the available storage.
D	Desired Rules: Rules may be dropped if insufficient storage is available.
S	Self-Sufficient: If permitted by the value of the `SelfSufficent` setting, templates may be added to the storage pool if insufficient storage is available.
T	Template Combination: Different template combinations may be used if insufficient storage is available.
<	Indicates the precedence in choosing an action when attempting to allocate storage. "X < Y" means that Y is chosen in preference to X. The rightmost items in the list are chosen before those to the left.

For example, the default behavior of ISP (equivalent to setting allocation_policy_order=7), A < S < T < D, has the following meaning:

T < D	First, drop desired rules in preference to using a different template combination.
S < T	Secondly, use a different template combination in preference to adding more templates to the storage pool.
A < S	Finally, add more templates to the storage pool in preference to growing the storage pool.

If a set of capabilities are specified for a volume during creation, ISP may find that there are multiple sets of templates that provide the same set of capabilities. These sets are referred to as template combinations. Some of the available template combinations may provide additional capabilities (extraneous capabilities) to those that were requested for the volume. By preferring not to select a different template combination, you can avoid implicitly specifying the extraneous capabilities. This assumes that you would request these capabilities explicitly if you required them.

The templates that are associated with a storage pool define the characteristics of the volumes that can be created in that pool. The selfsufficient policy of the storage pool allows you to control whether ISP can redefine the pool definition automatically, and, if so, to what extent.


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