HP XP External Storage Software - Manual (Page 25)

External LU groups
When mapping an external LU as an internal LDEV, you must register the external LU in an external
LU group.
You can classify external LUs set by External Storage XP into groups according to their use. The group
is called an external LU group (ExG). For instance, you can register several LUs in one external array
to one external LU group. Or, if data you want to manage in a chunk is stored in LUs in various
external arrays, you can register those LUs in one external LU group, and manage them as a block.
You must assign a number from 1 to 16,384 to each external LU group. For the XP12000, you can
create a maximum of 15,360 external LU groups. For the XP10000/SVS200, you can create a
maximum of 8,192 external LU groups. You can register a maximum of 256 volumes in one external
group.
External LU attributes set by mapping
When mapping an external LU as an internal LDEV, use the Add LU pane in External Storage XP to
set the external LU's attributes. For instructions, see
Mapping external LUs (Add LU) .
The following are the external LU's attributes:
• Emulation type
Set the mapped LU's emulation type by selecting any emulation type from the drop-down list.
However, if you plan to use the mapped LU for Flex Copy XP operations, or you plan to access
existing data in the external LU, you must select the OPEN-V emulation type to avoid data resizing.
If you plan to use existing data in the external LU from the local array after mapping, you must
select the OPEN-V emulation type. For example, to migrate existing data in the external LU to the
local array volume, you must set the OPEN-V emulation type when mapping the external LU.
You must also select the OPEN-V emulation type when VMA of LUN Security XP Extension is set
for the external LU on the external array side.
If you select an emulation type other than OPEN-V, additional space is taken for XP management
information. This means that after mapping, LU capacity is less than the actual external LU capacity
(and the original data must be considered lost). For more information about volume capacity, see
Limitations on External Storage XP operations .
• IO Suppression mode (Enable or Disable)
When mapping an LU, determine whether to suppress I/O operations from hosts (via the local
array) to the mapped external LU.
If you select Enable, you can use the mapped LU only for Flex Copy XP operations.
If you select Disable, a host connected to the local array can use the mapped external LU as if it
were an LU inside the local array, but you cannot use the LU for Flex Copy XP operations.
You can select Enable only when you set the OPEN-V emulation type for the mapped LU. When
you set an emulation type other than OPEN-V, the IO Suppression mode is automatically set to
Disable.
• Cache Mode (Enable or Disable)
Cache mode specifies if I/O from the host is propagated synchronously or asynchronously to the
external storage device. All I/O to and from the local array in both cache modes always uses
some amount of cache. Write operations are always backed up in duplex cache.
Data that is not written by the host (for example, data written by XP Business Copy) is
asynchronously destaged to the external subsystem regardless of the Cache Mode setting.
External Storage XP user guide for the XP12000, XP10000, SVS200 25