HP XP P9500 Storage - Manual (Page 23)

You cannot map an external volume whose capacity is smaller than the minimum available
capacity.
For example, you cannot map an external volume of 10 GB as an internal volume requiring at
least 30 GB.
The maximum or minimum available capacity of an external volume depends on the emulation
type that is set when the volume is mapped.
Registering a volume to an external volume group (ExG)
When you map an external volume as an internal volume, you need to register the external volume
to an external volume group.
External volumes, which are set by External Storage, can be classified into groups by usage. Any
group of this type is called an external volume group (ExG). For instance, you can register multiple
volumes in one external storage system to one external volume group. Or you can register the
volumes in one external volume group and manage them in block, even though the data you want
to manage in a lump is stored in volumes in the different external storage systems.
You need to assign numbers to external volume groups.
Configuring external volume attributes
When you map an external volume as an internal virtual volume, you set the attributes of the
external volume. External volume attributes can be set using the mapping policy or when you map
an external volume.
The attributes of the external volume are as follows:
Emulation type
Select an emulation type for the mapped external volume from the list.
The emulation type OPEN-V must be selected if, after the mapping, you are planning to use the
existing data in the external volume from the local storage system. For example, if you want to
migrate the existing data in the external volume to the local storage system volume, set the OPEN-V
emulation type when the external volume is mapped.
If you select the emulation type other than OPEN-V, the volume requires a specific area provided
for management data. Once this area is provided, volume capacity after the mapping becomes
less than the actual external volume capacity for the area (volume).
Cache mode (Enable or Disable)
Cache mode specifies whether the write data from the host to the external storage system is
propagated synchronously (Disable) or asynchronously (Enable). By default, cache mode is set to
Disable. All I/O to and from the local storage system (either Enable or Disable) always uses cache.
Write operations are always backed up in duplex cache.
•
If you select Enable, after receiving the data into the local storage system cache memory, the
local storage system signals the host that an I/O operation has completed and then
asynchronously destages the data to the external storage system.
•
When emulating OPEN volumes and selecting Disable, the local storage system signals the
host that an I/O operation has completed only after the local storage system has synchronously
written the data to the external storage system.
When emulating mainframe volumes and selecting Disable, after receiving the data into the
local storage system cache memory, the local storage system signals the host that an I/O
operation has completed and then asynchronously destages the data to the external storage
system.
If you perform the Cache Residency operation on the external volume, which the Cache Mode
is set to Disable, the bind mode of Cache Residency cannot be specified. For the Cache
Configuring External Storage 23