HP XP External Storage Software - Manual (Page 18)

set to Disable. All I/O to and from the local storage system (either Enable or Disable) always
uses cache mode. 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, when you select 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 Manager operation on an external volume with
Cache Mode set to Disable, the bind mode of Cache Residency Manager cannot be
specified. For Cache Residency Manager operations, see the HP StorageWorks
XP24000/XP20000 Cache Residency Manager User Guide.
When you set the cache mode, note the following:
◦ Data that is not written by the host (for example, data written by HP StorageWorks XP
Continuous Access Software) is asynchronously destaged to the external storage system
regardless of the Cache Mode setting.
◦ If you set the emulation type for a mainframe system (such as 3390-x), data that is written
by a host using a command such as Format Write is asynchronously destaged to the
external storage system regardless of the Cache Mode setting. Data that is written by a
host using a command such as Update Write is destaged to the external storage system
as configured in the Cache Mode setting.
•
Inflow Control (Enable or Disable)
Inflow control specifies whether the writing operation to the cache memory is stopped (Enable)
or continued (Disable) when the writing operation to the external volume is impossible. By
default, inflow control is set to Disable.
◦ If you select Enable, the writing operation to cache is stopped and the I/O from the host
is not accepted when the writing operation to the external volume is impossible.
◦ If you select Disable, the I/O from the host during the retry operation is written to the
cache memory even after the writing operation to the external volume is impossible. Once
the writing operation to the external volume becomes normal, all the data in the cache
memory is written to the external volume (all the data is destaged).
•
CLPR
When the cache memory is partitioned using HP StorageWorks XP Disk/Cache Partition
Software, configure a cache logical partition (CLPR) to access the mapped volume. It is strongly
recommended that you place External Storage array groups in a CLPR other than CLPR0. See
the HP StorageWorks XP24000/XP20000 Disk/Cache Partition User Guide for detailed
information on CLPR.
Cross-subsystem Paths
A cross-subsystem path is a route from a local storage system port to an external storage system
port. To prepare for possible failures of the cable, the switch, or the channel processor, HP
recommends that you create redundant cross-subsystem paths. This redundancy allows you to
continue performing the I/O operations to the external volumes when you maintain the cable. You
can set up to eight paths.
18 About XP External Storage Operations