Alternate paths – HP StorageWorks XP Remote Web Console Software User Manual
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NOTE:
When you set the cache mode, note the following:
◦
Data that is not written by the host (for example, data written by XP Business Copy
Software) is asynchronously destaged to the external array 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 array 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 array as configured in the
Cache Mode setting.
•
CLPR
When using XP Disk/Cache Partition to partition cache memory, set the cache logical partition
(CLPR) used for accessing the mapped LU. You can also specify whether the CU selected at
mapping is restricted to the storage management logical partition (SLPR) the CLPR belongs to.
For more information about CLPRs and SLPRs, see the HP XP Disk/Cache Partition user guide
for XP12000/XP10000 Disk Arrays and SVS 200.
Alternate paths
When mapping an external LU as an internal LDEV, paths are set from the internal LDEV to the
external LU.
If two or more paths to the external LU are equipped from different clusters, the number of paths
you set when mapping the volume are available. If one path is equipped, only that path is available.
You can set up to eight paths to each external LU, including paths automatically set. Among the
paths to the external LU, the path with the highest priority is called the primary path, and other
paths are alternate paths.
Alternate path modes include Single mode and Multi mode. The alternate path mode, Single mode
or Multi mode, depends on the connected external array. For Single mode, only the path with the
highest priority (primary path) is used to execute I/Os to the external LU. When an error occurs
in the primary path, the path with the second highest priority is used (that is, no per-LU dynamic
load balancing across paths). For Multi mode, all set paths are used at the same time. The paths
are used to execute I/Os to the external LU, distributing the work load (round-robin processing).
For example, when an external LU volume with an alternate path in Single mode is mapped to an
internal LDEV using XP External Storage, host I/O operations to the external LU via the local array
are normally enabled using the mapped path. If the mapped path is not available (for instance,
during array maintenance or following a failure in the channel processor), the path is switched
automatically to the alternate path (if available). As long as an alternate path is available, host
I/O operations continue as usual, even when an error occurs in the original path.
NOTE:
When the primary path cannot be used for the length of the Path Blockade Watch timer
(for example, 180 seconds), the path is switched to an alternate path.
If you have not configured any alternate paths, host I/O operations are suspended when the
primary path becomes unavailable (such as during array maintenance operations or following a
failure in the channel processor).
HP recommends configuring alternate paths for safer operation and increased bandwidth. For
instructions, see
Setting alternate paths for external LUs
You can set alternate paths when the external LUs are mapped as the internal LDEVs (see
). You can also set alternate paths after completing the mapping operation
(see
Setting alternate paths for external LUs
).
Preparing for XP External Storage settings
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