HP StorageWorks External Storage XP user guide (T1706-96006, June 2006)

External Storage XP user guide 45
3 Managing cache with external storage
XP disk arrays and storage virtualization systems use cache to facilitate host and internal I/O. When cache
mode is enabled, host I/O is accepted as quickly as it is received into the array’s cache where the
associated read or write is buffered. When cache mode is disabled, the array limits caching to one block
of read and write data per LDEV. Cache is also used by Data Mirroring and Data Migration products that
generate internal I/O to copy and move data between devices, such as:
• Business Copy XP
• Continuous Access XP software products
• Business Copy XP’s Snapshot feature
• Auto LUN XP
• XP Tiered Storage Manager
Having cache mode enabled for external storage devices that are slower than the XP array can provide a
performance benefit for some very specific and well-defined workloads, such as single-threaded sequential
access to a device. HP strongly recommends that you configure all external storage devices with cache
mode disabled unless you have determined that you meet these exact workload descriptions. When you
deviate from these workload descriptions, performance might not be as good as when cache is disabled.
Consult your HP account support representative to discuss the availability of tuning services for your XP
array.
This chapter explains how to balance cache usage between internal and external storage and protect the
potential benefits of using cache.
Guidelines for using cache with external storage
• Disable cache for external devices.
• Always disable cache mode for external LUs in an MSA or other storage array that is slower than
the XP or SVS200 external ports.
• Always disable cache mode for external LUs that are used by XP data replication or data migration
software (for example, P-VOLs, S-VOLs, pool volumes, and Auto LUN XP or XP Tiered Storage
Manager target volumes).
See ”Determining, setting, or changing the external LU cache mode” on page 45.
• Partition cache for external storage.
• Always put external volumes and internal XP volumes in separate cache partitions (CLPRs).
• Always partition external storage LUNs that are targets for XP data replication or migration software
(Business Copy XP, Continuous Access XP software products, Snapshot XP, Auto LUN XP, or XP
Tiered Storage Manager).
• Partition any external LUs that have cache enabled. External LUNs with cache enabled can use
large amounts of cache.
See ”Partitioning cache for external storage” on page 49.
• Consult HP account support representative to verify or change storage system modes. See ”Changing
storage system modes” on page 53.
Determining, setting, or changing the external LU cache mode
You set the cache mode when you define external LUs. HP strongly recommends that you set cache mode
to Disabled for external LUs. For instructions on mapping external LUs in a new configuration, see
”Mapping external LUs (Add LU)” on page 73.
CAUTION: Before you delete any LUs that you want to remap, carefully record necessary configuration
information. If you remap existing LUs with changed configuration settings (other than cache mode), you
can lose access to the LUs and the data they contain.