HP XP External Storage Software - Manual (Page 55)

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 .
• 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 .
• Consult HP account support representative to verify or change storage system modes. See Changing
storage system modes .
External Storage XP user guide for the XP12000, XP10000, SVS200 55