HP StorageWorks External Storage XP user guide for the XP12000, XP10000, SVS200 (T1706-96009, October 2007)

3Managingcachewithexternal
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
associatedreadorwriteisbuffered.Whencachemodeisdisabled,thearraylimitscachingtooneblock
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
aperformancebenefit for some very specific and well-defined workloads, such as single-threaded
sequential access to a device. HP strongly recommends that you configureallexternalstoragedevices
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
• Disabl
e 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.
•Alway
s disable cache mode for external LUs that are used by XP data replication or data
migrationsoftware(forexample,P-VOLs,S-VOLs,poolvolumes,andAutoLUNXPorXP
Tiered Storage Manager target volumes).
See De
termining, 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).
•Alwa
ys partition external storage LUNs that are targets for XP data replication or migration
soft
ware (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
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