HP XP7 Storage - Manual (Page 79)

2. Calculate the required cache size according to the Cache Residency operation mode:
1.
Bind mode:
Required cache size (in KB) = (Number of slots Ч ((128 Ч (512 +
16)) Ч 3) / 1,024
2. Priority mode:
Required cache size (in KB) = (Number of slots Ч (128 Ч (512 +
16))) / 1,024
If the area is changed from priority mode to bind mode and no cache is added, then only 33%
of the user data will fit in the area previously assigned for priority mode, and the remaining 67%
is used to save read/write data.
CAUTION: If the area is changed and no cache is added, set the Cache Residency again.
Cautions and restrictions for Cache Residency
•
“Cautions and restrictions for cache residency” (page 79)
•
“Cautions and restrictions for prestaging” (page 80)
Cautions and restrictions for cache residency
•
Allocating cache
Do not attempt to allocate Cache Residency cache beyond the allocated capacity. ◦
◦ Do not attempt to allocate Cache Residency cache redundantly over a cache area that
is already allocated to an LDEV.
•
Maximum capacity limits
Specify 128 GB or less as the capacity of the BIND-mode cache residency extent per
parity group. If the capacity is too large, destaging will require too much time, thus
resulting in possible failure in planned outage or cache maintenance. ◦
◦ Do not specify too large a capacity for the cache residency extent allocated per processor
blade. Specify a value as follows:
[(capacity for BIND) x 3] + (capacity for PRIO) ≤ 190 GB
Specifying too large a capacity will result in too much overhead in cache management,
and thus cache residency performance may not improve as expected.
•
Unsupported volumes
Pool-VOLs and V-VOLs. You cannot allocate pool-VOLs and V-VOLs for Cache Residency.
For more information about pool-VOLs and V-VOLs, see the HP XP7 Fast Snap User Guide, ◦
the HP XP7 Provisioning for Open Systems User Guide, or the HP XP7 Provisioning for
Mainframe Systems User Guide
◦ Journal volumes. You cannot allocate the journal volumes for Cache Residency. For
additional information about the journal volumes, see the HP XP7 Continuous Access
Journal User Guide or the HP XP7 Continuous Access Journal for Mainframe Systems User
Guide.
◦ Remote command device. You cannot allocate the remote command device for Cache
Residency. For more information about the remote command device, see the HP XP7
External Storage for Open and Mainframe Systems User Guide.
◦ FlashCopy SE TSE-VOLs. Cache Residency cannot be assigned to a TSE-VOL of Compatible
FlashCopy SE.
Cautions and restrictions for Cache Residency 79