Cautions And Restrictions For Cache Residency - HPE XP7 User Manual

2. Calculate the required cache size according to the Cache Residency operation mode:
1. Bind mode:
Required cache size (in KB) = (Number of slots × ((128 × (512 +
16)) × 3) / 1,024
2. Priority mode:
Required cache size (in KB) = (Number of slots × (128 × (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:

Cautions and restrictions for Cache Residency

"Cautions and restrictions for cache residency" (page 144)
"Cautions and restrictions for prestaging" (page 145)
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) ≤ 192 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 XP7 Fast Snap
User Guide, the XP7 Provisioning for Open Systems User Guide, or the 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 XP7 Continuous Access
Journal User Guide or the XP7 Continuous Access Journal for Mainframe Systems User
Guide.
144 Cache Residency operations
If the area is changed and no cache is added, set the Cache Residency again.