HP XP P9500 Storage - Manual (Page 38)

10 Estimating cache size
This chapter describes how to estimate the cache size required for using Cache Residency.
About cache size
The required cache size for using Cache Residency differs according to operation modes or RAID
levels. For example, if the bind mode is set, RAID1 storage systems require twice the size of cache
for the user data to use Cache Residency. However, RAID5 or RAID6 storage systems require three
times the size of cache. If external volumes are used, twice the cache size for the user data is
required to use Cache Residency.
NOTE: If a RAID5 or RAID6 volume 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.
If a RAID1 volume area is changed from priority mode to bind mode and no cache is added, then
only 50% of the user data will fit in the area previously assigned for priority mode, and the
remaining 50% is used to save read/write data.
Changing the mode without cache extension requires reconfiguring Cache Residency.
If the priority mode or the bind mode is set, the cache size is calculated assuming that one slot has
the following values.
•
For open-systems volumes:
For OPEN-V, one slot is 264 KB (512 LBAs). ◦
◦ For other than OPEN-V, one slot is 66 KB (128 LBAs).
•
For mainframe (3390) volumes:
◦ One slot is 66 KB (128 LBAs).
38 Estimating cache size