HP XP RAID Manager Software - Manual (Page 11)

Table 1 Cache capacity requirements for CR priority mode
Standard cache capacity Settings of priority mode
16 GB Specified number of cache areas is 8,192 or less and the specified
capacity is 128 GB or less
32 GB Specified number of cache areas exceeds 8,192 or the specified
capacity exceeds 128 GB
1 GB = 1,073,741,824 bytes
Bind mode (read and write data)
In bind mode the Cache Residency extents are used to hold read and write data for specific extents
on volumes. Data written to the Cache Residency bind area is not destaged to the drives. To ensure
data integrity, write data is duplexed in the Cache Residency cache area, which consumes a
significant amount of the Cache Residency cache.
Bind mode provides the following advantages over priority mode:
•
The accessibility of read data is the same as Cache Residency priority mode.
•
Write operations do not have to wait for available cache segments.
•
There is no back-end contention caused by destaging data.
The required total cache capacity for bind mode is:
standard cache + Cache Residency cache
Cache Residency bind data that has write attributes is normally not destaged. However, the data
is destaged to disk in the following cases:
•
During cache blockage that is caused by certain maintenance operations (for example, cache
upgrades) or by cache failure.
•
When the storage system is powered off.
•
When the volume is deleted from Cache Residency bind mode.
The next table specifies the cache requirements for bind mode operations. Meeting these
requirements is important for preventing performance degradation. For more information about
calculating cache size for bind mode, see
“Estimating cache size” (page 69) .
Table 2 Bind mode cache requirements
Cache Residency Cache
Requirement Capacity Specifications RAID Level or Volume
Type System Type
3 times the space required for
user data: 1 slot = 3 × 264 Slot capacity: 264 KB
Cache segment capacity: 16.5 KB
RAID 5 (3390) or RAID
6 Open systems
KB = 792 KB = 48 cache
segments
Cache segments needed per slot: 48
(slot capacity / cache segment
capacity)
2 times the space required for
user data: 1 slot = 2 × 264 Slot capacity: 264 KB
Cache segment capacity: 16.5 KB
RAID 1, or external
volumes
KB = 528 KB = 32 cache
segments
Cache segments needed per slot: 32
(slot capacity / cache segment
capacity)
3 times the space required for
user data: 1 slot = 3 × 66 KB Slot capacity: 66 KB
Cache segment capacity: 16.5 KB
RAID 5 mainframe or
RAID 6 Mainframe (for
example, 3390-3,
3390-9)
Cache Residency overview 11