HP XP7 Storage - Manual (Page 73)

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 for Cache Residency” (page 74) .
Table 4 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)
= 198 KB = 12 cache
segments
Cache segments needed per slot: 12
(slot capacity / cache segment
capacity)
Note: Even though a mainframe track
is 56 KB, because cache is divided
into 16.5 KB segments, it requires 4
segments.
2 times the space required for
user data: 1 slot = 2 × 66 KB Slot capacity: 66 KB
Cache segment capacity: 16.5 KB
RAID 1 mainframe, or
external volumes
= 132 KB = 8 cache
segments
Cache segments needed per slot: 8
(slot capacity / cache segment
capacity)
Prestaging data in cache
Normally, the Cache Residency data is staged into the Cache Residency cache at the first access
from the host. From the second access, cache hit can be done. By using the Cache Residency
prestage function, the cache hit is available from the first host access, further improving data access
performance. Prestaging can be used for both priority mode and bind mode operations.
Overview of Cache Residency 73