HP XP Cache LUN Software - Manual (Page 11)

Cache LUN XP user guide for the XP1024/XP128 11
• While destaging to disk, read requests to the same block are delayed until the disk operation is
completed.
• Priority mode generates write traffic to disk, whereas bind mode does not.
General cache capacity
In addition to the Cache LUN XP cache, you should set the general cache capacity according to the
number of areas set in priority mode. This prevents a reduction in general cache performance.
If the general cache is overloaded (having very little cache space), this can result in a wait for free cache
space. This may degrade access to the general cache.
• If you set up to 512 areas in Cache LUN XP, set 512 MB x 2 (1024 MB) of standard cache.
• If you set 513-1024 areas in Cache LUN XP, set 1024 MB x 2 (2048 MB) of standard cache.
• For OPEN-V, one slot is 66 KB, which requires 4 cache segments (16.5 KB/segment). For example,
16 slots require 1,056 KB of reserved cache.
• For other than OPEN-V, one slot is 49.5 KB, which requires 3 cache segments. For example, 16 slots
require 792 KB of reserved cache.
Bind mode
Bind mode keeps both read and write data in cache, eliminating disk traffic (100% read/write hits). All
data stays in cache until the extent is removed. The data stored in the Cache LUN XP cache is duplexed for
data protection, which requires additional Cache LUN XP cache capacity. If AC power is lost, data is
maintained in cache for up to 48 hours, using battery power.
Bind mode details
• All reads following the initial read to a block will result in cache hits.
• Duplicated write blocks are stored in cache and are never “evicted.”
• All read or write blocks stay resident in cache until the extent is deleted.
• Bind mode requires more cache because duplicated write blocks are not destaged to disk.
Bind mode and RAID groups
For RAID-5 parity groups, a Cache LUN XP extent in bind mode requires a cache capacity three times
larger than the user data.
• For OPEN-V, one slot is 66 KB, which requires 12 cache segments (16.5 KB/segment). For example,
16 slots require 3,168 KB of reserved cache.
• For other than OPEN-V, one slot is 49.5 KB, which requires 9 cache segments. For example, 16 slots
require 2,376 KB of reserved cache.
For RAID-1 parity groups, a Cache LUN XP extent in bind mode requires a cache capacity twice as large
as user data.
• For OPEN-V, one slot is 66 KB, which requires 8 cache segments (16.5 KB/segment). For example,
16 slots require 2,112 KB of reserved cache.
• For other than OPEN-V, one slot is 49.5 KB, which requires 6 cache segments. For example, 16 slots
require 1,584 KB of reserved cache.
If you want to change a RAID-1 or RAID-5 parity group’s mode from priority mode to bind mode, reset and
re-allocate the affected Cache LUN XP extent. When you re-allocate, specify the greater capacity needed
for bind mode.
Pre-staging
As mentioned before, data assigned to a Cache LUN XP cache extent becomes resident (“staged”) in
cache after it is first accessed by the host. Then, the host finds the data in cache for the second and
subsequent accesses. The pre-staging function puts data in cache before the first read. Therefore,
pre-staging allows the host to find data in cache on the first access.
Specify pre-staging when you set up the cache extent. You can use this function with either priority or bind
mode.