HP XP Cache LUN Software - Manual (Page 12)

12 Cache LUN Introduction
You can select a CU group from the drop-down list for CU groups in the upper right on the XP Remote Web
Console main pane. After you select a CU group, you can use LDEVs only in the selected CU group. You
cannot apply or cancel Cache LUN XP settings on multiple CU groups. For more information about CU
groups, see the
HP StorageWorks XP Remote Web Console user guide for XP12000/XP10000/SVS200.
NOTE: Regardless of whether you use LDEVs from mainframe or open-system hosts, you can perform the
prestaging operation for all CU groups.
Cache LUN XP modes
Cache LUN XP has two modes:
• priority mode
•
bind mode
Priority mode
Priority mode requires fewer cache resources but has performance limitations over bind mode. Priority
mode keeps all read data in cache (until you delete the extent), but all writes are destaged to disk normally
(using the same algorithms as for regular, non-Cache LUN XP data). You can set the priority mode from the
operations box located on the lower-right of the Cache LUN pane.
Priority mode details
• All reads following the initial read to a block will result in cache hits (100% cache read hits).
•
After destaging of the write block to disk, the duplicated write areas in general cache become free.
• 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. You can set the bind mode from the
operations box located on the lower-right of the Cache LUN pane.
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.