HP StorageWorks XP Remote Web Console Software - Manual (Page 11)

Cache LUN XP user guide for the XP10000/XP12000 11
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.
Configuration Restrictions For example, in Figure 2 , the LUSE volume is composed of three LDEVs (00:01,
1,000 LBAs, 00:02, 1,002 LBAs, and 00:03, 1,020 LBAs). If you want to set
Cache LUN XP to 500 LBAs starting from LBA number 1,020, you can set
Cache LUN XP to 500 LBAs starting from LBA number 20 on the second LDEV,
because the first LDEV size is 1,000 LBAs.
Figure 2 Example of LBA value setting with LUSE volumes
• You cannot allocate Pool-VOLs and V-VOLs for Cache LUN XP. For more
information, see the HP StorageWorks Snapshot XP User Guide.
Usage Restrictions • If an LDEV is placed in cache, do not use it for BC/Continuous Access XP quick
restore operations or Auto LUN migration. These operations may swap the
internal locations of the primary and secondary volumes and cause a loss of
data integrity.
• You cannot change a Cache LUN XP extent after it is configured. To change the
specification, you must reset (
release ) the Cache LUN XP extent, and then
re-allocate ( set ) the Cache LUN XP extent.
VSC Automatic Resets In the following cases, a Cache LUN XP cache extent is deleted (reset or released)
automatically:
• If a fixed volume, part or all of which is assigned to Cache LUN XP, is
converted into free space by the VSC Volume to Space function.
• If a custom volume, part or all of which is assigned to Cache LUN XP, is
converted into free space using the VSC Volume to Space function.
• If an LDEV (or VDEV) containing fixed and custom volumes assigned to Cache
LUN XP is initialized by the VSC Volume Initialize function.
Table 3 Restrictions (continued)