HP XP Cache LUN Software - Manual (Page 10)

10 Cache LUN XP for the XP1024/XP128
Unit of cache area allocation
A Cache LUN XP extent is a range of logical blocks within an LDEV. A logical block is 512 bytes.
For OPEN-V, a cache extent must have at least 128 Logical Block Addresses (LBAs), or 66 KB. For all other
supported open system emulation types, a cache extent must have at least 96 LBAs, or 49.5 KB. This is
known as a cache slot. A cache extent can have a maximum size of an entire LDEV, up to the limits of
Cache LUN XP cache memory. Allocate data to Cache LUN XP with a minimum size of 128 LBAs for
OPEN-V and 96 LBAs for other than OPEN-V.
For mainframe systems, a cache extent must be at least one cache slot or track (48 KB or 64 KB). Its
maximum size may be an entire LDEV, up to the limits of Cache LUN XP cache memory. Allocate data to
Cache LUN XP in units of cylinders and tracks.
Number of cache areas allowed
• In DKC: 1,024
• In LDEV: 1,024
Restrictions
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).
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.
Table 3 Restrictions
Configuration Restrictions • Do not exceed the capacity of the Cache LUN XP cache.
• Do not configure an Auto LUN XP reserved volume for Cache LUN XP.
• Do not place an LDEV in cache twice.
• You cannot allocate Cache LUN XP cache space for an on-demand LDEV.
• To apply Cache LUN XP to a LUSE volume, you must apply it to all the LDEVs
that form the LUSE volume.
Usage Restrictions • If an LDEV is placed in cache, do not use it for BC/Continuous Access XP quick
restore operations or Auto LUN XP 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.
CVS 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 CVS 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 CVS Volume to Space function.
• If an LDEV (or VDEV) containing fixed and custom volumes assigned to Cache
LUN XP is initialized by the CVS Volume Initialize function.