HP XP P9500 Storage - Manual (Page 14)

Differential tables (tables managing differential bitmaps) and pair tables are required to create
pairs. You must supply an adequate number of tables to handle all the pairs you plan to create.
The number of differential and pair tables that are present in the system depends on additional
shared memory that is installed. Therefore, you must ensure that you have sufficient additional
shared memory to handle the number of pairs.
The following table shows the number of differential and pair tables and the number of volumes
allowed per additional shared memory. The maximum number of pairs is half the number of volumes
shown, when P-VOLs and S-VOLs are in a one-to-one relationship.
For example, if additional shared memory is not installed, since the number of volumes is 16,384,
then the number of pairs you can create is 8,192. This example assumes one S-VOL per P-VOL. If
there are more S-VOLs than P-VOLs, then the number of allowed pairs decreases.
However, for Extension 1 and Extension 2, the maximum number of pairs is 16,384, regardless
of the total number of system volumes.
Table 1 Differential tables, pair tables, additional shared memory
Number of system
volumes Number of pair tables Number of diff. tables Additional shared memory
16,384 8,192 26,176 Base (No additional shared memory)
65,536 16,384 104,768 Extension 1
65,536 16,384 209,600 Extension 2
You can determine the maximum number of pairs that can be created on your system by calculating
the number of differential and pair tables your system needs to create BC pairs. This number must
be equal to or less than the difference of the total number of differential and pair tables in the
storage system, minus the number of tables being used by the other products (for example,
Compatible FlashCopy).
The following software products also use differential tables:
•
BC Z
•
Compatible FlashCopy
•
Compatible FlashCopy SE
•
Auto LUN
•
SS
The following software products also use pair tables:
•
BC Z
•
Auto LUN (using migration plans)
NOTE: You can use RAID Manager’s inqraid command to query the number of the differential
tables required when you create BC pairs, though not for creating BC Z pairs. For BC, you can
also query the number of differential tables not used in the storage system with this command.
For more information about the inqraid command, see the HP XP P9000 RAID Manager User
Guide.
Calculating the number of differential tables and pair tables
Calculations in the following sections assume that you are only using BC in the system.
The emulation type of your volumes affects the number of differential and pair tables.
14 Requirements and planning