HP XP7 Storage - Manual (Page 37)

and the system will keep track of updates to the P-VOL. This allows the pair to be resynchronized
quickly. This setting also reduces the amount of time required to analyze S-VOL consistency
during disaster recovery.
This setting is functionally equivalent to the CRIT(YES PATHS) parameter for the CESTPAIR
command.
•
Never – the P-VOL is never fenced. This setting should be used when I/O performance
out-weighs data recovery. “Never” ensures that the P-VOL remains available to applications
for updates, even if all Continuous Access Synchronous Z copy operations have failed. The
S-VOL may no longer be in sync with the P-VOL, but the primary system keeps track of updates
to the P-VOL while the pair is suspended. Host failover capability is essential if this fence level
setting is used. For disaster recovery, the consistency of the S-VOL is determined by using the
sense information transferred via host failover or by comparing the S-VOL contents with other
files confirmed to be consistent.
This setting is functionally equivalent to the CRIT(NO) parameter for the CESTPAIR command.
Differential data
Differential data is update data collected while the pair is split.
With XP7, data is stored in units of tracks using bitmaps, and is then used to resynchronize the
pair.
•
If your primary system is a P9500 or earlier model, which support tracks and cylinders, make
sure that the unit of the differential data management is track.
•
If XP7 is the primary system and you are using RAID Manager, though RAID Manager allows
you to specify track or cylinder, only track will be used by the XP7.
Maximum number of pairs supported
Continuous Access Synchronous Z supports a maximum of 65,280 pairs, which is the maximum
number of pairs supported by the XP7 storage system. If BCM or RAID Manager is used, a command
device must be defined for each product; therefore, the maximum number of pairs in XP7 is
calculated by subtracting the number of command devices from 65,280.
Calculating maximum number of pairs
It is necessary to calculate the maximum number of pairs you can have on the XP7 storage system.
The maximum number is based on the following:
•
The number of cylinders in the volumes, which must be calculated.
•
The number of bitmap areas required for a Continuous Access Synchronous Z volume, which
is calculated using the number of cylinders.
NOTE: In the following formulas, ceil() indicates that the result within the parentheses should
be rounded up to the nearest integer, and floor() indicates that it should be rounded down to
the nearest integer.
Procedure 1 To calculate the maximum number of pairs
1. Calculate the number of cylinders.
1.
Calculate the system’s number of logical blocks.
Number of logical blocks = Volume capacity (bytes) / 512
2. Calculate the number of cylinders:
For 3390:
Number of cylinders = ceil ( (ceil (Number of logical blocks /
116) ) / 15)
Planning pairs and pair volumes 37