HP XP Racks - Manual (Page 21)

of the following Fence Level options during the initial copy and resync operations. You can also
change the Fence Level option outside these operations.
•
Data – the P-VOL is fenced if an update copy operation fails. This prevents the host from writing
to the P-VOL during a failure. This setting should be considered for the most critical volumes
for disaster recovery. This setting reduces the amount of time required to analyze the consistency
of S-VOL data with the P-VOL during disaster recovery efforts.
•
Status – the P-VOL is fenced only if the primary system is not able to change S-VOL status to
suspended when an update copy operation fails. If the primary system successfully changes
S-VOL pair status to suspended, subsequent write I/O operations to the P-VOL will be accepted,
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.
•
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 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.
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 supports a maximum of 65,280 pairs, which is the maximum
number of pairs supported by the XP7 storage system. If 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.
The number of LDEVs is used to determine the maximum number, not the number of LUs.
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 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.
Planning pairs and pair volumes 21