HP XP P9000 Continuous Access Synchronous for Mainframe Systems User Guide

• If system mode 20 is enabled, the VOLSER of the suspended S-VOL may be changed, which
allows the S-VOL to be online to the same host as the P-VOL, while the pair is suspended. All
other write I/Os to the S-VOL are rejected. The main system copies the P-VOL VOLSER onto
the S-VOL when the pair is resynchronized.
• For write-access to an S-VOL, the pair must be released.
Fence Level options for I/O to the P-VOL after suspension
The P-VOL Fence Level setting determines whether the host is denied access or continues to access
the P-VOL when the pair is suspended due to an error. You specify one 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
currency of the S-VOL during disaster recovery efforts. This setting is also designed for
applications that can continue to operate with another device pair.
This setting is functionally equivalent to the CRIT(YES-ALL) parameter for the CESTPAIR
command.
• Status – The P-VOL is fenced only if the main system is not able to change S-VOL status to
suspended when an update copy operation fails. If the main 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 currency 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
outweighs data recovery. “Never” ensures that the P-VOL remains available to applications
for updates, even if all Cnt Ac-S Z copy operations have failed. The S-VOL may no longer be
in sync with the P-VOL, but the main 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 currency of the S-VOL is determined by using the sense information transferred
through host failover or by comparing the S-VOL contents with other files confirmed to be
current.
This setting is functionally equivalent to the CRIT(NO) parameter for the CESTPAIR command.
Differential data options
You specify how differential data is managed during the Add Pair operation. Differential data is
update data that is collected when the pair is split. This data is stored in units of tracks or cylinders
until the pair is resynchronized. It is then restored to the S-VOL.
When you start the initial copy, you will specify whether to manage differential data by Tracks or
Cylinders. In Remote Web Console, you also have the Auto option. With Auto, either Cylinder or
Track is used, depending on the basic size of the LDEV.
• When managing differential data by cylinders, sharing volumes with Compatible FlashCopy
is restricted. See HP XP P9000 for Compatible FlashCopy Mirroring User Guide for more
information.
• When creating a pair with BCM, the Auto option is not available. If you do not specify
anything, Cylinder is used.
36 Planning for Continuous Access Synchronous Z