HP XP7 Storage - Manual (Page 36)

If the volumes that become S-VOLs are physically attached to the same system images as the
volumes that become the P-VOLs, the following error-prone situations may occur:
•
When a pair is established using the TSO CESTPAIR command, the secondary system might
be online (PPRC allows this; Continuous Access Synchronous Z does not). This could produce
a duplex secondary online to a host image, the results of which are not predictable.
•
When a pair is released, the old S-VOL is usually offline. Then, when a host system is IPL’d
(initial program loaded), the operator is offered both volumes and asked which volume should
be left offline. You can avoid duplication of the VOLSER as follows:
1.
Identify the volumes that are not accessed by the host system.
2. Perform CHP OFF or some other operation to ensure that the volumes are inaccessible.
3. When performing IPL, make sure to perform LOAD CLEAR.
To avoid such situations:
•
HP strongly recommends that you specify OFFLINE=YES if the secondary volumes are to be
generated in the production host’s IOCP and system generation.
•
If you cannot create a pair because the S-VOL is online with hosts, all paths must be offline
from all hosts. If you cannot identify the hosts that are online, contact your HP account team.
You can obtain the information about online paths from SSBLOG of the SVP.
About allowing I/O to the S-VOL
The secondary system rejects I/O to the S-VOL, unless the S-VOL-read option is enabled. The S-VOL
read option allows read-only access to an S-VOL while the pair is split. Differential data is managed
using bitmaps by track; this data is used when the pair is resynchronized.
•
The S-VOL read option (mode 20) must be enabled by a HP representative.
•
When you read an S-VOL from the host, the reference date in the VTOC is updated. In this
case, you must permit update of the VTOC.
•
The CSUSPEND command to the S-VOL of a split pair will be rejected when the S-VOL read
option is enabled, for I-2107 controller emulation.
•
If system mode 20 is enabled, the VOLSER of the split 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 split. All other write
I/Os to the S-VOL are rejected. The primary system copies the P-VOL VOLSER to the S-VOL
when the pair is resynchronized.
•
For write-access to an S-VOL, release the pairs, or enable the write option and then split the
pairs.
Allowing I/O to the P-VOL after a split—Fence Level options
You can specify whether the host is denied access or continues to access the P-VOL when the pair
is split due to an error. This is done with the Primary Volume Fence Level setting. 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 consistency
of S-VOL data with the P-VOL during disaster recovery efforts.
This setting is functionally equivalent to the CRIT(YES-ALL) parameter for the CESTPAIR
command.
•
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,
36 Planning for Continuous Access Synchronous Z