Accessing Continuous Access Xp P-Vols And S-Vols; Cache And Nonvolatile Storage (Nvs); Host Failover Software; Duplicate Volumes - HP StorageWorks XP12000 User Manual

Accessing Continuous Access XP P-VOLs and S-VOLs

To ensure maximum data integrity during normal Continuous Access XP operations, the RCU rejects
all write operations issued by a host to a Continuous Access XP S-VOL. If you need write access to a
Continuous Access XP S-VOL, split the pair using the S-VOL write option (see
you resume (resync) the split pair, the RCU sends the S-VOL cylinder bitmap to the MCU to ensure
proper resynchronization of the pair.

Cache and nonvolatile storage (NVS)

Cache and nonvolatile storage (NVS) must be operable for both the MCU and RCU of a Continuous
Access XP volume pair. If not, the Continuous Access XP paircreate operation fails. Configure the
remote disk array cache to adequately support Continuous Access XP remote copy workloads and
local workload activity.
NOTE:
NVS is the portion of cache with a battery backup.

Host failover software

Host failover software is a critical component of any disaster recovery effort. When an MCU fails to
maintain synchronization of a Continuous Access XP pair, the MCU generates sense information. This
information must be transferred to the remote site via the host failover software for effective disaster
recovery.

Duplicate volumes

Because the contents of the P-VOL and S-VOL of a Continuous Access XP pair are identical, the S-VOL
is considered a duplicate of the P-VOL. Because the host operating system does not allow duplicate
volumes, the host system administrator must take precautions to prevent system problems related to
duplicate volumes. You must define the Continuous Access XP S-VOLs so they do not auto-mount or come
online to the same host at the same time as the P-VOLs (see the following CAUTION).
Continuous Access XP does not allow the S-VOL to be online (except when the pair is split). If the S-VOL
is online, the Continuous Access XP paircreate operation fails.
CAUTION:
If Continuous Access XP S-VOLs are physically attached to the same host servers as Continuous Access
XP P-VOLs, a problem might occur. When a Continuous Access XP pair is deleted, the old S-VOL is
usually offline. If the host is then restarted, the system administrator may be offered both volumes and
asked which volume should be left offline. This can be confusing and is prone to error.
If Continuous Access XP S-VOLs and P-VOLs are connected to the same hosts, HP strongly recommends
that you define S-VOLs to remain offline (or create them with the RAID Manager 'no read' option)
to avoid this problem.

Host system crash

Contents of the P-VOL and S-VOL of a Continuous Access XP pair may become inconsistent if the host
system hangs (crashes) while a write operation is being performed on the Continuous Access XP P-VOL,
and the write operation cannot be retried because of the following reasons:
• A check condition is reported to the host due to a failure in the disk array when the write
command for the Continuous Access XP pair is issued.
• A reset command is issued from the host, and the write operation is stopped (write operation
is incomplete).
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Continuous Access XP for the Array
S-VOL write option). When