HP XP Racks - Manual (Page 16)

When using external storage systems with ESAM, please observe the following:
•
Optional external storage systems may be used to store pair data.
For supported external systems, see the HP XP P9000 External Storage for Open and Mainframe
Systems User Guide.
•
You can connect one external system per ESAM P-VOL, and one per S-VOL.
•
The maximum number of external systems that can be connected depends on the number of
the external ports that can be defined for a storage system.
Planning failover
Automatic failover of host operations to the secondary storage system is part of the ESAM system.
Failover occurs when the primary storage system cannot continue I/O operations due to a failure.
The multipath software in the host switches I/O to the secondary storage system.
•
The multipath software automatically configures the path to the P-VOL as the owner path when
you create an ESAM pair.
•
The path to the S-VOL is automatically configured as the non-owner path.
In the ESAM system, the quorum disk stores the information about the state of data consistency
between the P-VOL and S-VOL, which is used to check whether P-VOL or S-VOL contains the latest
data. If the P-VOL and S-VOL are not synchronized due to a failure, the MCU and RCU determine
which volume should accept host I/O based on the information stored in the quorum disk.
The following figure illustrates failover when a failure occurs at the MCU.
ESAM also performs the following checks to detect failures:
•
The RCU issues service information messages (SIMs) when the data path is blocked. The
multipath software issues messages about the failure in the host-MCU paths.
•
Health check of the quorum disk by the MCU and RCU. The primary or secondary storage
system issues a SIM if a failure in the quorum disk is detected. Host operations will not switch
to the S-VOL if the quorum disk fails. In this case, the failure must be cleared as soon as possible
and the quorum disk recovered.
•
If the multipath software detects a failure in the host-to-pair volume paths, the operation switches
to a different available path and no SIM is issued. To stay informed about path status, monitor
the path failure messages issued by the multipath software.
•
The multipath software issues message when all host-MCU paths fail. These messages must
then be checked and the cause corrected. If failover took place, host operations should be
switched back to the primary storage system.
16 System implementation planning and system requirements