HP XP Continuous Access Software - Manual (Page 98)

98 Continuous Access XP for the XP1024/XP128
NOTE: The P-VOL fence-level setting does not apply to Continuous Access XP Asynchronous pairs.
Continuous Access XP Async P-VOLs are never fenced due to suspension of the pair.
Data. When S-VOL Data is selected, the P-VOL is fenced if an update copy operation fails. This P-VOL
fence level setting ensures that the S-VOL remains identical to the P-VOL after the Continuous Access XP
volume pair is synchronized, but makes the P-VOL inaccessible for updates whenever Continuous Access
XP remote copy operations fail. 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 operating with
another device pair.
Status. When S-VOL Status is selected, the P-VOL is fenced only if the MCU cannot change the S-VOL pair
status to PSUE. If the MCU successfully changes the S-VOL pair status to PSUE, subsequent write I/O
operations to the P-VOL are accepted, and the MCU tracks updates to the P-VOL. This allows the volume
pair to be resumed quickly using the pairresync operation (out-of-sync-cylinders only). This setting also
reduces the amount of time required to analyze the S-VOL currency during disaster recovery.
Never. When Never is selected, the P-VOL is never fenced when the pair is suspended. This P-VOL
fence-level setting ensures that the P-VOL remains available to applications for updates, even if all
Continuous Access XP copy operations failed. The S-VOL might no longer be in sync with the P-VOL, but
the MCU tracks 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 using sense
information transferred via host failover or comparing the S-VOL contents with other files confirmed to be
current.
Setting the fence level
When a take-over by the S-VOL occurs as shown in Figure 57 (two errors have already occurred), Data(V)
remains in the rollback process at the secondary host, and full recovery cannot be performed. To avoid
this situation, define the fence level of the Redo log file as Data, so the P-VOL returns an error if a data
disagreement concerning a write request issued from the host is expected to occur. The Data fence-level
setting maintains full consistency between the Redo log file and data file because no data is written to the
data file due to the write error at the log file. However, when the fence level is defined as Data, a write
I/O causes an error even when processing is suspended due to an error at the S-VOL. In this case, a
take-over by the S-VOL occurs, and the significance of the duplex system is lost. Therefore, if you define
the fence level as Data, applications must be able to handle write I/O errors. Systems allowing disk errors
Table 19 Fence-level settings on Continuous Access XP Sync P-VOLs
Type of failure Fence-level setting
S-VOL Data S-VOL Status
Never
Update copy operation failed,
and the MCU changed the
status of the S-VOL to PSUE.
Write I/O operations to the
P-VOL will be:
REJECTED Accepted Accepted
Update copy operation failed,
and the MCU could NOT
change the status of the S-VOL
to PSUE.
Write I/O operations to the
P-VOL will be:
REJECTED REJECTED Accepted