Hitachi TrueCopy for IBM z/OS User and Reference Guide (T5211-96001, June 2007)

7-4 TrueCopy for z/OS Usage Scenarios
Hitachi Universal Storage Platform V TrueCopy for IBM z/OS User’s Guide
Considering the M-VOL Fence Level Setting
The M-VOL fence level setting for each TrueCopy for z/OS Sync pair
determines whether the M-VOL will be fenced when TCz remote copy
operations fail. Table 7-1 summarizes the effect of the fence level setting on a
TCz Synchronous M-VOL.
Note: The M-VOL fence level setting does not apply to TCzA pairs. The TCzA
M-VOL is never fenced due to suspension of the pair.
Table 7-1 Effect of the Fence Level Setting on a TrueCopy for z/OS M-
VOL
Write I/O operations to the M-VOL will be (On Each Fence Level Setting):
Type of Failure Data
(CRIT=YES-ALL)
Status
(CRIT=YES-PATHS)
Never
(CRIT=NO)
The update copy operation failed,
and the MCU was able to change the
status of the R-VOL to suspended.
REJECTED Accepted Accepted
The update copy operation failed,
and the MCU was NOT able to
change the status of the R-VOL to
suspended.
REJECTED REJECTED Accepted
Data (CRIT=YES-ALL). When this fence level setting is selected, the M-VOL
will be fenced if an update copy operation fails. This M-VOL fence level setting
makes the M-VOL inaccessible to applications for updates whenever TCz
remote copy operations fail. This setting should be considered for the most
critical volumes for disaster recovery. This setting will reduce the amount of
time required to analyze the currency of the R-VOL during disaster recovery
efforts. This setting is also designed for applications which can continue to
operate with another device pair (e.g., IMS logger dual write log files).
Status (CRIT=YES-PATHS). When this fence level is selected, the M-VOL is
fenced only if the MCU is not able to change the R-VOL pair status to
suspended. If the MCU successfully changes the R-VOL pair status to
suspended, subsequent write I/O operations to the M-VOL will be accepted,
and the MCU will keep track of updates to the M-VOL. This allows the pair to
be resynchronized quickly using the resync (out-of-sync-cylinders) copy
operation (MODE=RESYNC). This setting will also reduce the amount of time
required to analyze the R-VOL currency during disaster recovery.
Never (CRIT=NO). When this fence level is selected, the M-VOL is never
fenced when the pair is suspended. This fence level setting ensures that the
M-VOL remains available to applications for updates, even if all TCz copy
operations have failed. The R-VOL may no longer be in sync with the M-VOL,
but the MCU will keep track of M-VOL updates while the pair is suspended.
ERC is essential if this fence level setting is used. For disaster recovery, the
currency of the R-VOL is determined by using the sense information
transferred via ERC or by comparing the R-VOL contents with other files
confirmed to be current.