IBM System storage DS6000 Series, Redbook

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Chapter 13. Metro Mirror options and configuration
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The ICKDSF PPRCOPY FREEZE and PPRCCOPY RUN commands provide the same
functionality as the TSO CGROUP FREEZE and CGROUP RUN commands.
If you are using the DS Command-Line Interface (DS CLI), you can use the
freezepprc
and
unfreezepprc
commands to provide the same functionality as the TSO CGROUP FREEZE
and CGROUP RUN commands. It is not possible to issue Consistency Group
(freeze/unfreeze) type commands from the DS Storage Manager Graphical User Interface.
13.3.4 Critical attribute
The critical attribute determines what will happen after a failed I/O to a secondary volume
occurs. What happens will depend on how this attribute was set when the pair was
established, and how the DS6000 was configured.
The critical attribute is set when establishing the volume pair. If using TSO commands, the
CRIT parameter of the CESTPAIR command is used for this. If you are using the DS CLI, the
critical attribute is set using the
mkpprc -critmode parameter. If using ICKDSF, specify the
CRIT(YES) parameter of the PPRCOPY ESTPAIR command. If using the DS Storage
Manager GUI, select the
Enable critical volume mode option in the Select copy options
page when defining volume pairs.
If CRIT(NO) was specified on the CESTPAIR command (or equivalent options through the
other interfaces) when the pairs were established, then following an I/O error completion to
the secondary volume, Metro Mirror will suspend the volume and allow subsequent write
requests to the Metro Mirror primary volume. The pair is suspended, the secondary volume
does not receive any more updates, and the primary DS6000 will perform change recording,
thus keeping track of the primary updates for subsequent resynchronization.
If CRIT(YES) was specified with the CESTPAIR command (or equivalent options through the
other interfaces), and if an I/O error to the secondary volume occurs, Metro Mirror will either
allow or disallow subsequent writes to the primary, depending on how the DS6000 was
configured. For z/OS, the
IEA491E message will present the reason SUSPENDED ,
CRITICAL-STATE, ALL WRITES FAILED . The Metro Mirror pair then remains in a suspended
state until the problem is corrected and a CESTPAIR RESYNC or CDELPAIR command is
later issued.
Whether subsequent writes to the primary volumes are allowed or not will depend on how the
DS6000 LCU is configured:
򐂰
DS CLI:
chlcu -critmode enable
򐂰 DS Storage Manager GUI: Copy Services
→
Paths
→
Select LSS Copy Options from
the pull down list
→
Go
→
Accept or modify the selected LSSs from the Select LSS
pull-down list
→
Select the Critical Mode option to enable the setting
→
OK.
Note: In order to re-synchronize your Metro Mirror environment after the problem has been
resolved, you will need to redefine your Metro Mirror paths (they are removed as part of the
CGROUP FREEZE command). You can then issue CESTPAIR commands with
MODE(RESYNC) (or equivalent commands in the other interfaces).
Tip: We recommend using FlashCopy against your secondary volumes prior to a resyn-
chronization. This is because you do not have data consistency until all volumes have
reached the Duplex state.