Preserve Mirror option
This topic presents recommendations for using the Preserve Mirror option in FlashCopy® and Metro Mirror sessions.
When the source of the FlashCopy relationship
is a source of a Metro Mirror relationship, and the target of the FlashCopy relationship is
the source of a Metro Mirror relationship, the Preserve Mirror option
attempts to preserve the consistency of the Metro Mirror relationship
at the target of the FlashCopy relationship,
preventing a full copy from being performed over the Metro Mirror
link. Instead, parallel flashes are performed (if possible) on both
sites. If the consistency cannot be preserved, the Flash for the FlashCopy relationships fails,
and the data of the Metro Mirror relationship at the target of the FlashCopy relationship is
not changed.
Note: This option is available only on DS8000® storage devices with the required code
levels installed.
When you use
Preserve Mirror, where the sources of the FlashCopy relationships are sources in a multi-target
Metro Mirror relationship, you must currently specify which of the Metro Mirror relationships for
the Preserve Mirror feature to
use. To make this
specification, add the following property to the rmserver.properties file:
com.ibm.csm.<sessionName>.<rolepair>.userpfc. When the Metro Mirror pairs
are established, any pairs that are marked with this
property are set up so that when the
source is flashed the Preserve Mirror feature knows to use these pairs. For more information, see
the rmserver.properties file.
Notes:
- This property default is set to false. It must be set to true to enable the option on the role pair.
- This property is only used when establishing the Metro Mirror relationships. If the relationships are already established on the hardware, they need to be suspended and restarted to set up for Preserve Mirror. Alternatively, the DSCLI can be used to mark the pairs for Preserve Mirror. Use the DSCLI chpprc command with the -action enable option (to turn it on), and -action disable (to turn it off).
- When a pair is set up for Preserve Mirror in Copy Services Manager, there is currently no way to unset the pair. If there is a requirement to switch Preserve Mirror between the two Metro Mirror multi-target relationships, use the DSCLI to unset the current pair before you change the property to handle the other pair. Alternatively the Metro Mirror pair can be terminated, which removes the association on the DS8000.
However, in some instances, the Preserve Mirror option can cause a Metro Mirror session to go into a Preparing state, or even a Suspended state. This topic describes the recommended usage of the Preserve Mirror feature. Using this feature in other ways might lead a Metro Mirror session to go into a Preparing or Suspended state.
FlashCopy session
You
can use the Preserve Mirror option in FlashCopy sessions in two different methods:
- Perform an incremental resynchronization
- To perform an incremental resynchronization, select the Incremental and Persistent options in the FlashCopy session: do not select the No Copy option.
- Perform a single full copy
- To perform a single full copy, ensure that the Incremental, Persistent, and No Copy options are not selected before you issue a Flash command. If you use the No Copy option, issue either an Initiate Background Copy command or Terminate command before you issue the Flash command.
See your DS8000 documentation for more information about the Preserve Mirror function.
Metro Mirror session
You can set up your Metro Mirror pairs in two different ways, depending on the level of
consistency you need, and your preferences.
Note: For the examples in this section, the source pair
is H1a->H2a and the target pair is H1b->H2b. The source pair contains volumes that are the source of
the FlashCopy relationship and the target pair always
contains volumes that are the target of the FlashCopy
relationship.
- Create one Metro Mirror session, and add the Metro Mirror pairs as copy sets to that session
- The benefit to this approach is that you do not need to worry about whether the host considers
the H1a->H2a and H1b->H2b volumes to be consistent with one another. Copy Services
Manager ensures that all of the volumes remain consistent.
A drawback to this approach is that when you use the Attempt to preserve Metro Mirror consistency, but fail FlashCopy if Metro Mirror target consistency cannot be preserved option (Preserve Mirror Required), there is a chance that the target pair (H1b->H2b) might suspend unexpectedly. This situation causes all other pairs in the Metro Mirror session to suspend (including H1a->H2a). This can occur when a FlashCopy establish or withdraw fails unexpectedly on the remote (H1b->H2b) site. If the host requires the H1a->H2a and H1b->H2b volumes to be consistent, then you should suspend all other volumes.
- Create one Metro Mirror session for the H1a->H2a volumes, and another Metro Mirror session for the H1b->H2b volumes
- Use this option when the hosts and applications do not require the H1a->H2a and H1b->H2b volumes to be consistent with one another. In this case, you should create one Metro Mirror session for all of the H1a->H2a volumes, and another Metro Mirror session for the H1b->H2b volumes. The H1a->H2a pair is added to the first session, while the H1b->H2b pair is added to the second Metro Mirror session. If the host does not require consistency between the H1a and H1b volumes, this option benefits you when you use the Attempt to preserve Metro Mirror consistency, but fail FlashCopy if Metro Mirror target consistency cannot be preserved option (Preserve Mirror Required). The benefit is that if one pair is suspended (such as H1a->H2a), the pairs in the other session are not affected, since it is in a different Metro Mirror session. Using this method, you can avoid the situation in which a critical application is writing to the source pair (H1a->H2a), while a batch job is writing to the target pair (H1b->H2b), and both pairs are in the same Copy Services Manager session. These factors cause both applications to receive extended long busy signals, instead of just the batch job.