HP XP RAID Manager Software - Manual (Page 143)

Continuous Access Synchronous/Continuous Access Asynchronous and Continuous
Access Journal fence level settings
Continuous Access Synchronous volume pairs are assigned a fence level for write I/Os to ensure
mirroring consistency of critical volumes. When the secondary volume takes over from the primary
volume, the takeover action is determined according to the pair status and fence level of the
corresponding secondary volume.
Table 33 (page 143) shows the relationship between Continuous
Access Synchronous pair statuses and fence levels.
•
Mirror consistency = Identity and sequence of data is assured via error notification after an
I/O completion.
•
Data consistency = Sequence of data is assured in I/O order based on host.
Table 33 Relationship between Continuous Access Synchronous pair statuses and fence levels
Fence level and write response Continuous Access Synchronous pair status of
volume
Async
4
Never
3
Status
2
Data
1
OK OK OK OK
Data consistency
assured Mirroring
consistency
assured Mirroring
consistency
assured Mirroring
consistency
assured
OK OK OK ERROR
Data consistency
assured Mirroring
consistency not
assured Mirroring
consistency not
assured Mirroring
consistency
assured
OK OK ERROR ERROR
Data consistency
assured Mirroring
consistency not
assured Mirroring
consistency
assured Mirroring
consistency
assured
1
When the fence level is data: Mirroring consistency is assured, since a write error is returned if mirror consistency with the
remote S-VOL is lost. The secondary volume can continue operation, regardless of the status. Note: A P-VOL write that discovers
a link down situation will, in addition to returning an error to the host, likely be recorded on [only] the P-VOL side.
2
When the fence level is status: If there is a mirror consistency problem (i.e., PSUE) and it is possible to set the S-VOL to PSUE,
the P-VOL write completes OK. If the S-VOL cannot be set to PSUE for any reason, the P-VOL write completes with an error. The
mirror consistency of the S-VOL depends on its status:
PSUE: The secondary volume is dubious.
PAIR: The secondary volume can continue operation.
3
When the fence level is never: Writing to the P-VOL is still enabled in the state where mirror consistency to the S-VOL is lost,
regardless of whether the secondary volume status is updated or not. Thus, the secondary could have these states:
PSUE: The secondary volume is dubious.
PAIR: The secondary volume is substantially dubious, since it can continue operation and is also dubious. The P-VOL status must
be checked to confirm the mirroring consistency.
4
When the fence level is async: Continuous Access Asynchronous/Cnt Ac-J uses asynchronous transfers to ensure the sequence
of write data between the P-VOL and S-VOL. Writing to the P-VOL is enabled, regardless of whether the S-VOL status is updated
or not. Thus the mirror consistency of the secondary volume is dubious (similar to the “Never” fence):
•
PSUE: The S-VOL mirroring consistency is not assured, but the PSUE suspended state ensures the sequence of data for the
consistency group; thus, data consistency is also assured during a PSUE state. At a PSUE state, the P-VOL writes still complete
Continuous Access Synchronous, Business Copy, and Continuous Access Journal operations 143