HP 3PAR Operating System Software - Manual (Page 157)

HP 3PAR Remote Copy uses low timeout values for synchronous volume groups to ensure that
remote copy stops replication before the host I/O times out.
Timeouts and Transient Network Problems
If remote copy is using a network that has transient problems, HP 3PAR Remote Copy might display
repeated link failure alerts or even stop all synchronous volume groups.
If remote copy uses asynchronous periodic volume groups, the higher timeout values allow HP
3PAR Remote Copy to continue functioning even if there are transient network problems.
Failure of a Single Link
When a link between storage systems fails, the system issues an alert on each system as soon as
it detects the link failure.
If at least one additional link between the two systems remains active, HP 3PAR Remote Copy
remains in normal operation and sends all data through the remaining links.
The system might experience a slight reduction in throughput (bandwidth), but a single link failure
in a multiple-link remote-copy pair does not incur errors under normal operating conditions other
than the link failure itself.
Target failure
When all links between remote copy targets in a remote-copy relationship fail, neither side can
communicate. Each side reports that its remote copy target has failed. Following a remote-copy
target failure, the following actions occur:
•
Both systems declare the other system to be down.
•
Both systems generate alerts regarding the other system’s failure.
The system handles complete link failure differently depending on whether the links are used for
synchronous or asynchronous periodic mode volume groups.
For more information about synchronous and asynchronous periodic volume groups, see the
following sections.
Target Failure and Synchronous Volume Groups
For synchronous volume groups, when all links between a remote-copy pair fail:
•
HP 3PAR Remote Copy continues to allow writes to the primary volume group after snapshots
of the primary volumes have been taken.
Therefore, applications that are writing data remain active even though the primary and
secondary volume groups go out of synchronization.
•
Remote copy stops replication from the primary to the backup system.
•
Remote copy creates snapshots of all primary virtual volumes that completed the initial
synchronization process.
•
While replication is stopped, remote copy does not create snapshots for secondary virtual
volumes.
•
The offset and length of the I/O that failed to complete on the secondary virtual volumes
because the link failures are recorded in the nonvolatile memory.
The system records failed I/O replication separately because remote copy creates the snapshots
of the primary virtual volumes on the primary system after I/O is written to the primary virtual
volumes, but before remote copy can write the I/O to the secondary virtual volumes. These
recorded failed I/O operations are complete when the replication links return and before
resynchronization of the volumes occurs.
Failure of a Single Link 157