Building Disaster Recovery Serviceguard Solutions Using Metrocluster with Continuous Access XP P9000 for Linux B.01.00.00

Continuous Access Synchronous Replication
In Continuous Access Synchronous replication, all write operations on the primary volume are
replicated to the secondary volume before the write is acknowledged to the host. This synchronous
replication mode ensures the highest level of data currency possible. Host I/O performance is
directly impacted by the distance between the primary and secondary volumes.
Continuous Access Synchronous replication supports three types of fence levels: DATA, STATUS
and NEVER for a device group. The fence level of the device group is set when you define it. All
devices defined in a given device group must be configured with the same fence level.
NOTE: Metrocluster environment does not support the STATUS fence level.
Fence level NEVER
The fence level NEVER must only be used when the availability of the application is more important
than the currentness of data on the remote XP or P9000 disk arrays. In case all Continuous Access
links fail, the application continues to modify the data on PVOL side, however the new data is not
replicated to the SVOL side. The SVOL only contains a copy of the data up to the point of Continuous
Access links failure. If an additional failure, such as a system failure, occurs before the Continuous
Access link is fixed, the application fails over to the SVOL side, and is forced to deal with non-current
data.
Fence Level DATA
The fence level DATA is recommended to ensure a current and consistent copy of the data on all
sides. In the case of Continuous Access link failure, all I/Os on the PVOL side are refused. The
fence level DATA ensures that the data at the SVOL side is always consistent and current.
The disadvantage of enabling fence level DATA is when the Continuous Access link fails, or if the
entire remote (SVOL) data center fails, all I/Os are refused until the Continuous Access link is
restored. To give the PVOL read/write capability even if the SVOL is unavailable with a fence level
of data, PVOL takeover function is used from the PVOL side.
NOTE: If the fence level DATA is enabled and the Continuous Access link fails, then the
applications might fail or might continuously retry the I/Os (depending on the application).
Continuous Access Asynchronous replication
In Continuous Access Asynchronous replication, all write operations on the primary volume are
time stamped and stored in the array system cache, also known as the side file, before the write
is acknowledged to the host. The data is then asynchronously replicated to the secondary disk
array and re-applied in sequence to the secondary devices. Data is not always current, but due
to the unique timestamp implementation, data is always consistent. The fence level of the Continuous
Access Asynchronous device group is set to “async”.
NOTE: The P9000 disk array family does not support asynchronous replication using side file.
Continuous Access link timeout
In Continuous Access asynchronous replication, when there is a Continuous Access link failure,
both the PVOL and SVOL sides change to a PSUE state. When the SVOL side detects missing data
blocks from the PVOL side, it waits for the data blocks from the PVOL side until the Continuous
Access link times out (set in the SVP). After the timeout, the SVOL side changes to a PSUE state.
The default Continuous Access link timeout value is 5 minutes (300 seconds).
Consistency Group
An important property of asynchronous mode volumes is the Consistency Group (CT group). A CT
group is a grouping of LUNs that must be treated the same from the perspective of data consistency
Overview of Continuous Access P9000 and XP concepts 7