Designing Disaster Recovery Clusters using Metroclusters and Continentalclusters, Reprinted October 2011 (5900-1881)

Journal Cache, Journal Volumes, and Inflow Control
When a primary array performs an update (host-requested write I/O) on PVOL, the primary array
creates the journal data (metadata and new write data) to be transferred to secondary array. The
journal data is stored in the journal cache or journal volumes depending on an amount of data in
cache. If available cache memory for Continuous Access Journal is low, the journal data is stored
in the journal volumes. A secondary array receives the journal data that is transferred from the
primary array according to the read journal command. The received journal data is stored in the
journal cache or the journal volumes depending on the “Use of Cache” parameter and/or amount
of data in cache. If the “Use of Cache” is set to “Use”, journal data will be stored into the journal
cache. If it is set to “No Use”, journal data will bypass the cache and move directly to the journal
volumes. In addition, if available cache memory for Continuous Access Journal is low, the journal
data is stored in the journal volume.
For Continuous Access Journal processing, Continuous Access Journal allows the usage rate of
journal volume to be specified. The Journal volume stores journal data to be transferred to the
secondary array asynchronously using host write I/Os to PVOL. However, if the hosts transfer
excessive amounts of data, the journal volume may become full. Consequently, if the journal
volumes remains full for the specified period of time, the journal group will be suspended due to
a failure. To specify the period of time for how long the journal volume can remain full, use the
Data Overflow Watch option.
The XP12000 array uses the following parameters to control the inflow of data into journal group
and state change of the journal group:
• Inflow Control: Indicates whether to restrict inflow of update I/Os (slow down the host response)
to the journal volume, when the journal volume is full. ‘Yes’ indicates inflow will be restricted.
‘No’ indicates inflow will not be restricted.
• Data Overflow Watch: Indicates the time (in seconds) to implement Inflow Control before
suspending the journal group.
If the amount of data in the journal volume, in the primary array, reaches the capacity, the disk
array I/Os will be delayed. If journal volume remains full for the period of time specified by the
Data Overflow Watch parameter, the primary array suspends the affected journal groups due to
a failure.
Continuous Access Journal Pair State
If the amount of data in the journal cache, in the secondary subsystem, reaches the specified journal
cache capacity, the secondary subsystem stores the received journal data into the restore journal
volume, and then issues the next read-journal command to the primary subsystem. This suppresses
the cache usage rate increase.
To accommodate, the Continuous Access Journal retains the PAIR state when the Continuous Access
links fail as long as the journal volumes has enough space while the Continuous Access
Asynchronous switches to PSUE state.
In addition, this allows host write-data to be kept continuously as journal data in the journal volumes
while the updated data is not being replicating to the remote array. Once the links are recovered,
the data replication of the primary and secondary arrays is resumed automatically. The journal
data accumulated in the primary journal volumes is replicated to the secondary site automatically.
NOTE: If the journal volumes get full, the pair state will be switched to PFUS and the data written
to the data volume is tracked in bitmap.
Limitations of Continuous Access Journal
The following two sections describe the “One-to-One Volume Copy Operations” and “One-to-One
Journal Group Operations” limitations of the Continuous Access Journal.
Overview of Continuous Access P9000 and XP Concepts 161