HP XP Continuous Access Software - Manual (Page 20)

Figure 1 Continuous Access XP Journal components for Fibre Channel connections
Journal obtain
This function stores pre-existing primary volume data contnets as the base-journal in the primary site
journal volume. This function then stores write data in the journal volume every time the host updates
the primary data volume. The journal obtain operation is performed when a paircreate or pairresync
operation command is issued from the primary site. The XP array assigns a sequence number to
each Continuous Access XP Journal primary volume write operation received from a host. Using this
information, write sequence consistency at the secondary site is maintained. After the pre-existing
primary volume data has been used to populate the initial contents of the journal, host write updates to
the primary volume are then continually moved from write cache to the primary side (master) journal.
Journal copy
This function copies data in the master journal volume at the primary site to the restore journal volume at
the secondary site. When a paircreate or pairresync operation command is issued from the primary
site, the secondary array issues the command to the primary array to request a transfer of data stored in
the master journal volume. If there is journal data to be sent, the primary array transfers journal volume
data to the secondary site according to the read journal command. If the primary array has no un-sent
journal data, notification of that is sent. The secondary array stores journal volume data sent from the
primary site in the journal volume at the secondary site. Read journal commands are issued repeatedly
and regularly from the secondary site to the primary site as long as the journal functionality operates.
After data is restored, journal sequence numbers are returned from the secondary site to the primary
site in response to the read journal command. Based on this “remote replication complete” information,
journal data at the primary site can be safely discarded.
Journal restore
This function applies stored data in the remote site restore journal volume to the secondary data volume.
Data in the restore journal volume is restored (de-staged) to the secondary data volume according to
the ordering dictated by the write sequence numbers. This ensures write sequence consistency between
the primary and secondary data volumes even if the replication link infrastructure allows writes to get
out of order. After journal data is restored to the secondary data volume, journal data is discarded
at the primary and secondary sites.
20 Overview of Continuous Access XP Journal