HP XP Continuous Access Software - Manual (Page 11)

Figure 1 XP Continuous Access Journal components for Fibre Channel connections
Journal obtain
This function stores pre-existing primary volume data contents 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 XP Continuous Access 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.
Overview of functionality 11