HP XP Continuous Access Software - Manual (Page 32)

Table 3 Metadata information
Type Description
Journal type Journal type (for example, base-journal or update journal)
LDEV No. (data) Number of the primary data volume that stores the original data
Original data storing position Primary data volume's slot number, and start and end of sub-block
number (data length)
LDEV No. (journal) Volume number of the master journal volume that stores the journal data
Journal data storing position Master journal volume's slot number, and start sub-block number
Journal sequence number Sequence number assigned when the journal is obtained
Timestamp Time journal data is obtained
The journal sequence number indicates the primary data volume write sequence that the primary array
created for each journal group. Journal data is transferred to the secondary array asynchronously with
regards to the host I/O. The secondary array updates the secondary data volume in the same order as
the primary data volume according to the sequence number information in the journal.
NOTE:
Continuous Access XP Journal processing continues uninterrupted if the SVP reboots or fails.
Copying journals to secondary arrays
When a primary array receives a read journal command from a secondary array, the primary array
sends available journal data to the secondary array. The secondary array's initiator ports act as host
processor channels and issue special I/O operations, called remote I/Os (RIOs), to the primary array.
The RIO transfers journal data in FBA format using a single channel command. The primary array can
send several journal data entries using a single RIO, even if their sequence numbers are not contiguous.
Therefore, journal data is often sent to the secondary array out of order. The secondary array re-sorts
journal entries by sequence numbers to ensure that journal data is applied to the secondary data
volume in the correct sequence. This remote I/O method provides the most efficient use of primary
array-to-secondary array link resources.
NOTE:
Ensure that channel extenders are capable of supporting remote I/O. For more information, contact
your HP account support representative.
Storing journals at secondary arrays
A secondary array receives journal data transferred from a primary array by way of the read journal
command. Journal data is stored in cache first, and then in the journal volume.
NOTE:
The primary array does not remove target journal data from its master journal volume until it receives the
restored journal's sequence numbers provided by the read journal command from the secondary array.
This is true even if the primary and secondary arrays are connected through a channel extender product.
Selecting and restoring journals at secondary arrays
The secondary array selects journal data to be promoted to formal data (or restored) as follows:
32 Overview of Continuous Access XP Journal