HP XP Continuous Access Software - Manual (Page 23)

Table 2 Metadata information
Description Type
Journal type (for example, base-journal or update journal) Journal type
Number of the primary data volume that stores the original
data LDEV No. (data)
Primary data volume's slot number, and start and end of
sub-block number (data length) Original data storing position
Volume number of the master journal volume that stores
the journal data LDEV No. (journal)
Master journal volume's slot number, and start sub-block
number Journal data storing position
Sequence number assigned when the journal is obtained Journal sequence number
Time journal data is obtained Timestamp
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: XP Continuous Access 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:
Journal processing 23