HP StorageWorks XP Remote Web Console Software - Manual (Page 29)

Continuous Access XP Journal user guide 29
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 restores
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:
1. The secondary array provides a status indicator number as management information to distinguish (by
sequence number), those data entries that have successfully been received at the remote site. If the
number is 1, that journal data has arrived at the secondary array. If the number is 0, journal data has
not arrived yet. The secondary array determines whether or not journal data still needs to be settled
according to this number. If journal data has not arrived yet, the secondary array waits for the journal
data.
2. When the journal group’s top queue indicates journal data has arrived, the secondary array selects
the journal data that has the lowest sequence number, and settles (destages) that journal data.
3. The secondary array repeats step 1 and step 2 to select and settle journal data.
Figure 6 illustrates journal data selection and settling at the secondary array. This diagram shows that
journal data S1 arrived at the secondary array because the management information indicates 1. The
Journal sequence number Sequence number assigned when the journal is obtained
Timestamp Time journal data is obtained
Table 3 Metadata information (continued)
Type Description