HP XP Continuous Access Software - Manual (Page 33)

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 status is 1, that journal data has arrived at the secondary array. If the status 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 status indicator 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
secondary array selects journal data S1 to be settled, because S1 is the lowest sequence number. When
S1 is removed from the sequence number queue, journal data S2 becomes the top entry, but it has not
arrived yet. Journal data S2's management information is 0. The secondary array waits for journal data
S2. When journal data S2 arrives, the secondary array selects S2 as the next journal data to be settled.
The journal data that the secondary disk selects is marked as “host-dirty” and treated as formal data.
Figure 6 Selecting and settling journal at the secondary array
The secondary array settles and restores journal data to the secondary data volume as follows:
• Journal data stored in cache
Journal data is copied to the corresponding cached track and promoted to formal data.
• Journal data stored in the restore journal volume
Journal data is read from the restore journal volume to cache. Journal data that is read to cache is
copied to the existing cache track and promoted to formal data. After that, the space for the restore
journal volume is released.
Journal types
In addition to sending journal data for updating, the primary array sends control information to the
secondary array. This control information:
• Indicates volume pair status changes.
• Indicates when a primary array power-off sequence is has been initiated.
Continuous Access XP Journal user guide 33