HP XP Continuous Access Software - Manual (Page 25)

Difference Management
The differential data (updated by write I/Os during a split or suspension) between the primary
data volume and the secondary data volume is stored in each track bitmap. When a split/suspended
pair is resumed (pairresync), the primary storage system merges the primary data volume and
secondary data volume bitmaps, and the differential data is copied to the secondary data volume.
The number of bitmap areas affects the maximum possible number of pairs that can be created in
the storage system. For details on the maximum possible number of pairs, see
“Maximum Number
of Pairs” (page 47) .
Journal Processing
The XP Continuous Access Journal data contains the primary data volume updates and the metadata
information (associated control information), which enables the secondary storage system to
maintain update consistency of the XP Continuous Access Journal secondary data volumes. XP
Continuous Access Journal processing includes:
•
Creating and storing journals at the primary storage system (see “Journal Processing at the
Primary Storage System” (page 25) )
•
Copying journals to the secondary storage system (see “Journal Processing at the Secondary
Storage System” (page 26) )
•
Storing journals at the secondary storage system (see “Storing Journals at the Secondary
Storage System” (page 26) )
•
Selecting and restoring journals at the secondary storage system (see “Selecting and Restoring
a Journal at the Secondary Storage System” (page 26) )
•
Types of journals (see “Types of Journals” (page 26) ).
Journal Processing at the Primary Storage System
When a primary storage system performs an update (host-requested write I/O) on an XP Continuous
Access Journal primary data volume, the primary storage system creates the journal data to be
transferred to the secondary storage system. The journal data will be stored in the cache first, and
then in the journal volume.
Metadata information will be attached to the journal data (see “Metadata Information” (page 25) ).
When the base-journal is obtained, only metadata information is created and stored in the XP
Continuous Access Journal cache or the journal volume.
Table 3 Metadata Information
Description Type
Type of journal (for example, base-journal or update journal) Journal type
The number of the primary data volume that stores the original data LDEV No. (data)
The primary data volume slot number, and the start and end of the sub-block
number (data length) Original data storing position
The volume number of the master journal volume that stores the journal data LDEV No. (journal)
The slot number of the master journal volume, and the start sub-block number Journal data storing position
The sequence number that is assigned when the journal is obtained Journal sequence number
The time when the journal data is obtained Timestamp
The journal sequence number indicates the primary data volume write sequence that the primary
storage system has created for each journal group. The journal data is transferred to the secondary
storage system asynchronously with the host I/O. The secondary storage system updates the
Journal Processing 25