HP StorageWorks XP Remote Web Console Software - Manual (Page 26)

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 even
if the SVP fails.
Types of Journals
In addition to the journal data for updating, the primary storage system sends control information
to the secondary storage system. This control information indicates when the volume pair status
changes and when a primary storage system power-off sequence is initiated, and also maintains
the sequence numbers in periods of low host activities.
Journal Processing at the Secondary Storage System
When a primary storage system receives a read journal command from a secondary storage
system, the primary storage system sends the journal data to the secondary storage system. The
secondary storage system’s initiator ports act as host processor channels and issue special I/O
operations, called remote I/Os (RIOs), to the primary storage system. The RIO transfers the journal
data in FBA format using a single channel command. The primary storage system can send several
journal data using a single RIO, even if their sequence numbers are not contiguous. Therefore, the
journal data is usually sent to the secondary storage system in a different order than the journal
data was created at the primary storage system. The secondary storage system ensures that the
journal data is applied to the secondary data volume in the correct sequence. This method of
remote I/O provides the most efficient use of the primary storage system-to-secondary storage
system link resources.
Note: Make sure that your channel extenders are capable of supporting remote I/O. For further
details, contact your HP service representative.
Storing Journals at the Secondary Storage System
A secondary storage system receives the journal data that is transferred from a primary storage
system according to the read journal command. The journal data will be stored in the cache at
first, and then in the journal volume.
Note: The primary storage system does not remove the target journal data from its master journal
volume until it receives the sequence numbers of the restored journal that it sends to the read journal
command from the secondary storage system. This is true even if the primary storage system and
secondary storage system are connected via a channel extender product.
Selecting and Restoring a Journal at the Secondary Storage System
The secondary storage system selects the journal data to be promoted to formal data (or restored)
as follows:
1. The secondary storage system assigns a management information bitmap according to the
sequence number, and uses that information to determine whether journal data has arrived.
The secondary storage system examines the management information for journal data that
has the lowest sequence number in the journal group. If the bitmap is 0, it means that journal
data has not arrived yet, and the secondary storage system waits.
2. When the journal data that has the lowest sequence number has arrived, which is when the
bitmap is 1, the secondary storage system settles that journal data.
3. The secondary storage system repeats steps 1 and 2 to select and settle the journal data.
Figure 6 (page 27) illustrates journal data selection and settling at the secondary storage system.
This diagram shows that journal data S1 arrived at the secondary storage system because the
management information indicates 1. The secondary storage system selects journal data S1 to be
settled because S1 is the lowest sequence number. When S1 is removed from the queue of sequence
numbers, journal data S2 becomes the top entry, but it has not arrived yet. The management
information for journal data S2 is 0. The secondary storage system waits for journal data S2.
26 About XP Continuous Access Journal Operations