HP StorageWorks XP Remote Web Console Software - Manual (Page 31)

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 are usually sent to the secondary storage system in a different order than the journal
data were created at the primary storage system. The secondary storage system ensures that the
journal data are applied to the secondary data volume in the correct sequence. This method of
remote I/O provides the most efficient use of primary storage system-to-secondary storage system
link resources.
NOTE: You must make sure that your channel extenders are capable of supporting remote I/O.
For further details, contact your HP service representative.
Storing Journal 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 into the cache at
first, and then into 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 which is given 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 Journal at the Secondary Storage System
The secondary storage system selects journal data to be promoted to formal data (or restored) as
follows:
1. The secondary storage system gives the number as the management information to distinguish
the journal data arrival to the sequence number that is assigned to the journal data from the
primary storage system. If the number is 1, the journal data arrived at the secondary storage
system. If the number is 0, the journal data has not arrived yet. The secondary storage system
determines whether the journal data should be settled or not according to this number. If the
journal data has not arrived yet, the secondary storage system waits for the journal data.
2. When the top of the queue in the journal group indicates the journal data arrival, the secondary
storage system selects the journal data which has the lowest sequence number, and then settles
this journal data.
3. The secondary storage system repeats steps (1) and (2) to select and settle the journal data.
The following figure illustrates journal data selection and settling at the secondary storage system.
This diagram shows that journal data S1 arrives 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. When journal data S2 arrives, the secondary storage system selects S2 as the next journal
data to be settled. The journal data selected by the secondary storage system is marked as host-dirty
and treated as formal data.
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