HP StorageWorks XP Remote Web Console Software - Manual (Page 22)

URz processing continues uninterrupted if the SVP reboots or even if the SVP fails.
Copying Journals to the Secondary Storage System
When a primary subsystem receives a read journal command from a secondary subsystem, the
primary subsystem sends the journal data to the secondary subsystem. The secondary subsystem’s
initiator ports act as host processor channels and issue special I/O operations, called remote I/Os
(RIOs), to the primary subsystem. The RIO transfers the journal data in FBA format using a single
channel command. The primary subsystem 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 subsystem in a different order than the journal data were created at the primary
subsystem. The secondary subsystem 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 subsystem-to-secondary subsystem link resources.
NOTE: You must make sure that your channel extenders are capable of supporting remote I/O.
For further details, contact your HP account team.
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.
The primary storage system does not remove the target journal data from its master journal volume
until it receives the sequence numbers of restored journal which is give 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 Subsystem
The secondary subsystem selects journal data to be promoted to formal data (or restored) as follows:
1. The secondary subsystem 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 subsystem. If the number is 1, the journal data arrived at the secondary subsystem.
If the number is 0, the journal data has not arrived yet. The secondary subsystem determines
whether the journal data should be settled or not according to this number. If the journal data
has not arrived yet, the secondary subsystem waits for the journal data.
2. When the top of queue in the journal group indicates the journal data arrival, the secondary
subsystem selects the journal data which has the lowest sequence number, and then settles
this journal data.
3. The secondary subsystem repeats steps (1) and (2) to select and settle the journal data.
This diagram shows that journal data S1 arrives at the secondary subsystem because the
management information indicates 1. The secondary subsystem 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 subsystem waits journal data
S2. When journal data S2 arrives, the secondary subsystem selects S2 as the next journal data to
be settled. The journal data selected by the secondary subsystem is marked as “host-dirty” and
treated as formal data.
22 Overview of Universal Replicator for Mainframe Operations