HP StorageWorks XP24000 Continuous Access Journal Software User and Reference Guide, v01 (T5278-96001, June 2007)

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 Secondar y Storage System
When a p rimary storage system receives a read journal comma nd from a seconda ry 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 da ta is applied to the secondar y d ata
volume in the correct sequence. This metho d of remote I/O p rovides the most efficient use of the p rimar y
storage sy stem-to-secondary storage system link resources.
Note: You must m ake sure that your channel extenders are capable of supporting remote I/O. For further
details, contact your HP service representative.
Storing J
ournals at the Secondary Storage System
Aseconda
ry storage system receives the journal data that is transferred from a primary storage system
accordi
ng
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 which 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 secondar y storage system sends a number as m a nag em ent information to d istinguish the
journal data arrival from 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
that
journal data.
3. The secondary storage system repeats steps (1) and (2) to select and settle the journal data.
Figure 6 on page 32 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 thelowest sequencenumber. When S1 is removedfromthe queueofsequencenumbers,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 da ta 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-dirt y and treated as formal data.
XP24000 Continuous Access Journal Software User a nd Reference Guide
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