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Chapter 2 About Universal Replicator Operations 2-3
Hitachi Universal Replicator for IBM /OS User’s Guide
Primar
y
site
Primar
y
host
Secondar
y
site
Secondar
y
host
Write instruction
Primar
y
stora
g
e s
y
stem Secondary storage system
Primary
data
volume
Issuing Read
Journal command
Journal restore
function
Journal obtain
function
Journal co
py
function
Secondary
data
volume
Master
j
ournal
volume
Restore
j
ournal
volume
Figure 2-1 UR Components for Fibre-Channel Connection
Journal Obtain
Journal obtain is the function to store the already stored data in the primary
data volume as a base-journal in the journal volume at the primary site. And
then, this function stores the write data as a journal data in the journal
volume with every update of the primary data volume according to the write
instruction from the host. The journal obtain operation is performed according
to the instruction of add pair or Resume Pair operation from the primary site.
The write sequence number from the host is assigned to the journal data.
According to this information, the write sequence consistency at the secondary
site can be maintained. The update data from the host is kept in the cache.
Therefore, the journal obtain function for the update data is performed
asynchronously from the time the storage system receives the update data
from the host and stores the update data to the data volume.