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3-36 Chapter 3 Preparing for Universal Replicator z/OS Operations
Hitachi Universal Replicator for IBM /OS User’s Guide
As illustrated in Figure 3-23, if a host in the primary site issues an I/O request,
data will be stored into the TCz Synchronous M-VOL. The data will also be
stored into the TCz Synchronous R-VOL (i.e., the URz primary data volume).
After that, the fact that data has been stored will be reported to the primary
site, and then the fact that I/O has been finished will be reported to the host.
Also, data in the URz primary data volume (i.e., TCz Synchronous R-VOL) will
be stored as journal data into the master journal volume in the intermediate
site. Sequence numbers, which indicates the order that data will be written,
will be added to these journal data. Also, journal data in the master journal
volume in the intermediate site will be transferred, independently from and
asynchronously with the behavior mentioned above, to the restore journal
volume in the secondary site. Next, the journal data in the restore journal
volume will be restored to the URz secondary data volume in the secondary
site; the journal data will be restored in the order of sequence numbers that
are added to journal data. These sequence numbers ensure that updates to
URz secondary data volumes in the secondary site will be made in the same
order that updates to TCz Synchronous M-VOL in the primary site are made.
As explained above, when the host in the primary site issues an I/O request to
the TCz Synchronous M-VOL, the data will be restored asynchronously to the
URz secondary data volume in the secondary site in a remote location.
R-VOL/
Primary data
volume
Master
JNLVOL
Restore
JNLVOL
Primary site
Intermediate
site
Secondary
site
M-VOL
Secondary
data volume
I/O
I/O finished
Transfer
asynchronously
Transfer
synchronously
Finished storing
data
Figure 3-23 Basic Behavior in 3DC Cascading Configuration
If a TCz Synchronous pair is suspended in a 3DC cascading configuration, you
will be able to resynchronize the pair by using differential copy. If a URz pair is
suspended, you will be able to resynchronize the pair by using differential copy.
Note: To perform disaster recovery from a failure in the primary site in a 3DC
cascading configuration, you must ensure that the fence level of the TCz
Synchronous M-VOL is Data (CRIT=Y (ALL))). If this fence level is specified,
you can perform disaster recovery by using the URz secondary data volume
that contains the data stored in the TCz Synchronous M-VOL.