HP XP7 Storage - Manual (Page 10)

Replication occurs in the following sequence:
1.
Journal obtain - When the host sends an update to the primary volume (P-VOL), the primary
system’s journal-obtain function triggers a copy of the updated data to the master journal
volume.
•
The host assigns write-sequence numbers to the data sent to the master journal volume.
•
Write-sequence numbers and other metadata attached to journal data ensure consistency
with the data in the P-VOL.
2. Journal copy - Data is copied from the master journal to the restore journal.
•
When the master journal has data, the data is transferred to the restore journal. When
data transfer is complete and the secondary volume (S-VOL) is updated, master journal
data is discarded.
•
Data copy to the restore journal is initiated by the read-journal command issued by the
secondary system.
•
Data copy occurs on a continual basis unless there is no data in the master journal. The
request for data from the secondary system is repeated as soon as the previous read
operation is completed.
•
Journal data is removed from the master journal only when the primary system receives
sequence numbers for the data from the restore journal.
3. Journal-restore - The S-VOL is updated with changed data from the restore journal.
•
Data is copied to the S-VOL according to the write sequence numbers, ensuring data
consistency.
•
When journal-restore is completed, the data in the restore journal is discarded.
Performance is affected because of journal-to-journal copying. Usage rates are also affected. See
“Read and write I/O during remote copy” (page 14) for more high level information on Continuous
Access Journal Z operations.
Hardware and software components
A typical configuration consists of a XP7 system or externally attached storage system on both
primary and secondary sites, a host or hosts connected to the systems, Continuous Access Journal
Z software on both systems, data path connections, and interface tools for configuring and managing
Continuous Access Journal Z.
•
The primary and secondary XP7 systems are connected using dedicated fibre-channel data
paths that can include fibre-channel switches. Data paths are routed from the fibre-channel
ports on the primary system to the ports on the secondary system.
•
The host is connected to the XP7 using a FICON channel port.
•
Remote Web Console, whose GUI is used to view and operate Continuous Access Journal Z,
is connected via a management LAN.
A Continuous Access Journal Z system consists of the following:
•
P-VOLs and S-VOLs on the primary and secondary XP7
•
Master and restore journal volumes on the primary and secondary XP7
•
Master and restore journals on the primary and secondary XP7
The master journal consists of the primary volumes and master journal volumes. ◦
◦ The restore journal consists of the secondary volumes and restore journal volumes.
When Continuous Access Journal Z journals consist of journal volumes located in multiple storage
systems, a host I/O time stamping function is required. This is provided by MVS DFSMSdfp.
10 Continuous Access Journal Z overview