HP XP7 Storage - Manual (Page 9)

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.
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The host assigns write-sequence numbers to the data sent to the master journal volume.
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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.
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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.
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Data copy to the restore journal is initiated by the read-journal command issued by the
secondary system.
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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.
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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.
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Data is copied to the S-VOL according to the write sequence numbers, ensuring data
consistency.
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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 13) for more high level information on Continuous
Access Journal 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
software on both systems, data path connections, and interface tools for configuring and managing
Continuous Access Journal.
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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.
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The host is connected to the XP7 using a fibre-channel or fibre-channel-over-Ethernet (FCoE)
target port.
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Remote Web Console, whose GUI is used to view and operate Continuous Access Journal,
is connected via a management LAN.
A Continuous Access Journal system consists of the following:
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P-VOLs and S-VOLs on the primary and secondary XP7
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Master and restore journal volumes on the primary and secondary XP7
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Master and restore journals on the primary and secondary XP7
The master journal consists of the primary volumes and master journal volumes.
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◦ The restore journal consists of the secondary volumes and restore journal volumes.
Hardware and software components 9