HP XP7 Storage - Manual (Page 13)

Update copy operation
When a host has new or changed information, the following occurs in the primary system:
•
The update is written to the P-VOL
•
The update is copied to the master journal along with metadata that includes sequence and
other consistency information.
The secondary system issues the read-journal command (independent of host I/O activity). At
this time, the following occurs:
◦
Any data in the master journal is sent to the restore journal.
◦ The updated data is copied to the S-VOL.
◦ Journal data on the primary and secondary systems is discarded when data consistency
is established in the copy.
NOTE: Journal data is transferred using special I/O operations initiated by the secondary
system, called RIO (remote I/O). RIO provides the most efficient type of data transfer. Make
sure that your channel extenders are capable of supporting RIO. Contact HP Technical Support
for more information.
If an update copy operation fails, the secondary system suspends the affected pair or all pairs
in the journal, depending on the type of failure. The suspended pair or journal returns to Paired
status when the primary and secondary systems are re-synchronized.
Read and write I/O during remote copy
The primary system reads from the P-VOL when it receives a read I/O command. If the read fails,
the redundancy provided by RAID-1 or RAID-5 technology recovers the failure. The primary system
does not read the S-VOL for recovery.
When a primary system receives a write I/O command for a P-VOL in PAIR status, the system
performs the write operation and performs the update copy operation. The write operation completes
independently of the update copy operations on the S-VOL.
The secondary system updates the S-VOL according to the write sequence number in the journal
data. This maintains data consistency between P-VOL and S-VOL.
If the P-VOL write operation fails, the primary system reports a unit check and does not create the
journal data for this operation. As mentioned, if the update copy operation fails, the secondary
system suspends either the affected pair or all Continuous Access Journal pairs in the journal,
depending on the type of failure. When the suspended pair or journal is resynchronized, the
primary and secondary systems negotiate the resynchronization of the pairs.
During normal operations, the secondary system does not allow S-VOLs to be online (mounted).
Therefore, hosts cannot read from and write to S-VOLs. However, if the S-VOL write option is
enabled, write access to an S-VOL is allowed while the pair is split. The pair must be split from the
primary system for the option to take effect.
To reduce the overhead associated with remote copy activities and to maximize rate of data
transfer, the XP7 uses a special write command for initial and update copy operations. This
command transfers the control parameters and the fixed-block architecture (FBA) format data for
consecutive updated records in a track using a single write operation. It eliminates the overhead
required for performing FBA-to-count-key-data (CKD) and CKD-to-FBA conversions.
Differential data management
Differential data is the data that is changed in the P-VOL when a pair is split or suspended and
that is not reflected in the S-VOL. This data is stored in a track bitmap. When the pair is
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