HP XP7 Storage - Manual (Page 14)

Reference Information
The following topics describe Cnt Ac-J Z copy operations and other features.
Overview of copy operations
initial and update copy operations including the underlying operations, such as journal processing
and differential data management.
Initial copy operation
The initial copy is executed when the primary system copies all the data in sequence from the
P-VOL directly to the S-VOL. Though journal volumes are not used during the initial copy, the copy
data in this operation is referred to as “base journal data”.
•
Creating or resynchronizing two or more pairs within the same journal results in the base
journal data being copied to the respective S-VOLs, one at a time. This extends the time
required for all the operations to be completed.
•
An initial copy operation can be performed to establish the pair — with no data copied
between the volumes. This can be done when data in the P-VOLs and S-VOLs are identical.
•
Continuous Access Journal Z pair data can also be copied using a Continuous Access
Synchronous initial copy operation. Doing this reduces the time to complete the copy operation.
See
“Planning pair volumes ” (page 27) for more information.
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 Duplex 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.
14 Continuous Access Journal Z overview