How Universal Replicator works

In the Universal Replicator system, remote replication is accomplished through the use of journal volumes on the primary and secondary systems.

Note the following:

Universal Replicator does not have a feature to copy the data from one P-VOL to multiple S-VOLs or to copy the data from multiple P-VOLs to one S-VOL.

If the primary system fails reading a P-VOL, the redundancy of RAID-1, RAID-5, or RAID-6 enables the recovery from the failure. The primary system never reads the S-VOL for data recovery.

Universal Replicator copies the journal data from the primary site to secondary site in the following sequence:

Obtain journal: - The primary system writes journal data to the master journal volume. When the host writes the updated data to the P-VOL, the primary system copies the updated data. This copy of the updated data is called the "journal data". The volume that contains the journal data at the primary site is called the "master journal volume". If a P-VOL write operation fails, the primary system does not create the journal data for the failed write operation.

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 of the data in the P-VOL.

Journal copy (initial and update copy) - Journal data is copied from the master journal volume to the restore journal volume. Copying all of the data first is called "initial copy". Copying only the differential data after the initial coy is called "update copy". The volume that restores the journal data in the secondary system is called the "restore journal volume".

When a read-journal command is issued to the primary system from the secondary system, the primary system transfers the journal data stored in the master journal volume to the restore journal volume at secondary site.

Data copy occurs on a continual basis unless there is no data in the master journal volume. The request for data from the secondary system is repeated as soon as the previous read operation is completed.

Universal Replicator is "pull" type replication software. Universal Replicator starts copying journal data to secondary site when the secondary system sends an update (when the read-journal command is issued). The primary system operates as the resource for the secondary system's transaction processor.

Journal-restore - The S-VOL is updated based on the journal data stored in the restore journal volume.

The journal volume data in the restore journal volume is copied to the S-VOL according to the write sequence numbers, ensuring data consistency.

When journal restore to the S-VOL is completed, the data in the restore journal volume is discarded.

When journal restore to the S-VOL is completed, the data in the master journal volume is also discarded.

The primary system discards journal data in the master journal volume when it receives the sequence number of the restore journal, which is attached to the read journal command from the secondary system. This applies also when the primary and secondary systems are connected through a channel extender.