InfoScale™ 9.0 Replication Administrator's Guide - AIX

Synchronous mode considerations when the synchronous attribute is set to fail

When the synchronous attribute is set to fail, VVR ensures that writes do not succeed if they do not reach the Secondary. If the RLINK is disconnected, the writes fail and are not written either to the SRL or the data volumes. However, if the RLINK was connected but disconnects during the process of sending the writes to the Secondary, it is possible that the writes are written into the SRL and applied to the data volumes even though the application correctly receives failure for these writes. This happens because the data volume writes are asynchronous regardless of the mode of replication.

See How VVR uses kernel buffers for replication.

The state of the running application on the Primary at this time is no different from that of the application brought up on the Secondary after changing its role to Primary. However, the actual contents of the Primary data volumes and the Secondary data volumes differ, and the Primary data volumes are ahead by these last writes.

Note that as soon as the synchronous RLINK connects, these writes will reach the Secondary, and then the data volumes on the Primary and the Secondary have the same contents. Also, note that at no time is the data consistency being compromised.

If the application is stopped or crashes at this point and is restarted, it recovers using the updated contents of the data volumes. The behavior of the application on the Primary could be different from the behavior of the application when it is brought up on the Secondary after changing its role of the Secondary to Primary, while the RLINK was still disconnected.

In the case of a database application, these writes might be the ones that commit a transaction. If the application tries to recover using the data volumes on the Primary, it will roll forward the transaction because the commit of the transaction is already on the data volume. However, if the application recovers using the data volumes on the Secondary after changing its role to Primary, it will roll back the transaction.

This case is no different from that of an application directly writing to a disk that fails just as it completes part of a write. Part of the write physically reaches the disk but the application receives a failure for the entire write. If the part of the write that reached the disk is the part that is useful to the application to determine whether to roll back or roll forward a transaction, then the transaction would succeed on recovery even though the transaction was failed earlier.

It could also happen that a write was started by the application and the RLINK disconnected and now before the next write is started, the RLINK reconnects. In this case, the application receives a failure for the first write but the second write succeeds.

Different applications, such as file systems and databases, deal with these intermittent failures in different ways. The Veritas File System handles the failure without disabling the file or the file system.