Order-Dependent Writes and Volume Set Grouping

For the Sun SNDR Version 2.0 software, write ordering is only maintained by the Sun SNDR software within a logical volume. In Version 3.0, write ordering is also maintained for groups of asynchronously replicating volume sets. (The general definition of write ordering here is that write operations directed to the target volume occur in the same order as write operations to the source.) The group of target volumes is a point-in-time copy of the group of source volumes.

This feature is especially valuable in those cases where you can avoid application requirements that limit operations. For example, a database application might limit partition sizes to no greater than 2 Gbytes. In this case, you might group volume sets to create a virtual large “volume” that preserves write operations. Otherwise, you might risk having inconsistent data by trying to update volume sets individually instead of as a group.

When an application has multiple logical volumes assigned, application data integrity can be maintained by one of the following:

■Specifying all Sun SNDR software volumes associated with that application as

SYNC

■Using Sun StorEdge Instant Image software to take periodic recoverable point-in-time copies

If you use Instant Image software, the remote point-in-time is taken while the application is in the recoverable state. For example, most database applications allow for a hot backup. If a remote point-in-time copy were made of the entire replicated database while the primary was in hot backup mode, then a consistent remote database is available by using the point-in-time copy and the log files taken while the database was in hot backup mode.

Chapter 2 Configuration Considerations 13