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Chapter 2 Configuration Considerations 13
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.