HP StorageWorks Storage Mirroring for Linux User's Guide (T2558-96317, June 2009)

9 - 12
Configuring automatic reconnections
While disk queues are user configurable and can be extensive, they are limited. If the amount of disk
space specified for disk queuing is met, additional data could not be added to the queue and data
would be lost. To avoid any data loss, the auto-disconnect and auto-reconnect processes occur.
 Exhausted queues on the source—If disk queuing is exhausted on the source, Storage
Mirroring will automatically start disconnecting connections. This is called auto-disconnect. The
transaction logs and system memory are flushed allowing Storage Mirroring to begin processing
anew. The auto-reconnect process ensures that any connections that were auto-disconnected are
automatically reconnected. Then, if configured, Storage Mirroring will automatically remirror the
data. This process is called auto-remirror. The remirror re-establishes the target baseline to
ensure data integrity, so disabling auto-remirror is not advised.
 Exhausted queues on the target—If disk queuing is exhausted on the target, the target instructs
the source to pause. The source will automatically stop transmitting data to the target and will
queue the data changes. When the target recovers, it will automatically tell the source to resume
sending data. If the target does not recover by the time the source queues are exhausted, the
source will auto-disconnect as described above. The transaction logs and system memory from
the source will be flushed then Storage Mirroring will auto-reconnect. If configured, Storage
Mirroring will auto-remirror. The remirror re-establishes the target baseline to ensure data
integrity, so disabling auto-remirror is not advised.
 Queuing errors—If there are errors during disk queuing on either the source or target, for
example, Storage Mirroring cannot read from or write to the transaction log file, the data integrity
cannot be guaranteed. To prevent any loss of data, the source will auto-disconnect and
auto-reconnect. If configured, Storage Mirroring will auto-remirror. The remirror re-establishes
the target baseline to ensure data integrity, so disabling auto-remirror is not advised.
 Target server interruption—If a target machine experiences an interruption (such as a cable
or NIC failure), the source/target network connection is physically broken but both the source and
target maintain the connection information. The Storage Mirroring source, not being able to
communicate with the Storage Mirroring target, stops transmitting data to the target and queues
the data changes, similar to the exhausted target queues described above. When the interruption
is resolved and the physical source/target connection is reestablished, the source begins sending
the queued data to the target. If the source/target connection is not reestablished by the time
the source queues are exhausted, the source will auto-disconnect as described above.
 Target daemon shutdown—If the target daemon is stopped and restarted, there could have
been data in the target queue when the daemon was stopped. To prevent any loss of data, the
Storage Mirroring daemon will attempt to persist to disk important target connection information
(such as the source and target IP addresses for the connection, various target queue information,
the last acknowledged operation, data in memory moved to disk, and so on) before the daemon
is stopped. If Storage Mirroring is able to successfully persist this information, when the Storage
Mirroring daemon on the target is restarted, Storage Mirroring will pick up where it left off, without
requiring an auto-disconnect, auto-reconnect, or auto-remirror. If Storage Mirroring cannot
successfully persist this information prior to the restart (for example, a server crash or power
failure where the target daemon cannot shutdown gracefully), the source will auto-reconnect
when the target is available, and if configured, Storage Mirroring will auto-remirror. The remirror
re-establishes the target baseline to ensure data integrity, so disabling auto-remirror is not
advised.
Use the following steps to configure automatic reconnections.
1. Right-click the source server on the left pane of the Management Console and select
Properties.
2. Select the Setup tab.
3. Verify that the check box Automatically Reconnect During Source Initialization is marked
to enable the auto-reconnect feature.
4. Click OK to save the settings.
NOTE: If you are experiencing frequent auto-disconnects, you may want to increase the amount
of disk space on the volume where the Storage Mirroring queue is located or move the
disk queue to a larger volume. See Queuing Storage Mirroring data on page 9-8 for
more information.
If you have changed data on the target while not failed over, for example if you were
testing data on the target, Storage Mirroring is unaware of the target data changes. You
must manually remirror your data from the source to the target, overwritting the target
data changes that you caused, to ensure data integrity between your source and target.