Core operations

Mirroring is the process of transmitting user-specified data from the source to the target so that an identical copy of data exists on the target. When Double-Take Availability initially performs mirroring, it copies all of the selected data, including file attributes and permissions. Mirroring creates a foundation upon which Double-Take Availability can efficiently update the target server by replicating only file changes.

If subsequent mirroring operations are necessary, Double-Take Availability can mirror specific files or blocks of changed data within files. By mirroring only files that have changed, network administrators can expedite the mirroring of data on the source and target servers. Mirroring has a defined end point - when all of the selected files from the source have been transmitted to the target. When a mirror is complete, the target contains a copy of the source files at that point in time.

1. Identical files are not mirrored.
2. New files are mirrored.
3. Different files can be mirrored.
4. Checksums can calculate blocks of data to be mirrored.

Replication is the real-time transmission of file changes. Unlike other related technologies, which are based on a disk driver or a specific application, the Double-Take Availability replication process operates at the file system level and is able to track file changes independently from the file’s related application. In terms of network resources and time, replicating changes is a more efficient method of maintaining a real-time copy of data than copying an entire file that has changed.

After a source and target have been connected through Double-Take Availability, file system changes from the user-defined data set are tracked. Double-Take Availability immediately transmits these file changes to the target server. This real-time replication keeps the data on the target up-to-date with the source and provides high availability and disaster recovery with minimal data loss. Unlike mirroring which is complete when all of the files have been transmitted to the target, replication continuously captures the changes as they are written to the source. Replication keeps the target up-to-date and synchronized with the source.

1. A user or application updates part of a file.
2. Only the changed portion of the file is replicated to the target.
3. An up-to-date copy of the file is maintained on the target.

Failover is the process in which a target stands in for a failed source. As a result, user and application requests that are directed to the failed source are routed to the target.

Double-Take Availability monitors the source status by tracking network requests and responses exchanged between the source and target. When a monitored source misses a user-defined number of requests, Double-Take Availability assumes that the server has failed. Double-Take Availability then prompts the network administrator to initiate failover, or, if configured, it occurs automatically.

The failover target assumes the network identity of the failed source. When the target assumes the identity of the source, user and application requests destined for the source server or its IP address(es) are routed to the target.

When partnered with the Double-Take Availability data replication capabilities, failover routes user and application requests with minimal disruption and little or no data loss. In some cases, failover may be used without data replication to ensure high availability on a server that only provides processing services, such as a web server.

1. User and application requests are sent to the source name or IP address.
2. Data on the source is mirrored and replicated to the target.
3. The target monitors the source for failure.
4. In the event the source fails, the target stands in for the source. User and application requests are still sent to the source name or IP address, which are now running on the target.

Restoration provides an easy method for copying replicated data from the target back to its original location on the source. The process only requires you to select the source, target, and the appropriate replication set. There is no need to select files or to remember where the data came from on the source since that information is maintained by Double-Take Availability.

Restoration can be used if the source data is lost due to a disk crash or when the most up-to-date data exists on the target due to failover. At the time of a source server failure, your Double-Take Availability target will contain the same data as your Double-Take Availability source. If you are using the Double-Take Availability failover capabilities, users can continue updating data on the target server while the problems on the source are resolved. Because of the continued updates on the target, when the source server is ready to come back online, the two servers will no longer contain the same data. Restoration is the process of copying the up-to-date data from the target back to the original source or a new source. When a restoration is complete, the source and target are again synchronized. Replication continues from the target to the source, keeping the two servers synchronized, until you disconnect the restoration connection.

1. Identical files are not restored.
2. New files are restored.
3. Different files can be restored.
4. Checksums can calculate blocks of data to be restored.