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Data queues

During the Double-Take Availability installation, you identified the amount of disk space that can be used for Double-Take Availability queuing. Queuing to disk allows Double-Take Availability to accommodate high volume processing that might otherwise fill up system memory. For example, on the source, this may occur if the data is changing faster than it can be transmitted to the target, or on the target, a locked file might cause processing to back up.

The following diagram will help you understand how queuing works. Each numbered step is described after the diagram.

If data cannot immediately be transmitted to the target, it is stored, or queued, in system memory. You can configure how much system memory you want to use for queuing. By default, 128 or 512 MB of memory is used, depending on your operating system.
When the allocated amount of system memory is full, new changed data bypasses the full system memory and is queued directly to disk. Data queued to disk is written to a transaction log. Each transaction log can store 5 MB worth of data. Once the log file limit has been reached, a new transaction log is created. The logs can be distinguished by the file name which includes the target IP address, the Double-Take Availability port, the connection ID, and an incrementing sequence number.

Note:

You may notice transaction log files that are not the defined size limit. This is because data operations are not split. For example, if a transaction log has 10 KB left until the limit and the next operation to be applied to that file is greater than 10 KB, a new transaction log file will be created to store that next operation. Also, if one operation is larger than the defined size limit, the entire operation will be written to one transaction log.

When system memory is full, the most recent changed data is added to the disk queue, as described in step 2. This means that system memory contains the oldest data. Therefore, when data is transmitted to the target, Double-Take Availability pulls the data from system memory and sends it. This ensures that the data is transmitted to the target in the same order it was changed on the source. Double-Take Availability automatically reads operations from the oldest transaction log file into system memory. As a transaction log is depleted, it is deleted. When all of the transaction log files are deleted, data is again written directly to system memory (step 1).
To ensure the integrity of the data on the target, the information must be applied in the same order as it was on the source. If there are any delays in processing, for example because of a locked file, a similar queuing process occurs on the target. Data that cannot immediately be applied is queued to system memory. By default, 128 or 512 MB of memory is used, depending on your operating system.
When the allocated amount of system memory on the target is full, new incoming data bypasses the full system memory and is queued directly to disk. Data queued to disk is written to a transaction log. On the target, the transaction logs are identified with the source IP address, the Double-Take Availability port, the connection ID, and an incrementing sequence number.

Like the source, system memory on the target contains the oldest data so when data is applied to the target, Double-Take Availability pulls the data from system memory. Double-Take Availability automatically moves operations from the oldest transaction log file to system memory. As a transaction log is depleted, it is deleted. When all of the transaction log files are deleted, data is again written directly to system memory (step 4).