Double-Take Availability uses a file system filter driver to integrate itself within the I/O stack on a server. This filter driver allows the Double-Take Availability process to see each I/O operation that flows through the stack to the file system. It should be noted that the filter driver will support NTFS file systems. Any read/write operation performed by any application or by the Windows operating system will be visible to the Double-Take Availability driver and therefore, may be subject to replication depending on how Double-Take Availability is configured.
The Double-Take Availability filter driver is a kernel-mode driver. This means that it exists within Windows kernel memory – a subset of system memory reserved for high-level operations that cannot be accessed directly by general applications or the end-users. This will permit the Double-Take Availability filter driver to act with minimal impact on the server itself, while still ensuring that each I/O operation is able to be examined. Since filters exist within the server’s OS kernel, several safety features have been implemented to ensure they do not adversely impact the system in the event of an error.