Paragon Software’s Snapshot for Mac is designed to get instant copies of a disk or several disks at a specific point in time. This technology enables to take consistent snapshots of both inactive and in-use partitions, e.g. those that host Mac OS X, providing a coherent state of all open files involved in the process and taking into account that applications may still keep writing to disks. These snapshots then are provided to a third-party backup engine to allow it make consistent backup images of in-use files, running applications and OS.
Hard Disk Manager for Mac uses this technology for creating backup archives.
The concept of the Snapshot for Mac technology is based on embedding a special filter driver into a kernel input-output (I/O) stack between a block device and a file system. It is aimed at saving initial (at the moment of taking a snapshot) state of data blocks on a disk to provide backup data consistency, while OS or applications keep modifying data on this disk. So, when attempting to write something to a block device, which snapshot has been taken, the filter driver first copies existing data from the targeted blocks to a special temporary file called the backstore and only then the writing operation is allowed. This way Snapshot for Mac doesn’t prohibit re-writing data on the “snapshotted” block device, but only postpone it until the old data is copied to the backstore. The above trick enables to ensure consistency of backup data at a specific point-in-time, while allowing standard read/write operations for target storage devices during a backup process. Thus, for changed data blocks after taking a snapshot, Snapshot for Mac provides a backup engine the initial data from the backstore, while for unchanged blocks – those stored directly on the block device.