Cloning files and directory trees

The file clone feature allows for the quick copying of a file, which is space efficient when the source file and its copies ("clones") do not diverge significantly from each other over time. This feature is intended for situations in which long-lived and mostly unchanging copies of a large master file are required (for example, deploying many copies of an OS disk image for virtualized desktops). When a file clone is created, a snapshot of the master file is created, preserving an image of the source file at that point in time. The snapshot is only of the file, it is accessible only by the NAS server, and it is read-only. This snapshot allows the source file and clone to share common data blocks. If additional clones of the master file are created, new snapshots are created if necessary.

The maximum number of clones is limited by the availability of space in the file system. File cloning is most space efficient when related clones do not diverge significantly from each other. File clones are less space efficient when some clones are short-lived and others are long-lived, because the data blocks that were once shared between the source (master) file and all related clones are not freed until all related clones are deleted.

The directory tree cloning feature provides a way to quickly create space-efficient writeable copies of a directory tree. Space efficiency is achieved by creating clones of the source files in the destination tree. Space efficiency is reduced over time, as the source files and clones diverge. The source directory's structure is recreated in the destination directory, and all of the contained files are cloned. Security information from the source files and directories is applied to the destination.

During a directory tree cloning operation, timing can have an effect on what is cloned, because the directory tree is not protected against modification during the clone operation. The cloned directory tree is not a point-in-time replica (like a snapshot), because the source tree is online and may be in use during the clone operation, so the directory tree (or the files in the tree) may be modified while the tree clone operation is in progress. When a directory tree is modified during a tree clone operation, none, some, or all of the modifications may be included in the clone. To ensure that the directory tree is cloned precisely as it is at the time the clone operation is initiated (a consistent copy), you must ensure that the directory tree is not modified until after the clone operation is complete.

Tree cloning uses the same mechanism as file cloning to clone individual files within the tree, so the same limitations apply, and a File Clone license is required to enable file or directory tree clones features.