Overview of ZFS Clones
A clone is a writable volume or file system whose initial contents are the same as the dataset from which it was created. As with snapshots, creating a clone is nearly instantaneous, and initially consumes no additional disk space. In addition, you can snapshot a clone.
Clones can only be created from a snapshot. When a snapshot is cloned, an implicit dependency is created between the clone and snapshot. Even though the clone is created somewhere else in the dataset hierarchy, the original snapshot cannot be destroyed as long as the clone exists. The origin property exposes this dependency, and the zfs destroy command lists any such dependencies, if they exist.
Clones do not inherit the properties of the dataset from which it was created. Use the zfs get and zfs set commands to view and change the properties of a cloned dataset. For more information about setting ZFS dataset properties, see Setting ZFS Properties.
Because a clone initially shares all its disk space with the original snapshot, its used property is initially zero. As changes are made to the clone, it uses more space. The used property of the original snapshot does not consider the disk space consumed by the clone.
Creating a ZFS Clone
To create a clone, use the zfs clone command, specifying the snapshot from which to create the clone, and the name of the new file system or volume. The new file system or volume can be located anywhere in the ZFS hierarchy. The type of the new dataset (for example, file system or volume) is the same type as the snapshot from which the clone was created. You cannot create clone of a file system in a pool that is different from where the original file system snapshot resides.
In the following example, a new clone named tank/home/ahrens/bug123 with the same initial contents as the snapshot tank/ws/[email protected] is created.
# zfs snapshot tank/ws/[email protected] # zfs clone tank/ws/[email protected] tank/home/ahrens/bug123
In the following example, a cloned workspace is created from the projects/[email protected] snapshot for a temporary user as projects/teamA/tempuser. Then, properties are set on the cloned workspace.
# zfs snapshot projects/[email protected] # zfs clone projects/[email protected] projects/teamA/tempuser # zfs set sharenfs=on projects/teamA/tempuser # zfs set quota=5G projects/teamA/tempuser
Destroying a ZFS Clone
# zfs destroy tank/home/ahrens/bug123
Clones must be destroyed before the parent snapshot can be destroyed.
Replacing a ZFS File System With a ZFS Clone
You can use the zfs promote command to replace an active ZFS file system with a clone of that file system. This feature facilitates the ability to clone and replace file systems so that the “origin” file system become the clone of the specified file system. In addition, this feature makes it possible to destroy the file system from which the clone was originally created. Without clone promotion, you cannot destroy a “origin” file system of active clones. For more information about destroying clones, see Destroying a ZFS Clone.
In the following example, the tank/test/productA file system is cloned and then the clone file system, tank/test/productAbeta becomes the tank/test/productA file system.
# zfs create tank/test # zfs create tank/test/productA # zfs snapshot tank/test/[email protected] # zfs clone tank/test/[email protected] tank/test/productAbeta # zfs list -r tank/test NAME USED AVAIL REFER MOUNTPOINT tank/test 314K 8.24G 25.5K /tank/test tank/test/productA 288K 8.24G 288K /tank/test/productA tank/test/[email protected] 0 - 288K - tank/test/productAbeta 0 8.24G 288K /tank/test/productAbeta # zfs promote tank/test/productAbeta # zfs list -r tank/test NAME USED AVAIL REFER MOUNTPOINT tank/test 316K 8.24G 27.5K /tank/test tank/test/productA 0 8.24G 288K /tank/test/productA tank/test/productAbeta 288K 8.24G 288K /tank/test/productAbeta tank/test/[email protected] 0 - 288K -
In the above zfs -list output, you can see that the space accounting of the original productA file system has been replaced with the productAbeta file system.
Complete the clone replacement process by renaming the file systems. For example:
# zfs rename tank/test/productA tank/test/productAlegacy # zfs rename tank/test/productAbeta tank/test/productA # zfs list -r tank/test NAME USED AVAIL REFER MOUNTPOINT tank/test 316K 8.24G 27.5K /tank/test tank/test/productA 288K 8.24G 288K /tank/test/productA tank/test/[email protected] 0 - 288K - tank/test/productAlegacy 0 8.24G 288K /tank/test/productAlegacy
Optionally, you can remove the legacy file system. For example:
# zfs destroy tank/test/productAlegacy