Mounting and Sharing ZFS File Systems

This section describes how mount points and shared file systems are managed in ZFS.

Managing ZFS Mount Points

By default, all ZFS file systems are mounted by ZFS at boot by using SMF's svc://system/filesystem/local service. File systems are mounted under /path, where path is the name of the file system.

You can override the default mount point by setting the mountpoint property to a specific path by using the zfs set command. ZFS automatically creates this mount point, if needed, and automatically mounts this file system when the zfs mount -a command is invoked, without requiring you to edit the /etc/vfstab file.

The mountpoint property is inherited. For example, if pool/home has mountpoint set to /export/stuff, then pool/home/user inherits /export/stuff/user for its mountpoint property.

The mountpoint property can be set to none to prevent the file system from being mounted.

If desired, file systems can also be explicitly managed through legacy mount interfaces by setting the mountpoint property to legacy by using zfs set. Doing so prevents ZFS from automatically mounting and managing this file system. Legacy tools including the mount and umount commands, and the /etc/vfstab file must be used instead. For more information about legacy mounts, see Legacy Mount Points.

When changing mount point management strategies, the following behaviors apply:

  • Automatic mount point behavior

  • Legacy mount point behavior

Automatic Mount Points

  • When changing from legacy or none, ZFS automatically mounts the file system.

  • If ZFS is currently managing the file system but it is currently unmounted, and the mountpoint property is changed, the file system remains unmounted.

You can also set the default mount point for the root dataset at creation time by using zpool create's m option. For more information about creating pools, see Creating a ZFS Storage Pool.

Any dataset whose mountpoint property is not legacy is managed by ZFS. In the following example, a dataset is created whose mount point is automatically managed by ZFS.

# zfs create pool/filesystem
# zfs get mountpoint pool/filesystem
NAME             PROPERTY      VALUE                      SOURCE
pool/filesystem  mountpoint    /pool/filesystem           default
# zfs get mounted pool/filesystem
NAME             PROPERTY      VALUE                      SOURCE
pool/filesystem  mounted       yes                        -

You can also explicitly set the mountpoint property as shown in the following example:

# zfs set mountpoint=/mnt pool/filesystem
# zfs get mountpoint pool/filesystem
NAME             PROPERTY      VALUE                      SOURCE
pool/filesystem  mountpoint    /mnt                       local
# zfs get mounted pool/filesystem
NAME             PROPERTY      VALUE                      SOURCE
pool/filesystem  mounted       yes                        -

When the mountpoint property is changed, the file system is automatically unmounted from the old mount point and remounted to the new mount point. Mount point directories are created as needed. If ZFS is unable to unmount a file system due to it being active, an error is reported and a forced manual unmount is necessary.

Legacy Mount Points

You can manage ZFS file systems with legacy tools by setting the mountpoint property to legacy. Legacy file systems must be managed through the mount and umount commands and the /etc/vfstab file. ZFS does not automatically mount legacy file systems on boot, and the ZFS mount and umount command do not operate on datasets of this type. The following examples show how to set up and manage a ZFS dataset in legacy mode:

# zfs set mountpoint=legacy tank/home/eschrock
# mount -F zfs tank/home/eschrock /mnt

In particular, if you have set up separate ZFS /usr or /var file systems, you must indicate that they are legacy file systems. In addition, you must mount them by creating entries in the /etc/vfstab file. Otherwise, the system/filesystem/local service enters maintenance mode when the system boots.

To automatically mount a legacy file system on boot, you must add an entry to the /etc/vfstab file. The following example shows what the entry in the /etc/vfstab file might look like:

#device         device        mount           FS      fsck    mount   mount
#to mount       to fsck       point           type    pass    at boot options

tank/home/eschrock -		/mnt		   zfs		-		yes		-	

Note that the device to fsck and fsck pass entries are set to -. This syntax is because the fsck command is not applicable to ZFS file systems. For more information regarding data integrity and the lack of need for fsck in ZFS, see Transactional Semantics.

Mounting ZFS File Systems

ZFS automatically mounts file systems when file systems are created or when the system boots. Use of the zfs mount command is necessary only when changing mount options or explicitly mounting or unmounting file systems.

The zfs mount command with no arguments shows all currently mounted file systems that are managed by ZFS. Legacy managed mount points are not displayed. For example:

# zfs mount
tank                            /tank
tank/home                       /tank/home
tank/home/bonwick               /tank/home/bonwick
tank/ws                         /tank/ws

You can use the a option to mount all ZFS managed file systems. Legacy managed file systems are not mounted. For example:

# zfs mount -a

By default, ZFS does not allow mounting on top of a nonempty directory. To force a mount on top of a nonempty directory, you must use the O option. For example:

# zfs mount tank/home/lalt
cannot mount '/export/home/lalt': directory is not empty
use legacy mountpoint to allow this behavior, or use the -O flag
# zfs mount -O tank/home/lalt

Legacy mount points must be managed through legacy tools. An attempt to use ZFS tools results in an error. For example:

# zfs mount pool/home/billm
cannot mount 'pool/home/billm': legacy mountpoint
use mount(1M) to mount this filesystem
# mount -F zfs tank/home/billm

When a file system is mounted, it uses a set of mount options based on the property values associated with the dataset. The correlation between properties and mount options is as follows:


Mount Options









The mount option nosuid is an alias for nodevices,nosetuid.

Temporary Mount Properties

If any of the above options are set explicitly by using the o option with the zfs mount command, the associated property value is temporarily overridden. These property values are reported as temporary by the zfs get command and revert back to their original settings when the file system is unmounted. If a property value is changed while the dataset is mounted, the change takes effect immediately, overriding any temporary setting.

In the following example, the read-only mount option is temporarily set on the tank/home/perrin file system:

# zfs mount -o ro tank/home/perrin

In this example, the file system is assumed to be unmounted. To temporarily change a property on a file system that is currently mounted, you must use the special remount option. In the following example, the atime property is temporarily changed to off for a file system that is currently mounted:

# zfs mount -o remount,noatime tank/home/perrin
# zfs get atime tank/home/perrin
NAME             PROPERTY      VALUE                      SOURCE
tank/home/perrin atime         off                        temporary

For more information about the zfs mount command, see zfs ( 1M ) .

Unmounting ZFS File Systems

You can unmount file systems by using the zfs unmount subcommand. The unmount command can take either the mount point or the file system name as arguments.

In the following example, a file system is unmounted by file system name:

# zfs unmount tank/home/tabriz

In the following example, the file system is unmounted by mount point:

# zfs unmount /export/home/tabriz

The unmount command fails if the file system is active or busy. To forceably unmount a file system, you can use the f option. Be cautious when forceably unmounting a file system, if its contents are actively being used. Unpredictable application behavior can result.

# zfs unmount tank/home/eschrock
cannot unmount '/export/home/eschrock': Device busy
# zfs unmount -f tank/home/eschrock

To provide for backwards compatibility, the legacy umount command can be used to unmount ZFS file systems. For example:

# umount /export/home/bob

For more information about the zfs umount command, see zfs ( 1M ) .

Sharing ZFS File Systems

Similar to mount points, ZFS can automatically share file systems by using the sharenfs property. Using this method, you do not have to modify the /etc/dfs/dfstab file when a new file system is added. The sharenfs property is a comma-separated list of options to pass to the share command. The special value on is an alias for the default share options, which are read/write permissions for anyone. The special value off indicates that the file system is not managed by ZFS and can be shared through traditional means, such as the /etc/dfs/dfstab file. All file systems whose sharenfs property is not off are shared during boot.

Controlling Share Semantics

By default, all file systems are unshared. To share a new file system, use zfs set syntax similar to the following:

# zfs set sharenfs=on tank/home/eschrock

The property is inherited, and file systems are automatically shared on creation if their inherited property is not off. For example:

# zfs set sharenfs=on tank/home
# zfs create tank/home/bricker
# zfs create tank/home/tabriz
# zfs set sharenfs=ro tank/home/tabriz

Both tank/home/bricker and tank/home/tabriz are initially shared writable because they inherit the sharenfs property from tank/home. Once the property is set to ro (readonly), tank/home/tabriz is shared read-only regardless of the sharenfs property that is set for tank/home.

Unsharing ZFS File Systems

While most file systems are automatically shared and unshared during boot, creation, and destruction, file systems sometimes need to be explicitly unshared. To do so, use the zfs unshare command. For example:

# zfs unshare tank/home/tabriz

This command unshares the tank/home/tabriz file system. To unshare all ZFS file systems on the system, you need to use the a option.

# zfs unshare -a

Sharing ZFS File Systems

Most of the time the automatic behavior of ZFS, sharing on boot and creation, is sufficient for normal operation. If, for some reason, you unshare a file system, you can share it again by using the zfs share command. For example:

# zfs share tank/home/tabriz

You can also share all ZFS file systems on the system by using the a option.

# zfs share -a

Legacy Share Behavior

If the sharenfs property is off, then ZFS does not attempt to share or unshare the file system at any time. This setting enables you to administer through traditional means such as the /etc/dfs/dfstab file.

Unlike the traditional mount command, the traditional share and unshare commands can still function on ZFS file systems. As a result, you can manually share a file system with options that are different from the settings of the sharenfs property. This administrative model is discouraged. Choose to either manage NFS shares completely through ZFS or completely through the /etc/dfs/dfstab file. The ZFS administrative model is designed to be simpler and less work than the traditional model. However, in some cases, you might still want to control file system sharing behavior through the familiar model.