fsfreeze

fsfreeze [-f | -u] mountpoint

-f, --freeze
    Freeze the filesystem after syncing data.

-u, --unfreeze
    Unfreeze a previously frozen filesystem.

mountpoint
    Path to the mounted filesystem (required).

fsfreeze is a utility from the util-linux package that freezes or unfreezes a mounted filesystem, suspending new write operations to create a quiescent state. This is essential for consistent live backups, LVM snapshots, or online fsck checks without unmounting.

Freezing issues the FIFREEZE ioctl to the block device, blocking writes while allowing reads. Ongoing writes complete, but no new ones start. Unfreezing uses FIUNFREEZE to resume normal operations.

Run as root only. Supported on local filesystems like ext4, XFS, Btrfs; unsupported on NFS, FUSE, or tmpfs. Freezing fails if the filesystem is busy (open files, mounts) or unsupported.

Common workflow: sync data, freeze, snapshot, unfreeze. Ensures data consistency across the filesystem without downtime.

Requires root privileges.
Root filesystem hard to freeze (ongoing kernel writes).
Only local filesystems with FIFREEZE support (ext4, XFS, etc.).
Fails if already frozen/busy.
Default is freeze without options.

Without -f or -u, freezes the filesystem.

0: success
1: failure
32: already frozen
33: not frozen (unfreeze attempt).

fsfreeze /home
# Take snapshot here
fsfreeze -u /home

Introduced in util-linux 2.17 (September 2009) to support filesystem snapshots via ioctls.

mount(8), umount(8), fsck(8), sync(8), fstrim(8)