fsfreeze

fsfreeze [-f | -u]

-f
    Freezes the specified filesystem, suspending all I/O operations.

-u
    Unfreezes the specified filesystem, resuming all I/O operations.


    The path to the mounted filesystem to be frozen or unfrozen (e.g., /mnt/data).

The fsfreeze command is used to suspend and resume access to a filesystem, effectively pausing all I/O operations on it. This state is known as "frozen." It's primarily employed to create consistent backups or snapshots of a filesystem without applications writing data during the snapshot process. When a filesystem is frozen, any new write attempts will block until the filesystem is unfrozen. This ensures data integrity for operations like LVM snapshots or raw disk imaging (e.g., with dd). It typically works by issuing FIFREEZE and FITHAW ioctl calls to the kernel. After the snapshot operation is complete, the filesystem must be unfrozen to allow normal I/O to resume.

Requires root privileges: The command must be run as root or with sufficient capabilities (CAP_SYS_ADMIN).
Temporary I/O suspension: Filesystems should only be frozen for the shortest possible duration to avoid significant application disruption or potential deadlocks.
Filesystem support: Not all filesystems support freezing. Modern journaling filesystems like ext4, XFS, and Btrfs typically do.
System crashes: If the system crashes while a filesystem is frozen, it might remain in a frozen state upon reboot, requiring manual unfreezing.

fsfreeze utilizes ioctl commands (FIFREEZE and FITHAW) that communicate directly with the kernel's filesystem driver to suspend or resume I/O.

The command returns 0 on success and a non-zero value on failure.

fsfreeze was introduced as a generic tool in the util-linux package to provide filesystem freezing capabilities independent of the specific filesystem, unlike its predecessor xfs_freeze which was specific to XFS. Its development aimed to standardize the interface for pausing filesystem I/O across various Linux filesystems, making it easier for backup and snapshot tools to integrate.

xfs_freeze(8), mount(8), lvm(8), dd(1)