What is NILFS?

NILFS is a new implementation of a log-structured file system (LFS) supporting continuous snapshotting. In addition to versioning capability of the entire file system, users can even restore files mistakenly overwritten or destroyed just a few seconds ago. Since NILFS can keep consistency like conventional LFS, it achieves quick recovery after system crashes.

NILFS creates a number of checkpoints every few seconds or per synchronous write basis (unless there is no change). Users can select significant versions among continuously created checkpoints, and can change them into snapshots which will be preserved until they are changed back to checkpoints.

There is no limit on the number of snapshots until the volume gets full. Each snapshot is mountable as a read-only file system. It is mountable concurrently with a writable mount and other snapshots, and this feature is convenient to make consistent backups during use.

Snapshot administration is easy and quickly performable. NILFS will make snapshotting or versioning of the POSIX filesystem much familiar to you. The possible use of NILFS includes, versioning, tamper detection, SOX compliance logging, and so forth. It can serve as an alternative filesystem for Linux desktop environment, or as a basis of advanced storage appliances.

The current major version of NILFS is version 2, which is referred to as NILFS2. NILFS2 realized online garbage collection that reclaims disk space with keeping multiple snapshots.

Other NILFS features include:

Using NILFS

Here we illustrate by examples how to use NILFS version 2:

  1. Format a disk partition with mkfs(8). 

    e.g.
# mkfs -t nilfs2 /dev/sdb1
mkfs.nilfs2 ver 2.0
Start writing file system initial data to the device
       Blocksize:4096  Device:/dev/sdb1  Device Size:73402366464
File system initialization succeeded !!

  2. Mount it on a mount-point with mount(8) command,

    # mkdir /nilfs
# mount -t nilfs2 /dev/sdb1 /nilfs

    This will invoke a garbage collector (GC) through mount.nilfs2(8) helper program. The garbage collector is separately implemented as a user space daemon nilfs_cleanerd(8) and can be executed manually.

  3. Use the nilfs mount point as usual

    It's available as an ordinary POSIX filesystem.

  4. Make snapshots

    NILFS makes ``checkpoints'' at regular intervals (unless there is no change) or with synchronous writings. Each checkpoint represents a consistent state of NILFS filesystem, and a number of checkpoints are created continuously. There is no practical limit on the number of checkpoints and snapshots.

    These checkpoints can be listed with lscp(1) command. 

    $ lscp
       CNO        DATE     TIME  MODE  FLG   NBLKINC       ICNT
         1  2008-05-08 14:45:49   cp    -         11          3
         2  2008-05-08 14:50:22   cp    -     200523         81
         3  2008-05-08 20:40:34   cp    -        136         61
         4  2008-05-08 20:41:20   cp    -     187666       1604
         5  2008-05-08 20:41:42   cp    -         51       1634
         6  2008-05-08 20:42:00   cp    -         37       1653
         7  2008-05-08 20:42:42   cp    -     272146       2116
         8  2008-05-08 20:43:13   cp    -     264649       2117
         9  2008-05-08 20:43:44   cp    -     285848       2117
        10  2008-05-08 20:44:16   cp    -     139876       7357

    A snapshot is the checkpoint marked not to be deleted by GC. It is possible to make a snapshot of the current state by mkcp(8) command, and is also possible to change an existing checkpoint into a snapshot. Checkpoints and snapshots are manipulated through the following userland tools: 

     lscp     lists checkpoints.
 mkcp     makes a checkpoint.
 mkcp -s  makes a snapshot.
 chcp     changes an existing checkpoint to a snapshot or
          vice versa.
 rmcp     invalidates specified checkpoint(s).

    In the following example, an existing checkpoint with checkpoint number two, is changed into a snapshot after a period of time. 

    $ sudo chcp ss 2
$ lscp
       CNO        DATE     TIME  MODE  FLG   NBLKINC       ICNT
         1  2008-05-08 14:45:49   cp    -         11          3
         2  2008-05-08 14:50:22   ss    -     200523         81
         3  2008-05-08 20:40:34   cp    -        136         61
         4  2008-05-08 20:41:20   cp    -     187666       1604
         5  2008-05-08 20:41:42   cp    -         51       1634
         6  2008-05-08 20:42:00   cp    -         37       1653
         7  2008-05-08 20:42:42   cp    -     272146       2116
         8  2008-05-08 20:43:13   cp    -     264649       2117
         9  2008-05-08 20:43:44   cp    -     285848       2117
        10  2008-05-08 20:44:16   cp    -     139876       7357
        11  2008-05-08 21:05:23   cp    -         10       7357

    The recent checkpoints are protected from GC during the period given by a GC parameter ``protection_period''; GC never deletes checkpoints whose age from its creation time is less than the value given by the protection_period on the second time scale.

    GC parameters are described in /etc/nilfs_cleanerd.conf(5) and the behavior of GC is tunable by rewriting the config file.

  5. Mount snapshots.

    Snapshots are mounted with two options, a read-only option (``-r'' or ``-o ro'') and a ``cp'' option which specifies the number of checkpoint you want to mount. 

    # mount -t nilfs2 -r -o cp=2 /dev/sdb1 /nilfs-cp
# df -t nilfs2
Filesystem           1K-blocks      Used Available Use% Mounted on
/dev/sdb1             71679996   3203068  64888832   5% /nilfs
/dev/sdb1             71679996   3203068  64888832   5% /nilfs-cp
# mount -t nilfs2
/dev/sdb1 on /nilfs type nilfs2 (rw,gcpid=13296)
/dev/sdb1 on /nilfs-cp type nilfs2 (ro,cp=2)

    The ``current'' filesystem and snapshots are mountable independently.

  6. Unmount snapshots or rw-mount.

    # umount /nilfs
# umount /nilfs-cp

    The GC daemon will be shutdown on umount.

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