NILFS2¶
NILFS2 is a log-structured file system (LFS) supporting continuoussnapshotting. In addition to versioning capability of the entire filesystem, users can even restore files mistakenly overwritten ordestroyed just a few seconds ago. Since NILFS2 can keep consistencylike conventional LFS, it achieves quick recovery after systemcrashes.
NILFS2 creates a number of checkpoints every few seconds or persynchronous write basis (unless there is no change). Users can selectsignificant versions among continuously created checkpoints, and canchange them into snapshots which will be preserved until they arechanged back to checkpoints.
There is no limit on the number of snapshots until the volume getsfull. Each snapshot is mountable as a read-only file systemconcurrently with its writable mount, and this feature is convenientfor online backup.
The userland tools are included in nilfs-utils package, which isavailable from the following download page. At least “mkfs.nilfs2”,“mount.nilfs2”, “umount.nilfs2”, and “nilfs_cleanerd” (so calledcleaner or garbage collector) are required. Details on the tools aredescribed in the man pages included in the package.
- Project web page:
- Download page:
- List info:
Caveats¶
Features which NILFS2 does not support yet:
atime
extended attributes
POSIX ACLs
quotas
fsck
defragmentation
Mount options¶
NILFS2 supports the following mount options:(*) == default
barrier(*) | This enables/disables the use of write barriers. This |
nobarrier | requires an IO stack which can support barriers, andif nilfs gets an error on a barrier write, it willdisable again with a warning. |
errors=continue | Keep going on a filesystem error. |
errors=remount-ro(*) | Remount the filesystem read-only on an error. |
errors=panic | Panic and halt the machine if an error occurs. |
cp=n | Specify the checkpoint-number of the snapshot to bemounted. Checkpoints and snapshots are listed by lscpuser command. Only the checkpoints marked as snapshotare mountable with this option. Snapshot is read-only,so a read-only mount option must be specified together. |
order=relaxed(*) | Apply relaxed order semantics that allows modified datablocks to be written to disk without making acheckpoint if no metadata update is going. This modeis equivalent to the ordered data mode of the ext3filesystem except for the updates on data blocks stillconserve atomicity. This will improve synchronouswrite performance for overwriting. |
order=strict | Apply strict in-order semantics that preserves sequenceof all file operations including overwriting of datablocks. That means, it is guaranteed that noovertaking of events occurs in the recovered filesystem after a crash. |
norecovery | Disable recovery of the filesystem on mount.This disables every write access on the device forread-only mounts or snapshots. This option will failfor r/w mounts on an unclean volume. |
discard | This enables/disables the use of discard/TRIM commands. |
nodiscard(*) | The discard/TRIM commands are sent to the underlyingblock device when blocks are freed. This is usefulfor SSD devices and sparse/thinly-provisioned LUNs. |
Ioctls¶
There is some NILFS2 specific functionality which can be accessed by applicationsthrough the system call interfaces. The list of all NILFS2 specific ioctls areshown in the table below.
Table of NILFS2 specific ioctls:
Ioctl
Description
NILFS_IOCTL_CHANGE_CPMODE
Change mode of given checkpoint betweencheckpoint and snapshot state. This ioctl isused in chcp and mkcp utilities.
NILFS_IOCTL_DELETE_CHECKPOINT
Remove checkpoint from NILFS2 file system.This ioctl is used in rmcp utility.
NILFS_IOCTL_GET_CPINFO
Return info about requested checkpoints. Thisioctl is used in lscp utility and bynilfs_cleanerd daemon.
NILFS_IOCTL_GET_CPSTAT
Return checkpoints statistics. This ioctl isused by lscp, rmcp utilities and bynilfs_cleanerd daemon.
NILFS_IOCTL_GET_SUINFO
Return segment usage info about requestedsegments. This ioctl is used in lssu,nilfs_resize utilities and by nilfs_cleanerddaemon.
NILFS_IOCTL_SET_SUINFO
Modify segment usage info of requestedsegments. This ioctl is used bynilfs_cleanerd daemon to skip unnecessarycleaning operation of segments and reduceperformance penalty or wear of flash devicedue to redundant move of in-use blocks.
NILFS_IOCTL_GET_SUSTAT
Return segment usage statistics. This ioctlis used in lssu, nilfs_resize utilities andby nilfs_cleanerd daemon.
NILFS_IOCTL_GET_VINFO
Return information on virtual block addresses.This ioctl is used by nilfs_cleanerd daemon.
NILFS_IOCTL_GET_BDESCS
Return information about descriptors of diskblock numbers. This ioctl is used bynilfs_cleanerd daemon.
NILFS_IOCTL_CLEAN_SEGMENTS
Do garbage collection operation in theenvironment of requested parameters fromuserspace. This ioctl is used bynilfs_cleanerd daemon.
NILFS_IOCTL_SYNC
Make a checkpoint. This ioctl is used inmkcp utility.
NILFS_IOCTL_RESIZE
Resize NILFS2 volume. This ioctl is usedby nilfs_resize utility.
NILFS_IOCTL_SET_ALLOC_RANGE
Define lower limit of segments in bytes andupper limit of segments in bytes. This ioctlis used by nilfs_resize utility.
NILFS2 usage¶
To use nilfs2 as a local file system, simply:
# mkfs -t nilfs2 /dev/block_device# mount -t nilfs2 /dev/block_device /dir
This will also invoke the cleaner through the mount helper program(mount.nilfs2).
Checkpoints and snapshots are managed by the following commands.Their manpages are included in the nilfs-utils package above.
lscp
list checkpoints or snapshots.
mkcp
make a checkpoint or a snapshot.
chcp
change an existing checkpoint to a snapshot or vice versa.
rmcp
invalidate specified checkpoint(s).
To mount a snapshot:
# mount -t nilfs2 -r -o cp=<cno> /dev/block_device /snap_dir
where <cno> is the checkpoint number of the snapshot.
To unmount the NILFS2 mount point or snapshot, simply:
# umount /dir
Then, the cleaner daemon is automatically shut down by the umounthelper program (umount.nilfs2).
Disk format¶
A nilfs2 volume is equally divided into a number of segments exceptfor the super block (SB) and segment #0. A segment is the containerof logs. Each log is composed of summary information blocks, payloadblocks, and an optional super root block (SR):
______________________________________________________| |SB| | Segment | Segment | Segment | ... | Segment | ||_|__|_|____0____|____1____|____2____|_____|____N____|_|0 +1K +4K +8M +16M +24M +(8MB x N) . . (Typical offsets for 4KB-block) . ..______________________.| log | log |... | log ||__1__|__2__|____|__m__| . . . . . ..______________________________.| Summary | Payload blocks |SR||_blocks__|_________________|__|
The payload blocks are organized per file, and each file consists ofdata blocks and B-tree node blocks:
|<--- File-A --->|<--- File-B --->|_______________________________________________________________ | Data blocks | B-tree blocks | Data blocks | B-tree blocks | ..._|_____________|_______________|_____________|_______________|_
Since only the modified blocks are written in the log, it may havefiles without data blocks or B-tree node blocks.
The organization of the blocks is recorded in the summary informationblocks, which contains a header structure (nilfs_segment_summary), perfile structures (nilfs_finfo), and per block structures (nilfs_binfo):
_________________________________________________________________________| Summary | finfo | binfo | ... | binfo | finfo | binfo | ... | binfo |...|_blocks__|___A___|_(A,1)_|_____|(A,Na)_|___B___|_(B,1)_|_____|(B,Nb)_|___
The logs include regular files, directory files, symbolic link filesand several meta data files. The meta data files are the files usedto maintain file system meta data. The current version of NILFS2 usesthe following meta data files:
1) Inode file (ifile) -- Stores on-disk inodes2) Checkpoint file (cpfile) -- Stores checkpoints3) Segment usage file (sufile) -- Stores allocation state of segments4) Data address translation file -- Maps virtual block numbers to usual (DAT) block numbers. This file serves to make on-disk blocks relocatable.
The following figure shows a typical organization of the logs:
_________________________________________________________________________| Summary | regular file | file | ... | ifile | cpfile | sufile | DAT |SR||_blocks__|_or_directory_|_______|_____|_______|________|________|_____|__|
To stride over segment boundaries, this sequence of files may be splitinto multiple logs. The sequence of logs that should be treated aslogically one log, is delimited with flags marked in the segmentsummary. The recovery code of nilfs2 looks this boundary informationto ensure atomicity of updates.
The super root block is inserted for every checkpoints. It includesthree special inodes, inodes for the DAT, cpfile, and sufile. Inodesof regular files, directories, symlinks and other special files, areincluded in the ifile. The inode of ifile itself is included in thecorresponding checkpoint entry in the cpfile. Thus, the hierarchyamong NILFS2 files can be depicted as follows:
Super block (SB) | vSuper root block (the latest cno=xx) |-- DAT |-- sufile `-- cpfile |-- ifile (cno=c1) |-- ifile (cno=c2) ---- file (ino=i1) : : |-- file (ino=i2) `-- ifile (cno=xx) |-- file (ino=i3) : : `-- file (ino=yy) ( regular file, directory, or symlink )
For detail on the format of each file, please see nilfs2_ondisk.hlocated at include/uapi/linux directory.
There are no patents or other intellectual property that we protectwith regard to the design of NILFS2. It is allowed to replicate thedesign in hopes that other operating systems could share (mount, read,write, etc.) data stored in this format.