200 lines
7.6 KiB
Plaintext
200 lines
7.6 KiB
Plaintext
NILFS2
|
|
------
|
|
|
|
NILFS2 is 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 NILFS2 can keep consistency
|
|
like conventional LFS, it achieves quick recovery after system
|
|
crashes.
|
|
|
|
NILFS2 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
|
|
concurrently with its writable mount, and this feature is convenient
|
|
for online backup.
|
|
|
|
The userland tools are included in nilfs-utils package, which is
|
|
available from the following download page. At least "mkfs.nilfs2",
|
|
"mount.nilfs2", "umount.nilfs2", and "nilfs_cleanerd" (so called
|
|
cleaner or garbage collector) are required. Details on the tools are
|
|
described in the man pages included in the package.
|
|
|
|
Project web page: http://www.nilfs.org/en/
|
|
Download page: http://www.nilfs.org/en/download.html
|
|
Git tree web page: http://www.nilfs.org/git/
|
|
NILFS mailing lists: http://www.nilfs.org/mailman/listinfo/users
|
|
|
|
Caveats
|
|
=======
|
|
|
|
Features which NILFS2 does not support yet:
|
|
|
|
- atime
|
|
- extended attributes
|
|
- POSIX ACLs
|
|
- quotas
|
|
- fsck
|
|
- resize
|
|
- defragmentation
|
|
|
|
Mount options
|
|
=============
|
|
|
|
NILFS2 supports the following mount options:
|
|
(*) == default
|
|
|
|
barrier=on(*) This enables/disables barriers. barrier=off disables
|
|
it, barrier=on enables it.
|
|
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 be
|
|
mounted. Checkpoints and snapshots are listed by lscp
|
|
user command. Only the checkpoints marked as snapshot
|
|
are 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 data
|
|
blocks to be written to disk without making a
|
|
checkpoint if no metadata update is going. This mode
|
|
is equivalent to the ordered data mode of the ext3
|
|
filesystem except for the updates on data blocks still
|
|
conserve atomicity. This will improve synchronous
|
|
write performance for overwriting.
|
|
order=strict Apply strict in-order semantics that preserves sequence
|
|
of all file operations including overwriting of data
|
|
blocks. That means, it is guaranteed that no
|
|
overtaking of events occurs in the recovered file
|
|
system after a crash.
|
|
|
|
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 umount
|
|
helper program (umount.nilfs2).
|
|
|
|
Disk format
|
|
===========
|
|
|
|
A nilfs2 volume is equally divided into a number of segments except
|
|
for the super block (SB) and segment #0. A segment is the container
|
|
of logs. Each log is composed of summary information blocks, payload
|
|
blocks, 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 of
|
|
data 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 have
|
|
files without data blocks or B-tree node blocks.
|
|
|
|
The organization of the blocks is recorded in the summary information
|
|
blocks, which contains a header structure (nilfs_segment_summary), per
|
|
file 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 files
|
|
and several meta data files. The mata data files are the files used
|
|
to maintain file system meta data. The current version of NILFS2 uses
|
|
the following meta data files:
|
|
|
|
1) Inode file (ifile) -- Stores on-disk inodes
|
|
2) Checkpoint file (cpfile) -- Stores checkpoints
|
|
3) Segment usage file (sufile) -- Stores allocation state of segments
|
|
4) 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 split
|
|
into multiple logs. The sequence of logs that should be treated as
|
|
logically one log, is delimited with flags marked in the segment
|
|
summary. The recovery code of nilfs2 looks this boundary information
|
|
to ensure atomicity of updates.
|
|
|
|
The super root block is inserted for every checkpoints. It includes
|
|
three special inodes, inodes for the DAT, cpfile, and sufile. Inodes
|
|
of regular files, directories, symlinks and other special files, are
|
|
included in the ifile. The inode of ifile itself is included in the
|
|
corresponding checkpoint entry in the cpfile. Thus, the hierarchy
|
|
among NILFS2 files can be depicted as follows:
|
|
|
|
Super block (SB)
|
|
|
|
|
v
|
|
Super 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 include/linux/nilfs2_fs.h.
|