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3a36199114
Resize feature was supported by the commit 4e33f9eab0 but it was not
reflected to the list of unsupported features in nilfs2.txt file.
This updates the list to fix discrepancy.
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
209 lines
8.1 KiB
Plaintext
209 lines
8.1 KiB
Plaintext
NILFS2
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------
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NILFS2 is a log-structured file system (LFS) supporting continuous
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snapshotting. In addition to versioning capability of the entire file
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system, users can even restore files mistakenly overwritten or
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destroyed just a few seconds ago. Since NILFS2 can keep consistency
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like conventional LFS, it achieves quick recovery after system
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crashes.
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NILFS2 creates a number of checkpoints every few seconds or per
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synchronous write basis (unless there is no change). Users can select
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significant versions among continuously created checkpoints, and can
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change them into snapshots which will be preserved until they are
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changed back to checkpoints.
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There is no limit on the number of snapshots until the volume gets
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full. Each snapshot is mountable as a read-only file system
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concurrently with its writable mount, and this feature is convenient
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for online backup.
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The userland tools are included in nilfs-utils package, which is
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available from the following download page. At least "mkfs.nilfs2",
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"mount.nilfs2", "umount.nilfs2", and "nilfs_cleanerd" (so called
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cleaner or garbage collector) are required. Details on the tools are
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described in the man pages included in the package.
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Project web page: http://www.nilfs.org/en/
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Download page: http://www.nilfs.org/en/download.html
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Git tree web page: http://www.nilfs.org/git/
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List info: http://vger.kernel.org/vger-lists.html#linux-nilfs
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Caveats
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=======
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Features which NILFS2 does not support yet:
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- atime
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- extended attributes
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- POSIX ACLs
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- quotas
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- fsck
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- defragmentation
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Mount options
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=============
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NILFS2 supports the following mount options:
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(*) == default
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barrier(*) This enables/disables the use of write barriers. This
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nobarrier requires an IO stack which can support barriers, and
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if nilfs gets an error on a barrier write, it will
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disable again with a warning.
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errors=continue Keep going on a filesystem error.
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errors=remount-ro(*) Remount the filesystem read-only on an error.
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errors=panic Panic and halt the machine if an error occurs.
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cp=n Specify the checkpoint-number of the snapshot to be
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mounted. Checkpoints and snapshots are listed by lscp
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user command. Only the checkpoints marked as snapshot
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are mountable with this option. Snapshot is read-only,
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so a read-only mount option must be specified together.
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order=relaxed(*) Apply relaxed order semantics that allows modified data
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blocks to be written to disk without making a
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checkpoint if no metadata update is going. This mode
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is equivalent to the ordered data mode of the ext3
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filesystem except for the updates on data blocks still
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conserve atomicity. This will improve synchronous
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write performance for overwriting.
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order=strict Apply strict in-order semantics that preserves sequence
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of all file operations including overwriting of data
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blocks. That means, it is guaranteed that no
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overtaking of events occurs in the recovered file
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system after a crash.
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norecovery Disable recovery of the filesystem on mount.
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This disables every write access on the device for
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read-only mounts or snapshots. This option will fail
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for r/w mounts on an unclean volume.
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discard This enables/disables the use of discard/TRIM commands.
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nodiscard(*) The discard/TRIM commands are sent to the underlying
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block device when blocks are freed. This is useful
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for SSD devices and sparse/thinly-provisioned LUNs.
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NILFS2 usage
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============
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To use nilfs2 as a local file system, simply:
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# mkfs -t nilfs2 /dev/block_device
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# mount -t nilfs2 /dev/block_device /dir
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This will also invoke the cleaner through the mount helper program
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(mount.nilfs2).
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Checkpoints and snapshots are managed by the following commands.
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Their manpages are included in the nilfs-utils package above.
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lscp list checkpoints or snapshots.
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mkcp make a checkpoint or a snapshot.
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chcp change an existing checkpoint to a snapshot or vice versa.
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rmcp invalidate specified checkpoint(s).
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To mount a snapshot,
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# mount -t nilfs2 -r -o cp=<cno> /dev/block_device /snap_dir
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where <cno> is the checkpoint number of the snapshot.
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To unmount the NILFS2 mount point or snapshot, simply:
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# umount /dir
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Then, the cleaner daemon is automatically shut down by the umount
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helper program (umount.nilfs2).
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Disk format
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===========
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A nilfs2 volume is equally divided into a number of segments except
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for the super block (SB) and segment #0. A segment is the container
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of logs. Each log is composed of summary information blocks, payload
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blocks, and an optional super root block (SR):
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______________________________________________________
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| |SB| | Segment | Segment | Segment | ... | Segment | |
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|_|__|_|____0____|____1____|____2____|_____|____N____|_|
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0 +1K +4K +8M +16M +24M +(8MB x N)
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. . (Typical offsets for 4KB-block)
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. .
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.______________________.
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| log | log |... | log |
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|__1__|__2__|____|__m__|
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. .
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. .
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. .
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.______________________________.
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| Summary | Payload blocks |SR|
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|_blocks__|_________________|__|
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The payload blocks are organized per file, and each file consists of
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data blocks and B-tree node blocks:
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|<--- File-A --->|<--- File-B --->|
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_______________________________________________________________
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| Data blocks | B-tree blocks | Data blocks | B-tree blocks | ...
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_|_____________|_______________|_____________|_______________|_
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Since only the modified blocks are written in the log, it may have
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files without data blocks or B-tree node blocks.
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The organization of the blocks is recorded in the summary information
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blocks, which contains a header structure (nilfs_segment_summary), per
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file structures (nilfs_finfo), and per block structures (nilfs_binfo):
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_________________________________________________________________________
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| Summary | finfo | binfo | ... | binfo | finfo | binfo | ... | binfo |...
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|_blocks__|___A___|_(A,1)_|_____|(A,Na)_|___B___|_(B,1)_|_____|(B,Nb)_|___
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The logs include regular files, directory files, symbolic link files
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and several meta data files. The mata data files are the files used
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to maintain file system meta data. The current version of NILFS2 uses
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the following meta data files:
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1) Inode file (ifile) -- Stores on-disk inodes
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2) Checkpoint file (cpfile) -- Stores checkpoints
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3) Segment usage file (sufile) -- Stores allocation state of segments
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4) Data address translation file -- Maps virtual block numbers to usual
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(DAT) block numbers. This file serves to
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make on-disk blocks relocatable.
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The following figure shows a typical organization of the logs:
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_________________________________________________________________________
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| Summary | regular file | file | ... | ifile | cpfile | sufile | DAT |SR|
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|_blocks__|_or_directory_|_______|_____|_______|________|________|_____|__|
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To stride over segment boundaries, this sequence of files may be split
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into multiple logs. The sequence of logs that should be treated as
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logically one log, is delimited with flags marked in the segment
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summary. The recovery code of nilfs2 looks this boundary information
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to ensure atomicity of updates.
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The super root block is inserted for every checkpoints. It includes
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three special inodes, inodes for the DAT, cpfile, and sufile. Inodes
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of regular files, directories, symlinks and other special files, are
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included in the ifile. The inode of ifile itself is included in the
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corresponding checkpoint entry in the cpfile. Thus, the hierarchy
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among NILFS2 files can be depicted as follows:
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Super block (SB)
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v
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Super root block (the latest cno=xx)
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|-- DAT
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|-- sufile
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`-- cpfile
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|-- ifile (cno=c1)
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|-- ifile (cno=c2) ---- file (ino=i1)
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: : |-- file (ino=i2)
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`-- ifile (cno=xx) |-- file (ino=i3)
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: :
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`-- file (ino=yy)
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( regular file, directory, or symlink )
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For detail on the format of each file, please see include/linux/nilfs2_fs.h.
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