fb6e7113ae
This clarifies missing features of nilfs as a regular filesystem. Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
199 lines
7.6 KiB
Text
199 lines
7.6 KiB
Text
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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NILFS mailing lists: http://www.nilfs.org/mailman/listinfo/users
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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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- resize
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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=on(*) This enables/disables barriers. barrier=off disables
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it, barrier=on enables it.
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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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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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