| 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: https://nilfs.sourceforge.io/ |
| Download page: https://nilfs.sourceforge.io/en/download.html |
| List info: http://vger.kernel.org/vger-lists.html#linux-nilfs |
| |
| 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, and |
| if nilfs gets an error on a barrier write, it will |
| disable 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 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. |
| norecovery Disable recovery of the filesystem on mount. |
| This disables every write access on the device for |
| read-only mounts or snapshots. This option will fail |
| for 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 underlying |
| block device when blocks are freed. This is useful |
| for SSD devices and sparse/thinly-provisioned LUNs. |
| |
| Ioctls |
| ====== |
| |
| There is some NILFS2 specific functionality which can be accessed by applications |
| through the system call interfaces. The list of all NILFS2 specific ioctls are |
| shown in the table below. |
| |
| Table of NILFS2 specific ioctls |
| .............................................................................. |
| Ioctl Description |
| NILFS_IOCTL_CHANGE_CPMODE Change mode of given checkpoint between |
| checkpoint and snapshot state. This ioctl is |
| used 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. This |
| ioctl is used in lscp utility and by |
| nilfs_cleanerd daemon. |
| |
| NILFS_IOCTL_GET_CPSTAT Return checkpoints statistics. This ioctl is |
| used by lscp, rmcp utilities and by |
| nilfs_cleanerd daemon. |
| |
| NILFS_IOCTL_GET_SUINFO Return segment usage info about requested |
| segments. This ioctl is used in lssu, |
| nilfs_resize utilities and by nilfs_cleanerd |
| daemon. |
| |
| NILFS_IOCTL_SET_SUINFO Modify segment usage info of requested |
| segments. This ioctl is used by |
| nilfs_cleanerd daemon to skip unnecessary |
| cleaning operation of segments and reduce |
| performance penalty or wear of flash device |
| due to redundant move of in-use blocks. |
| |
| NILFS_IOCTL_GET_SUSTAT Return segment usage statistics. This ioctl |
| is used in lssu, nilfs_resize utilities and |
| by 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 disk |
| block numbers. This ioctl is used by |
| nilfs_cleanerd daemon. |
| |
| NILFS_IOCTL_CLEAN_SEGMENTS Do garbage collection operation in the |
| environment of requested parameters from |
| userspace. This ioctl is used by |
| nilfs_cleanerd daemon. |
| |
| NILFS_IOCTL_SYNC Make a checkpoint. This ioctl is used in |
| mkcp utility. |
| |
| NILFS_IOCTL_RESIZE Resize NILFS2 volume. This ioctl is used |
| by nilfs_resize utility. |
| |
| NILFS_IOCTL_SET_ALLOC_RANGE Define lower limit of segments in bytes and |
| upper limit of segments in bytes. This ioctl |
| is 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 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 nilfs2_ondisk.h |
| located at include/uapi/linux directory. |
| |
| There are no patents or other intellectual property that we protect |
| with regard to the design of NILFS2. It is allowed to replicate the |
| design in hopes that other operating systems could share (mount, read, |
| write, etc.) data stored in this format. |