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286 lines
11 KiB
286 lines
11 KiB
.. SPDX-License-Identifier: GPL-2.0 |
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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: https://nilfs.sourceforge.io/ |
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:Download page: https://nilfs.sourceforge.io/en/download.html |
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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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======================= ======================================================= |
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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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======================= ======================================================= |
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Ioctls |
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====== |
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There is some NILFS2 specific functionality which can be accessed by applications |
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through the system call interfaces. The list of all NILFS2 specific ioctls are |
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shown in the table below. |
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Table of NILFS2 specific ioctls: |
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============================== =============================================== |
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Ioctl Description |
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============================== =============================================== |
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NILFS_IOCTL_CHANGE_CPMODE Change mode of given checkpoint between |
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checkpoint and snapshot state. This ioctl is |
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used in chcp and mkcp utilities. |
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NILFS_IOCTL_DELETE_CHECKPOINT Remove checkpoint from NILFS2 file system. |
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This ioctl is used in rmcp utility. |
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NILFS_IOCTL_GET_CPINFO Return info about requested checkpoints. This |
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ioctl is used in lscp utility and by |
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nilfs_cleanerd daemon. |
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NILFS_IOCTL_GET_CPSTAT Return checkpoints statistics. This ioctl is |
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used by lscp, rmcp utilities and by |
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nilfs_cleanerd daemon. |
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NILFS_IOCTL_GET_SUINFO Return segment usage info about requested |
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segments. This ioctl is used in lssu, |
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nilfs_resize utilities and by nilfs_cleanerd |
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daemon. |
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NILFS_IOCTL_SET_SUINFO Modify segment usage info of requested |
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segments. This ioctl is used by |
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nilfs_cleanerd daemon to skip unnecessary |
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cleaning operation of segments and reduce |
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performance penalty or wear of flash device |
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due to redundant move of in-use blocks. |
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NILFS_IOCTL_GET_SUSTAT Return segment usage statistics. This ioctl |
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is used in lssu, nilfs_resize utilities and |
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by nilfs_cleanerd daemon. |
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NILFS_IOCTL_GET_VINFO Return information on virtual block addresses. |
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This ioctl is used by nilfs_cleanerd daemon. |
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NILFS_IOCTL_GET_BDESCS Return information about descriptors of disk |
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block numbers. This ioctl is used by |
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nilfs_cleanerd daemon. |
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NILFS_IOCTL_CLEAN_SEGMENTS Do garbage collection operation in the |
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environment of requested parameters from |
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userspace. This ioctl is used by |
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nilfs_cleanerd daemon. |
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NILFS_IOCTL_SYNC Make a checkpoint. This ioctl is used in |
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mkcp utility. |
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NILFS_IOCTL_RESIZE Resize NILFS2 volume. This ioctl is used |
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by nilfs_resize utility. |
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NILFS_IOCTL_SET_ALLOC_RANGE Define lower limit of segments in bytes and |
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upper limit of segments in bytes. This ioctl |
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is used by nilfs_resize utility. |
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============================== =============================================== |
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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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==== =========================================================== |
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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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==== =========================================================== |
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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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| 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 nilfs2_ondisk.h |
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located at include/uapi/linux directory. |
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There are no patents or other intellectual property that we protect |
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with regard to the design of NILFS2. It is allowed to replicate the |
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design in hopes that other operating systems could share (mount, read, |
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write, etc.) data stored in this format.
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