ESFS is a FUSE-based filesystem that supports snapshots (C)
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README.md

ESFS

ESFS is a FUSE-based snapshotting (versioning) filesystem for Linux

ESFS can maintain a series of read-only snapshots, and is optimised for speed when reading or writing the current versions of the files. It uses an underlying filesystem to carry out the file operations, and to save the data necessary to maintain the snapshots themselves.

Through the snapshots, it provides read-only access to previous states of the whole filesystem, which can be used to restore data following some corruption caused by human error or software bug. In this sense, it can provide a backup / archiving solution, although on the same storage as the main filesystem itself.

Although ESFS supports most of the main filesystem operations, its main aim is to provide file-based containers that are used as block (loop) devices for filesystems that need versioning implemented.

Using ESFS

Use the esfs binary to mount the filesystem. It needs two directories: an empty mount point where the filesystem will appear, and another directory under which ESFS will store its data. Run esfs [ FUSE_AND_MOUNT_OPTIONS ] [--local-log] (DATA_DIRECTORY) (MOUNTPOINT) to mount the filesystem. To get a list of the FUSE and mount options, run esfs -h.

You can now use the filesystem by creating, writing or reading files under (MOUNTPOINT). ESFS stores its internal data under (DATA_DIRECTORY). Do not write or modify files under this directory as that can break the ESFS filesystem (except in special circumstances - please see below).

When the filesystem is mounted, a (MOUNTPOINT)/snapshots directory is created. The snapshots are all represented as directories under snapshots/.

To create a new snapshot from the current state of the filesystem, use mkdir (MOUNTPOINT)/snapshots/(SNAPSHOT_NAME). To delete the earliest snapshot, simply run rmdir (MOUNTPOINT)/snapshots.

You can access files in the snapshots under snapshots/(SNAPSHOT_NAME)/. For example, the version of the file (MOUNTPOINT)/mydir/myfile at the time the snapshot named Monday was taken is at (MOUNTPOINT)/snapshots/Monday/mydir/myfile.

To un-mount the ESFS filesystem, run fusermount -u (MOUNTPOINT).

ESFS will also create a log file if it was compiled with a positive $$DEBUG value (see params.h). The default location of the log is /var/log/esfs.log, but if you use the --local-log argument, ESFS will try to open the log file in the directory it was started in.

Installation and troubleshooting

Download the release of the ESFS source you would like to install from GitHub here.

If you haven't already, you will need to install the tools gcc, make and pkg-config, as well as FUSE and its header files. On Debian, all of these can be installed using apt-get update; apt-get install gcc make pkg-config fuse libfuse-dev.

In the directory where the source files of ESFS are, run perl Makefile.PL to generate the Makefile; and run make to compile ESFS. A new executable file called esfs will be generated. Settings and constants are defined in params.h and types.h.

Run groups as the user you want to run ESFS as; if fuse is not listed, you may need to add the user to the fuse group. As root, run adduser (USERNAME) fuse to do this.

Start ESFS by running esfs (UNDERLYING_ROOT_DIR) (MOUNTPOINT), or see above for more options.

If you get a failed to open /dev/fuse: Permission denied error when starting esfs, please make sure that /dev/fuse is group readable/writeable and it is owned by the group fuse. To make it group readable/writeable, run, as root, chmod g+rw /dev/fuse; and to make it owned by the user root and the group fuse, run chown root:fuse /dev/fuse.

If you want ESFS to use /var/log/esfs.log as its log, you may need to create this file first as root and change its ownership or permissions so that ESFS could write it. Use touch /var/log/esfs.log to create the file, and chown (USERNAME):(USERNAME) /var/log/esfs.log to change the owner to the user ESFS will run as.

How does it work?

ESFS forwards most requests to the underlying filesystem, but it intercepts writes and other modifications, and uses copy-on-write to save the blocks of old data before they are actually overwritten. It only saves a block once, when the given block is modified for the first time after a snapshot was created.

+--------------------------+
|           User           |
+--------------------------+
              |
+--------------------------+
|           ESFS           |
+--------------------------+
        |          |
+-+-----------+-+--------+-+
| | Snapshots | | Mirror | |
| +-----------+ +--------+ |
|  Underlying filesystem   |
+--------------------------+

This way, it keeps a mirror of the files you are working on in the underlying filesystem, and subsequent read and write operations are almost as fast as if there were no snapshots present.

In the underlying filesystem, the data saved for the snapshots are kept in two types of files. The .dat files contain the blocks saved, while the .map files contain file metadata and information about which blocks have been saved, and where. Please see the documentation in the source for details.

Security considerations

Please note that ESFS accesses the underlying filesystem as the user it is running as. Among other things, this means:

  • Bugs in the code can be exploited to access other files in the system readable or writeable by this user. Because of this, ESFS aborts if it is started as root.

  • Other processes running as the same user as ESFS can access all underlying files ESFS creates to store the filesystem data.

Please also be aware of the security implications of all FUSE options you use.

Bugs and limitations

By design, ESFS has the following limitations:

  • Snapshots are read-only
  • Only the earliest snapshot can be deleted
  • Paths starting with /snapshots are reserved for the snapshots

The current version of ESFS has these additional limitations:

  • Renaming is not supported
  • Group permission bits are ignored when accessing files in the snapshots
  • Symlinks and hard links are not supported
  • Special files (other than files and directories) are not supported

Please see fuse_path_write.c for a plan for a path map that would allow to store renames efficiently.

If you find a bug in ESFS, please let me know by submitting it on GitHub, here.

Wiki

Disclaimer and licence

If you find ESFS useful, and would like to extend or improve it, please feel free to submit patches or fork the code.

ESFS is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.

ESFS is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for details.

ESFS is Copyright (C) 2013, 2014 Elod Csirmaz

ESFS is under heavy development. It is not suitable for mission-critical applications.

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