fuse - format and options for the fuse file systems
FUSE (Filesystem in Userspace) is a simple interface for userspace
programs to export a virtual filesystem to the Linux kernel. FUSE also aims
to provide a secure method for non privileged users to create and mount
their own filesystem implementations.
Some options regarding mount policy can be set in the file
/etc/fuse.conf. Currently these options are:
- mount_max =
NNN
- Set the maximum number of FUSE mounts allowed to non-root users. The
default is 1000.
- user_allow_other
- Allow non-root users to specify the allow_other or
allow_root mount options (see below).
Most of the generic mount options described in mount are
supported (ro, rw, suid, nosuid, dev,
nodev, exec, noexec, atime, noatime,
sync, async, dirsync). Filesystems are mounted with
nodev,nosuid by default, which can only be overridden by a privileged
user.
These are FUSE specific mount options that can be specified for
all filesystems:
- default_permissions
- By default FUSE doesn't check file access permissions, the filesystem is
free to implement it's access policy or leave it to the underlying file
access mechanism (e.g. in case of network filesystems). This option
enables permission checking, restricting access based on file mode. This
is option is usually useful together with the allow_other mount
option.
- allow_other
- This option overrides the security measure restricting file access to the
user mounting the filesystem. So all users (including root) can access the
files. This option is by default only allowed to root, but this
restriction can be removed with a configuration option described in the
previous section.
- allow_root
- This option is similar to allow_other but file access is limited to
the user mounting the filesystem and root. This option and
allow_other are mutually exclusive.
- kernel_cache
- This option disables flushing the cache of the file contents on every
open(2). This should only be enabled on filesystems, where the file
data is never changed externally (not through the mounted FUSE
filesystem). Thus it is not suitable for network filesystems and other
intermediate filesystems.
NOTE: if this option is not specified (and neither
direct_io) data is still cached after the open(2), so a
read(2) system call will not always initiate a read
operation.
- auto_cache
- This option enables automatic flushing of the data cache on
open(2). The cache will only be flushed if the modification time or
the size of the file has changed.
- large_read
- Issue large read requests. This can improve performance for some
filesystems, but can also degrade performance. This option is only useful
on 2.4.X kernels, as on 2.6 kernels requests size is automatically
determined for optimum performance.
- direct_io
- This option disables the use of page cache (file content cache) in the
kernel for this filesystem. This has several affects:
- 1.
- Each read(2) or write(2) system call will initiate one or
more read or write operations, data will not be cached in the kernel.
- 2.
- The return value of the read() and write() system calls will correspond to
the return values of the read and write operations. This is useful for
example if the file size is not known in advance (before reading it).
- max_read=N
- With this option the maximum size of read operations can be set. The
default is infinite. Note that the size of read requests is limited anyway
to 32 pages (which is 128kbyte on i386).
- max_readahead=N
- Set the maximum number of bytes to read-ahead. The default is determined
by the kernel. On linux-2.6.22 or earlier it's 131072 (128kbytes)
- max_write=N
- Set the maximum number of bytes in a single write operation. The default
is 128kbytes. Note, that due to various limitations, the size of write
requests can be much smaller (4kbytes). This limitation will be removed in
the future.
- async_read
- Perform reads asynchronously. This is the default
- sync_read
- Perform all reads (even read-ahead) synchronously.
- hard_remove
- The default behavior is that if an open file is deleted, the file is
renamed to a hidden file (.fuse_hiddenXXX), and only removed when
the file is finally released. This relieves the filesystem implementation
of having to deal with this problem. This option disables the hiding
behavior, and files are removed immediately in an unlink operation (or in
a rename operation which overwrites an existing file).
It is recommended that you not use the hard_remove option.
When hard_remove is set, the following libc functions fail on unlinked
files (returning errno of ENOENT): read(2),
write(2), fsync(2), close(2), f*xattr(2),
ftruncate(2), fstat(2), fchmod(2),
fchown(2)
- debug
- Turns on debug information printing by the library.
- fsname=NAME
- Sets the filesystem source (first field in /etc/mtab). The default
is the mount program name.
- subtype=TYPE
- Sets the filesystem type (third field in /etc/mtab). The default is
the mount program name. If the kernel suppports it, /etc/mtab and
/proc/mounts will show the filesystem type as fuse.TYPE
If the kernel doesn't support subtypes, the source filed will
be TYPE#NAME, or if fsname option is not specified, just
TYPE.
- use_ino
- Honor the st_ino field in kernel functions getattr() and
fill_dir(). This value is used to fill in the st_ino field
in the stat(2), lstat(2), fstat(2) functions and the
d_ino field in the readdir(2) function. The filesystem does
not have to guarantee uniqueness, however some applications rely on this
value being unique for the whole filesystem.
- readdir_ino
- If use_ino option is not given, still try to fill in the
d_ino field in readdir(2). If the name was previously looked
up, and is still in the cache, the inode number found there will be used.
Otherwise it will be set to -1. If use_ino option is given,
this option is ignored.
- nonempty
- Allows mounts over a non-empty file or directory. By default these mounts
are rejected to prevent accidental covering up of data, which could for
example prevent automatic backup.
- umask=M
- Override the permission bits in st_mode set by the filesystem. The
resulting permission bits are the ones missing from the given umask value.
The value is given in octal representation.
- uid=N
- Override the st_uid field set by the filesystem (N is
numeric).
- gid=N
- Override the st_gid field set by the filesystem (N is
numeric).
- blkdev
- Mount a filesystem backed by a block device. This is a privileged option.
The device must be specified with the fsname=NAME option.
- entry_timeout=T
- The timeout in seconds for which name lookups will be cached. The default
is 1.0 second. For all the timeout options, it is possible to give
fractions of a second as well (e.g. entry_timeout=2.8)
- negative_timeout=T
- The timeout in seconds for which a negative lookup will be cached. This
means, that if file did not exist (lookup retuned ENOENT), the
lookup will only be redone after the timeout, and the file/directory will
be assumed to not exist until then. The default is 0.0 second, meaning
that caching negative lookups are disabled.
- attr_timeout=T
- The timeout in seconds for which file/directory attributes are cached. The
default is 1.0 second.
- ac_attr_timeout=T
- The timeout in seconds for which file attributes are cached for the
purpose of checking if auto_cache should flush the file data on
open. The default is the value of attr_timeout
- intr
- Allow requests to be interrupted. Turning on this option may result in
unexpected behavior, if the filesystem does not support request
interruption.
- intr_signal=NUM
- Specify which signal number to send to the filesystem when a request is
interrupted. The default is hardcoded to USR1.
- modules=M1[:M2...]
- Add modules to the filesystem stack. Modules are pushed in the order they
are specified, with the original filesystem being on the bottom of the
stack.
Modules are filesystem stacking support to high level API.
Filesystem modules can be built into libfuse or loaded from shared
object
Perform file name character set conversion. Options are:
- from_code=CHARSET
- Character set to convert from (see iconv -l for a list of possible
values). Default is UTF-8.
- to_code=CHARSET
- Character set to convert to. Default is determined by the current
locale.
Prepend a given directory to each path. Options are:
- subdir=DIR
- Directory to prepend to all paths. This option is mandatory.
- rellinks
- Transform absolute symlinks into relative
- norellinks
- Do not transform absolute symlinks into relative. This is the
default.
The fusermount program is installed set-user-gid to fuse. This is
done to allow users from fuse group to mount their own filesystem
implementations. There must however be some limitations, in order to prevent
Bad User from doing nasty things. Currently those limitations are:
- 1.
- The user can only mount on a mountpoint, for which it has write
permission
- 2.
- The mountpoint is not a sticky directory which isn't owned by the user
(like /tmp usually is)
- 3.
- No other user (including root) can access the contents of the mounted
filesystem.
FUSE filesystems are unmounted using the fusermount(1)
command (fusermount -u mountpoint).
The main author of FUSE is Miklos Szeredi
<mszeredi@inf.bme.hu>.
This man page was written by Bastien Roucaries
<roucaries.bastien+debian@gmail.com> for the Debian GNU/Linux
distribution (but it may be used by others) from README file.