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.Dd July 31, 2019
.Dt FUSEFS 4
.Os
.Sh NAME
.Nm fusefs
.Nd "File system in USErspace"
.Sh SYNOPSIS
To link into the kernel:
.Bd -ragged -offset indent
.Cd "options FUSEFS"
.Ed
.Pp
To load as a loadable kernel module:
.Pp
.Dl "kldload fusefs"
.Sh DESCRIPTION
The
.Nm
driver implements a file system that is serviced by a userspace program.
.Pp
There are many uses for
.Nm .
Userspace daemons can access libraries or programming languages that cannot run
in kernel-mode, for example.
.Nm
is also useful for developing and debugging file systems, because a crash of
the daemon will not take down the entire operating system.
Finally, the
.Nm
API is portable.
Many daemons can run on multiple operating systems with minimal modifications.
.Sh SYSCTL VARIABLES
The following
.Xr sysctl 8
variables are available:
.Bl -tag -width indent
.It Va vfs.fusefs.kernelabi_major
Major version of the FUSE kernel ABI supported by this driver.
.It Va vfs.fusefs.kernelabi_minor
Minor version of the FUSE kernel ABI supported by this driver.
.It Va vfs.fusefs.data_cache_mode
Controls how
.Nm
will cache file data for pre-7.23 file systems.
A value of 0 will disable caching entirely.
Every data access will be forwarded to the daemon.
A value of 1 will select write-through caching.
Reads will be cached in the VFS layer as usual.
Writes will be immediately forwarded to the daemon, and also added to the cache.
A value of 2 will select write-back caching.
Reads and writes will both be cached, and writes will occasionally be flushed
to the daemon by the page daemon.
Write-back caching is usually unsafe, especially for FUSE file systems that
require network access.
.Pp
FUSE file systems using protocol 7.23 or later specify their cache behavior
on a per-mountpoint basis, ignoring this sysctl.
.It Va vfs.fusefs.stats.filehandle_count
Current number of open FUSE file handles.
.It Va vfs.fusefs.stats.lookup_cache_hits
Total number of lookup cache hits.
.It Va vfs.fusefs.stats.lookup_cache_misses
Total number of lookup cache misses.
.It Va vfs.fusefs.stats.node_count
Current number of allocated FUSE vnodes.
.It Va vfs.fusefs.stats.ticket_count
Current number of allocated FUSE tickets, which is roughly equal to the number
of FUSE operations currently being processed by daemons.
.\" Undocumented sysctls
.\" ====================
.\" vfs.fusefs.enforce_dev_perms: I don't understand it well enough.
.\" vfs.fusefs.iov_credit: I don't understand it well enough
.\" vfs.fusefs.iov_permanent_bufsize: I don't understand it well enough
.El
.Sh SEE ALSO
.Xr mount_fusefs 8
.Sh HISTORY
The
.Nm fuse
driver was written as the part of the
.Fx
implementation of the FUSE userspace file system framework (see
.Lk https://github.com/libfuse/libfuse )
and first appeared in the
.Pa sysutils/fusefs-kmod
port, supporting
.Fx 6.0 .
It was added to the base system in
.Fx 10.0 ,
and renamed to
.Nm
for
.Fx 12.1 .
.Sh AUTHORS
.An -nosplit
The
.Nm fuse
driver was originally written by
.An Csaba Henk
as a Google Summer of Code project in 2005.
It was further developed by
.An Ilya Putsikau
during Google Summer of Code 2011, and that version was integrated into the
base system by
.An Attilio Rao Aq Mt attilio@FreeBSD.org .
.Pp
This manual page was written by
.An Alan Somers Aq Mt asomers@FreeBSD.org .