Mount options

Aaron Jacobs edited this page Mar 19, 2015 · 2 revisions


By default, osxfuse limits volume access to the user that mounted the volume. Nobody, not even the superuser, can access another user's osxfuse volume. This blanket denial is a first line of defense against misbehaving (whether inadvertently or otherwise) user-space file systems that could, for one, "hang" system programs. If you do trust a file system or are otherwise confident, you can use allow_other to turn the blanket denial off. For example, if you wish to use Spotlight on a osxfuse volume, you'll need to use allow_other.

With allow_other, the volume will be accessible normally to all users, but usual permission checks will of course apply. Note that allow_other is a privileged option in that it can only be used by either the superuser or by a user belonging to the "osxfuse Admin" group. When osxfuse loads in the kernel, it sets this group's ID to be that of the admin group on Mac OS X. The superuser can view or change this ID through the sysctl interface (note that osxfuse allows you to set any group ID you specify, even those that do not exist):

$ sudo sysctl macfuse.tunables.admin_group # get
macfuse.tunables.admin_group: 80
$ sudo sysctl -w macfuse.tunables.admin_group=81 # set
macfuse.tunables.admin_group: 80 -> 81


By default, osxfuse does not allow you to mount a osxfuse volume on a directory that itself resides on a osxfuse volume. Such recursion can create interesting unmount-time issues in some cases. Unlike most other osxfuse restrictions, this is a "soft" check that's done only by the mount program. allow_recursion disables this restriction.


Please read the description for allow_other first. allow_root is similar, except that the set of "other" users contains only the superuser. You need to be a member of the osxfuse Admin group to use allow_root.


By default, if osxfuse detects a change in a file's size during getattr(), it will purge that file's buffer cache. When auto_cache is enabled, osxfuse will additionally detect modification time changes during getattr() and open() and will automatically purge the buffer cache and/or attributes of the file if necessary. It will also generate the relevant kqueue messages. All this is subject to the attribute timeout. That is, up to one purge can occur per attribute timeout window. As long as the user-space file system's getattr() callback returns up-to-date size and modification time information, this should work as intended. For user-space file systems that wish the kernel to keep up with "remote" changes, this should obviate the need for explicit purging. auto_cache is not enabled by default: it's opt-in.


By default, osxfuse provides a flexible and adaptive mechanism to handle extended attributes (including things such as Finder Info, Resource Forks, and ACLs). It will initially forward extended attributes calls up to the user-space file system. If the latter does not implement extended attribute functions, osxfuse will remember this and will not forward subsequent calls. It will store extended attributes as Apple Double (._) files. If the user-space file system does implement extended attribute functions, it can choose to handle all or only some extended attributes. If there are certain extended attributes that the user-space file system wants osxfuse (the kernel) to handle through ._ files, it should return ENOTSUP for such attributes. The auto_xattr option tells osxfuse to not bother with sending any extended attributes calls up to user-space, regardless of whether the user-space file system implements the relevant functions or not. With auto_xattr, the kernel will always_ use ._ files.


daemon_timeout=N, where N specifies the timeout in seconds.

When the kernel sends calls to a user-space file system, the latter must reply within some reasonably short time limit, otherwise those calls, and in turn the application that made those calls, will "hang". User programs cannot be completely trusted as they could inadvertently or maliciously fail to respond forever. A typical example is when the file system has a bug that has caused it to crash. In this case, the file system will never respond. Another typical example is that of a network operation performed by the file system taking too long. The kernel can't tell what is really going on, but it does have to guard against hangs. Such hangs may have limited harmful impact on other operating systems, but they are typically deadly on Mac OS X: many parts of the system, including the Finder, can throw a major fit. Often, end users will fail to see a difference between trivially recoverable hangs and unrecoverable system lockup. Therefore, such defensive measures are imperative on Mac OS X.

Still, if you want more control, you can tweak things on a per-mount basis. The daemon_timeout option lets you specify the aforementioned time limit in seconds. The default timeout is set by the FUSE_DEFAULT_DAEMON_TIMEOUT define in fusefs/common/fuse_param.h in the osxfuse kernel source tree. A good question is regarding what happens when there is a timeout. There are several possibilities. Currently, on Mac OS X "Tiger" (default timeout 30 seconds), you will see an alert dialog asking you to choose between force ejecting the volume immediately, continuing to wait until the next timeout, or continue to wait forever (the latter disables the timeout altogether for the given mount instance). On Mac OS X "Leopard" (default timeout 60 seconds), there is no alert dialog--the volume will be automatically ejected (data in any open files may be lost). This is because the kernel APIs to show the alert dialog are deprecated in Leopard. There may be more fine-grained and more fault-tolerant behavior in future versions of osxfuse.


The debug (or simply -d) option causes the user-space file system daemon to print debugging messages for requests and response. The daemon will also run in foregrounded mode with this option.

default_permissions and defer_permissions

As with extended attributes, osxfuse provides a flexible and adaptive mechanism for dealing with permission checks. By default, osxfuse will involve the user-space file system in permission checks by sending it access messages (see access(2)). If the file system doesn't implement the access function, osxfuse will perform permission checks entirely itself based on the attributes of file system objects. The default_permissions option also enables this behavior, wherein the kernel will attempt to do the "right thing" based on what it can "see" (the permission bits as reported by the file system). The file system may not always do the right thing itself though--for example, if the file system retrieves file information from another computer and reports user/group IDs as is, the kernel will not see any alien IDs as belonging to the user that mounted the volume. (This can happen if user ID translation is not enabled or sometimes doesn't work with sshfs.) The defer_permissions (formerly defer_auth) option is useful in such cases. It causes osxfuse to assume that all accesses are allowed--it will forward all operations to the file system, and it is up to somebody else to eventually allow or deny the operations. In the case of sshfs, it would be the SFTP server eventually making the decision about what to allow or disallow.


Some file systems may not know the sizes of files that they provide. This could be because a file's content is being streamed so it's difficult or impossible to know the "size" of the file. The content could be dynamically changing so it may not make sense to advertise a size at getattr time only to find that the size has changed at read or write time. procfs is a good example of a osxfuse file system with such needs. What these file systems would like is to be able to allow reads and writes without the file size mattering. This isn't normally possible in the normal I/O paths in the kernel. In particular, short reads from a user-space file system will be zero filled by osxfuse. The direct_io option causes osxfuse to use an alternative "direct" I/O path between the kernel and the user-space file system. This path makes the file size irrelevant--a read will go on until the file system keeps returning data. There is also no automatic zero filling. In particular, as an implementation side effect, the I/O path bypasses the unified buffer cache altogether.

direct_io is a rather abnormal mode of operation from Mac OS X's standpoint. Unless your file system requires this mode, I wouldn't recommend using this option.


osxfuse supports Mac OS X access control lists (ACLs). The extended_security option will enable it for a file system. To properly support ACLs, a user-space file system must correctly handle the extended attribute. It is possible to get by with storing ACLs in Apple Double (._) files, although that wouldn't be a very secure implementation of ACLs because such files can be manipulated directly. Still, it would work for experimental purposes.


The -f option causes a user-space file system to run in foregrounded mode.


fsid=N, where N is a 24-bit integer (0 and 0xFFFFFF not valid).

Normally, osxfuse will use a newly generated file system ID for every mount. This is fine except in a case where you do want the volume, when mounted again, to have a persistent file system ID (think aliases, for example). Since osxfuse volumes do not have "real" devices backing them, this isn't possible without extra help. The fsid option is that extra help: you use it to specify a 24-bit number that will be used to generate the file system ID.


fsname=NAME, where NAME is a string.

This option can be used to specify the "file system name", analogous to the device in the case of a disk-backed "real" file system. For example, the fsname in the following mount information is sshfs#singh@

$ mount
sshfs#singh@ on /private/tmp/ssh (fusefs, nodev, nosuid, synchronous, mounted by singh)

Note that a file system daemon can choose to specify its own fsname and can override (by strategically inserting its argument) a user-provided fsname.


fssubtype=N, where N is an integer.

You can use this option to specify the file system subtype identifier for osxfuse volume being mounted. The Finder uses this subtype to retrieve the string description (if any) for the file system. The description is what you see next to the Format: label in window that you get when you perform a "Get Info" Finder operation on the volume. This only works for subtype-string pairs that are already present in the osxfuse file system bundle (fusefs.fs) that's installed as part of osxfuse. You can add your own additional mappings to the bundle, or send me an email if you think your file system is useful enough to have an "official" entry.


fstypename=NAME, where NAME is a string.

You can use this option to specify the file system's type_ name. NAME can be at most 7 character (and if specified, must be at least one character). This would cause the in-kernel file system type to be fusefs_NAME. That is, the fusefs_ prefix will be implied and automatically added by the kernel. Thereafter, a user-space file system that uses this option must name its on-disk file system bundle fusefs_NAME.fs. The Finder and other programs/libraries that look into file system bundles will now look inside this bundle (instead of the usual fusefs.fs) for things such as subtypes and description strings. The fssubtype=N option will continue to exist and work. A file system that does not need or use fssubtypename=NAME will work as in the past. Please note that this option is doing something the side effects of which cannot be entirely predicted because arbitrary programs can depend on the true file system type--therefore, the use of this option is subject to a general "this might break things you don't know about" warning.


iosize=N, where N is the I/O size in bytes.

You can use this option to specify the I/O size osxfuse should use while accessing the hypothetical storage device corresponding to a osxfuse volume. The minimum possible I/O size is 512 bytes, whereas the largest is 1MB. The size must also be a power of 2.


This option can be used to "jail" symbolic links within the osxfuse volume itself. If an absolute symbolic link is encountered, osxfuse will prefix the mount path to it.


On Mac OS X 10.4.x, a user-space file system may not exit upon unmount. This is because it may be blocked in one or more operations whose cancellation is not supported by the operating system. The kill_on_unmount option can be used as a workaround to make the kernel send a SIGKILL to the file system daemon after the unmount call has been processed. Although I have not tested exhaustively, this should not be necessary on Mac OS X 10.5.x.


This option marks the volume being mounted as "local". By default, osxfuse volumes are marked as "nonlocal", which technically isn't necessarily the same as a "server" or "network" volume, but is treated as such by the Finder in some cases. For example, the Finder may not show "connected servers" on the Desktop or in the sidebar in some cases. If you use this option, you can get around this "limitation". However, wait! Don't be too tempted and think local is a magic pill that will solve all your problems. In fact, it may mess things up more than you realize. The operating system can be more aggressive in dealing with "local" volumes (a .Trashes. directory will be created, for one). You could run into mysterious problems with Disk Arbitration and other system components. I don't know (and possibly can't know--Mac OS X isn't all open source!) the side effects of using this option. Therefore, treat this as experimental and use with caution. Moreover, please do not file bug reports that involve this option--reproduce your issue without this option and then file a bug report.


modules=M1[:M2...], where Mi is the name of a module to pushed onto the file system stack.

The osxfuse user-space library supports stackable modules--entire file systems or file system shims that can be stacked atop other file systems or shims. For example, filename character set conversion can be handled through such a module. Custom volume icon support in osxfuse is handled through the volicon module. In fact, the volicon=PATH option is a shortcut for modules=volicon,iconpath=PATH option combination. Note that each module can have one or more of its own arguments (volicon has one option: iconpath).


This option enables negative vnode name caching in the kernel. That is, when a file system object name does not exist, osxfuse will remember this upon the first lookup of that name. Thereafter, osxfuse will not call the user-space file system upon subsequent lookups for that name--until an object of that name does get created. The negative cache is LRU managed by the kernel. This is an optimization. If you have a case where file system objects can appear "outside of osxfuse's knowledge" (say, on a remote server in the case of a osxfuse-based network file system), then you should NOT enable this option.


This option makes osxfuse deny all types of access to Apple Double (._) files and .DS_Store files. Any existing files will become apparently non-existent. New files that match the criteria will be disallowed from being created.


This option makes osxfuse deny all types of access to extended attributes that begin with the prefix. On Mac OS X 10.5.x, this is the preferred option if you wish to disallow entities such as resource forks and Finder information.


This marks a volume as non-browsable in that it indicates that the file system is not an appropriate path to user data. The Finder wouldn't automatically browse into such a volume.


The nolocalcaches option disables (in the kernel) the unified buffer cache (UBC), vnode name caching, attribute caching, and readaheads. This means the kernel will have to call the user-space file system upon every operation. The purported "benefit" is that the kernel would be able to "pick up" file changes that occur unbeknownst to the kernel.

For example, in the case of SSHFS, if a file was changed on the server, osxfuse has no way of knowing about it--with nolocalcaches, osxfuse will have no cached information and will be forced to talk to the sshfs program. The SSHFS program has its own cache, which you can also disable using the cache=no option. If all caching is thus disabled, all operations would end up going to go to the SFTP server, resulting in an apparently up-to-date view of the remote file system. There are major caveats, however. nolocalcaches makes the operating system work in an abnormal mode, so the relevant code paths may not have been well tested. The option also creates significant overhead--file operations could end up being much slower. Besides, since SFTP provides no synchronization or locking, there are no consistency or ordering guarantees if multiple clients write to the same file concurrently. My strong suggestion is to realize that SFTP is merely a utility to access remote data, and not a means of distributed file sharing in the same vein as NFS, AFS, Coda, and so on. Of course, you are welcome to create a true distributed file system based on osxfuse.


This turns off the unified buffer cache (UBC) for the entire osxfuse volume.


This turns off vnode name caching in the kernel.

ping_diskarb and noping_diskarb

Beginning with version 1.0.0, osxfuse always performs synchronous (foreground) mounting, so there's no race between the mounting process finishing and the Finder discovering a osxfuse volume. On Mac OS X 10.4.x, osxfuse still needs to tell the Finder that a osxfuse volume has appeared. The ping_diskarb option tells osxfuse to do that (ping_diskarb is enabled by default). The noping_diskarb turns the default behavior off. On Mac OS X 10.5.x, both these options are no-ops (they are silently ignored).


The osxfuse user-space library ensures at runtime that it is talking to a compatible version of the osxfuse kernel component. If there is a mismatch, osxfuse will show an alert dialog and will also send a notification to the Distributed Notification Center. The quiet option will suppress any such alert dialogs.


The rdonly (or simply, -r) option can be used to mount a osxfuse file system in read-only mode.


By default, the osxfuse user-space library runs a file system in multithreaded mode. You can use the -s option to make a user-space file system run in single-threaded mode.


volicon=PATH, where PATH is path to an icon (.icns) file.

You can use the volicon option to specify an icon file that would be used as the Desktop icon for the mounted osxfuse volume. osxfuse will then do the needful (simulating Finder information for the root folder, simulating a /.VolumeIcon.icns file, etc.) to make the custom volume icon work. Note that osxfuse uses library-level stacking for this--it generates a shim file system atop the regular user-space file system, with the shim's only purpose being to simulate what's necessary for the icon to work. NB: the volicon argument WILL NOT WORK correctly if you use any other library stack module along with the volicon module. To make custom volume icons work alongside other stack modules, you have to specify the modules and their arguments using the "long form" (volicon=PATH is a convenience shortcut). For details, see the note at the end of the CUSTOM_VOLUME_ICON wiki page.


volname=NAME, where NAME is a string.

You can use the volname option to specify a name for the osxfuse volume being mounted. This is the name that would show up on the Desktop. In the absence of this option, osxfuse will automatically generate a name that would incorporate the osxfuse device index and the user-space file system being used. For example, an SSHFS mount might have an automatically assigned name "osxfuse Volume 0 (sshfs)".