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posixovl(1) posixovl posixovl(1)

Name

posixovl — The POSIX Overlay Filesystem

Syntax

mount.posixovl [-FH] [-S source_dir] target_dir [-- fuse_opts]

Description

posixovl provides a filesystem view that supports various POSIX operations while using an otherwise incapable lower filesystem. Filesystems of various degrees of POSIXness can be utilitzed. VFAT is a common denominator when it comes to cross-compatibility, though NTFS — its features are unused in Linux — would be another possibility. Native POSIX-capable filesystems can be used too, though that seems like an unusual application of posixovl.

Options

If no source directory is given with an -S option, the target directory specifies both source and target (mountpoint), yielding an "overmount", whereby the contents of the lower filesystem may become inaccessible while the mount is active (which may be a good thing, depending on your viewpoint).

Disable permission and ownership checks on the lower filesystem. Normally these would be used to check for POSIX filesystems mounted inside a non-POSIX tree. (For example, where /vfat is vfat, and /vfat/xfs is a POSIX-behaving filesystem.)
When this option is enabled and a hardlink in the posixovl namespace is created, the contents of the file will be replicated in the lower filesystem rather than using zero-sized placeholder files. Note that the replicas will not be updated later on when editing the inode through posixovl.
This option may be used to specify an alternate path for the lower directory.

Supported operations

posixovl will emulate the following calls if the lower filesystem is incapable of supporting these operations:

  • chmod
  • chown
  • hard links
  • mkfifo
  • mknod
  • symbolic links

The following currently only work in passthrough mode (when using a lower filesystem that already supports these):

•
Linux ACLs/xattrs

Missing suppot:

•
Case-sensitivity

Notes

Using posixovl on an already POSIX-behaving filesystem (e.g. XFS) incurs some issues, since detecting whether a path is POSIX behaving or not is difficult. Hence, the following decision was made:

  • permissions will be set to the default permissions (see below) unless a HCB (hidden control block, the metadata files for posixovl) is found that can override these
  • all lower-level files will be operated on/created with the user who inititated the mount

If no HCB exists for a file or directory, the default permissions are rw-r--r-- or rwxr-xr-x, respectively. The owner and group of the inode will be the owner/group of the real file.

Each non-regular, non-directory virtual file will have a zero-size real file. (Simplifies handling, and makes it apprarent the object exists when using other operating systems.)

`df` will show:

$ df -Tah
Filesystem    Type    Size  Used Avail Use% Mounted on
/dev/hda5     vfat    5.9G  2.1G  3.9G  35% /windows/D
posix-overlay(/windows/D)


     fuse.posixovl    5.9G  2.1G  3.9G  35% /windows/D

In general, posixovl does not handle case-insensitivity of the underlying filesystem (in case of vfat, for example). If you create a file "X0" on vfat, it is usually lowercased to "x0", which may break some software, namely X.org. In order to make vfat behave more POSIX-like, the following mount options are recommended:

$ mount -t vfat /dev/hda1 /windows/D -o check=s,shortname=mixed

Authors

posixovl and this manpage were written by Jan Engelhardt.

Development of posixovl was sponsored by Slax (http://www.slax.org/).

2010-12-07 posixovl