posixovl(1)

posixovl(1) — The POSIX Overlay Filesystem 💅

okay bestie, welcome to the posixovl(1) manual — the one where a lower filesystem gets main character energy and cosplays as fully POSIX-capable. ✨

💖 NAME 💖

posixovl — The POSIX Overlay Filesystem. That's the name, and it's giving "I can fix him" for filesystems. 😭

✨ SYNTAX ✨

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

No invented flags here, periodt.

🦋 DESCRIPTION 🦋

posixovl provides a filesystem view that supports various POSIX operations while using an otherwise incapable lower filesystem. Filesystems with all kinds 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). 👀

• -F — 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.) 🫶
• -H — 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. 😭
• -S source_dir — 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

🦋 Lowkey important vibe check: that vfat lowercase situation is real, bestie — X0 becoming x0 can absolutely wreck X.org's whole day. 💀

🎀 AUTHORS 🎀

posixovl and this manpage were written by Jan Engelhardt. 💜

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

💖 That's the posixovl(1) page — source/version: posixovl, date: 2010-12-07. Mwah, bestie! 🫶