1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
|
tmpoverlay
==========
tmpoverlay is a small, almost-POSIX shell script to create tmpfs-backed
overlayfs mounts.
Features
--------
- minimal requirements (sh, mount, getopt)
Benefits over manually calling ``mkdir /tmp/x; mount ...``
- separate tmpfs allows size limit (``tmpoverlay -t size=SIZE``)
- upperdir and workdir automatically managed
- tmpfs cleanup after mount so that umount frees RAM
- synchronizes owner, permissions, and xattrs (including ACLs)
- autodetects optimization flags (redirect_dir, metacopy, index, volatile)
Overmounting
------------
Like any other Linux mount, an overlayfs mount only affects new directory
lookups. If a process has its current directory or has files open inside the
mount point, it continues to access the original directory, not the overlaid
one. Each process also has a cached root directory pointer, which can only be
modified by chroot (internally) or pivot_root (globally). The pivot_root(2)_
and pivot_root(8)_ man pages should be fully read and understood before using
tmpoverlay to overmount ``/``.
.. _pivot_root(2): https://man7.org/linux/man-pages/man2/pivot_root.2.html
.. _pivot_root(8): https://man7.org/linux/man-pages/man8/pivot_root.8.html
Changes to underlying filesystems
---------------------------------
Per `the kernel overlayfs documentation`_, changing underlying filesystems
while the overlay is mounted is not supported:
Changes to the underlying filesystems while part of a mounted overlay
filesystem are not allowed. If the underlying filesystem is changed, the
behavior of the overlay is undefined, though it will not result in a crash
or deadlock.
.. _the kernel overlayfs documentation: https://www.kernel.org/doc/html/latest/filesystems/overlayfs.html#changes-to-underlying-filesystems
Pseudo-filesystems
------------------
Pseudo-filesystems like procfs and sysfs are not intended to be used with
overlayfs. Therefore, running commands like ``tmpoverlay /proc`` may have
unexpected results.
POSIX compliance
----------------
With the following exceptions, tmpoverlay is intended to be functional on
POSIX-only shells:
- ``mount -t overlay`` is obviously required
- ``getopt --`` is required for proper handling of options containing spaces
- ``getfattr`` is used for xattr copying but in case of failure, the system is
assumed to not support xattrs and setfattr is skipped.
- ``realpath`` is required for canonicalizing paths if -c is not provided
|
