On Fri, Jun 29, 2018 at 05:59:00PM +0300, Gena Makhomed wrote:

> > Т.е. директории должны быть как бы зеркалами друг друга.
> >
> > Это возможно сделать?
>
> Теоретически - наверное возможно, если написать свой модуль ядра,
> который будет реализовывать эту логику на уровне файловой системы.
>

кажется в лялихе тоже такое было.

MOUNT_UNIONFS(8) FreeBSD System Manager's Manual MOUNT_UNIONFS(8)

NAME
mount_unionfs — mount union file systems

SYNOPSIS
mount_unionfs [-b] [-o options] directory uniondir

DESCRIPTION
The mount_unionfs utility attaches directory above uniondir in such a way
that the contents of both directory trees remain visible. By default,
directory becomes the upper layer and uniondir becomes the lower layer.

The options are as follows:

-b Deprecated. Use -o below instead.

-o Options are specified with the -o flag followed by an option.
The following options are available:

below Inverts the default position, so that directory becomes
the lower layer and uniondir becomes the upper layer.
However, uniondir remains the mount point.

copymode = traditional | transparent | masquerade
Specifies the way to create a file or a directory in the
upper layer automatically when needed. The traditional
mode uses the same way as the old unionfs for backward
compatibility, and transparent duplicates the file and
directory mode bits and the ownership in the lower layer
to the created file in the upper layer. For behavior of
the masquerade mode, see MASQUERADE MODE below.

whiteout = always | whenneeded
Specifies whether whiteouts should always be made in the
upper layer when removing a file or directory or only
when it already exists in the lower layer.

udir=mode
Specifies directory mode bits in octal for masquerade
mode.

ufile=mode
Specifies file mode bits in octal for masquerade mode.

gid=gid
Specifies group for masquerade mode.

uid=uid
Specifies user for masquerade mode.

To enforce file system security, the user mounting a file system must be
superuser or else have write permission on the mounted-on directory. In
addition, the vfs.usermount sysctl(8) variable must be set to 1 to permit
file system mounting by ordinary users. However, note that transparent
and masquerade modes require vfs.usermount to be set to 0 because this
functionality can only be used by superusers.

Filenames are looked up in the upper layer and then in the lower layer.
If a directory is found in the lower layer, and there is no entry in the
upper layer, then a shadow directory will be created in the upper layer.
The ownership and the mode bits are set depending on the copymode option.
In traditional mode, it will be owned by the user who originally did the
union mount, with mode 0777 (“rwxrwxrwx”) modified by the umask in effect
at that time.

If a file exists in the upper layer then there is no way to access a file
with the same name in the lower layer. If necessary, a combination of
loopback and union mounts can be made which will still allow the lower
files to be accessed by a different pathname.

Except in the case of a directory, access to an object is granted via the
normal file system access checks. For directories, the current user must
have access to both the upper and lower directories (should they both
exist).

Requests to create or modify objects in uniondir are passed to the upper
layer with the exception of a few special cases. An attempt to open for
writing a file which exists in the lower layer causes a copy of the
entire file to be made to the upper layer, and then for the upper layer
copy to be opened. Similarly, an attempt to truncate a lower layer file
to zero length causes an empty file to be created in the upper layer.
Any other operation which would ultimately require modification to the
lower layer fails with EROFS.

The union file system manipulates the namespace, rather than individual
file systems. The union operation applies recursively down the directory
tree now rooted at uniondir. Thus any file systems which are mounted
under uniondir will take part in the union operation. This differs from
the union option to mount(8) which only applies the union operation to
the mount point itself, and then only for lookups.

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