How to Use Multiple Lower Layers in Overlayfs

How to use Overlayfs in Linux 3.19 to mount multiple lowerdir

The syntax for specifying multiple lower directories is correct and works, as described in https://www.kernel.org/doc/Documentation/filesystems/overlayfs.txt. But apparently the commit that supports this feature went in only for 4.0 and not 3.19.

ravi@tuxpad linux$ git describe --contains  a78d9f0d5d5ca9054703376c7c23c901807ddd87
v4.0-rc1~2^2~17^2~8

How to use Overlayfs in Linux 3.19 to mount multiple lowerdir

The syntax for specifying multiple lower directories is correct and works, as described in https://www.kernel.org/doc/Documentation/filesystems/overlayfs.txt. But apparently the commit that supports this feature went in only for 4.0 and not 3.19.

ravi@tuxpad linux$ git describe --contains  a78d9f0d5d5ca9054703376c7c23c901807ddd87
v4.0-rc1~2^2~17^2~8

How provide nested mount of overlayfs

If you check dmesg output, you will see the kernel stating an attempt to exceed the max stacking depth:

overlayfs: maximum fs stacking depth exceeded

Indeed, the stacking depth is limited to 2, see the overlayfs dev branch:

• #define FILESYSTEM_MAX_STACK_DEPTH
• stacking depth check

If you want to go beyond this depth, you could try changing the constant and building your own kernel. However, bear in mind the risk of overflowing the stack (per the comments).

Overlayfs: restore removed or changed file from lower

Remove file from /overlay/upper, and then remount rootfs.

rm /overlay/upper/my/file
mount -o remount /

overlayfs inside docker container

Found something that worked! Mounting the workdir and upperdir as tmpfs does the trick for me.
Like so:

> mkdir /tmp/overlay
> mkdir /tmp/{low,merged}
> mount -t tmpfs tmpfs /tmp/overlay
> mkdir /tmp/overlay/{up,work}
> mount -t overlay overlay -o lowerdir=/tmp/low/,upperdir=/tmp/overlay/up/,workdir=/tmp/overlay/work/ /tmp/merged/ 

I'd still be interested in an explanation why creating an overlay w/o tmpfs fails within a docker container?

How can I overlayfs the root filesystem on linux?

According to what I've read I don't think it's possible to programmatically do this once the system is started.

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