Questions[overlayfs](ubuntu)

I Cannot overlay mount after creating an mount namespace with unshare

$ unshare -rm
# mount -t overlay overlay -o lowerdir=/bin,upperdir=/overlay/upper/bin,workdir=/overlay/work/bin/ /overlay/merged/bin/
mount: /overlay/merged/bin: permission denied.

Also tried

$ unshare -rmpf --mount-proc
# mount -t overlay overlay -o lowerdir=/bin,upperdir=/overlay/upper/bin,workdir=/overlay/work/bin/ /overlay/merged/bin/
mount: /overlay/merged/bin: permission denied.

Is this a limitation of wsl or because I'm not really root or am I missing something?

bind mount works

# mount --bind /bin /overlay/merged/bin/
# grep bin /proc/self/mountinfo
127 126 0:19 / /proc/sys/fs/binfmt_misc rw,relatime - binfmt_misc binfmt_misc rw
137 106 8:16 /bin /overlay/merged/bin rw,relatime - ext4 /dev/sdb rw,discard,errors=remount-ro,data=ordered

I have write permission on upper and work and all directories are on the same filesystem

find /overlay/{upper,work} -name bin -ls
    38672      4 drwxr-xr-x   2 badger   badger       4096 Nov  3 17:05 /home/badger/overlay/upper/bin
    38631      4 drwxr-xr-x   3 badger   badger       4096 Nov  4 23:08 /home/badger/overlay/work/bin

I have looked at How do I use OverlayFS? but it doesn't answer my question.

I need to install some third party applications and they require /opt to be writable but the device I am running on has /opt on a readable rootfs. Essentially, I have a directory /opt on a read-only filesystem (lower) and let's say I have a read-write directory /mnt/optw (writable). I would like to merge /opt & /mnt/optw and mount it to /opt.

Is this possible at all?

When I run the live image (Ubuntu 18.04), it shows the root filesystem being an overlay with the squashfs image (/cdrom/casper/filesystem.squashfs, containing the main data) as lower layer, and /cow/upper as upper layer. But /cow is not visible anymore as it is hidden by the overlay mounted on root.

Where (on what device) does the upper layer live? Is it accessible somewhere or is it a memory-only device?

I'm running Ubuntu 14.04 with the 3.19 kernel which should have OpenFS supported (since kernel 3.18). I've been reading about OverlayFS but am confused as to how to actually implement using it. It looks like it could be a really useful alternative to LVM in enabling changes to be written somewhere else whilst taking a backup of the original filesystem that is being overlayed?

Can someone give me the example mount commands to use (if possible) for the following cases:

• With a raw disk image (created using dd if=/dev/zero of=$HOME/filestystem1.img bs=4096 count=1024000) to act as the overlaying filesystem, mount it over the top of my host's filesystem at $HOME/mount which should already contain some random files such as file1.txt and file2.txt. Once mounted I believe that I should still be able to see my already existing files and any files I now create will actually be stored inside the filesystem1.img file that I could possibly move to another system?

• Create another raw disk image and mount this one on top of the other one we just created. Do I do this as a second mount command that is executed after mount command we ran in the previous example, or can I possibly specify both mounts in one go?

I can't tell if OverlayFS is an actual filesystem I need to create on the disk images with mkfs -t xxx /path/to/raw/disk/image, or is a special mounting method and the raw disk images are actually using something like ext4, but they are mounted in a special way with mount -t OverlayFS? When I run:

cd /sbin
ls mkfs*

I get the following which do not show an option to make an overlay filesystem.

mkfs      mkfs.cramfs  mkfs.ext3  mkfs.ext4dev  mkfs.minix  mkfs.ntfs
mkfs.bfs  mkfs.ext2    mkfs.ext4  mkfs.fat      mkfs.msdos  mkfs.vfat

This answer and email message indicate that something called "OverlayFS" is available in Ubuntu 11.10 and will forcefully replace aufs in Ubuntu 12.04.

How do I use it? Where is its documentation?