overlayfs mounted - but writing fails as “read-only file system”

Question

I try a simple overlay example by mounting: sudo mount -t overlay -o lowerdir=/tmp/lower,upperdir=/tmp/upper/upper, workdir=/tmp/upper/workdir none /tmp/overlay

(I run the full script from example in : https://blog.programster.org/overlayfs)

The mount is successful:

/tmp/lower-fs.img on /tmp/lower type ext4 (ro)
/tmp/upper-fs.img on /tmp/upper type ext4 (rw)
none on /tmp/overlay type overlay 
(rw,lowerdir=/tmp/lower,upperdir=/tmp/upper/upper,workdir=/tmp/upper/workdir)

Yet, I get error on trying to write into the readonly and overlayed area:

 echo "hello world" >> /tmp/lower/lower-file.txt
 bash: /tmp/lower/lower-file.txt: Read-only file system

I use the following kernel:

$ uname -a
Linux user-VirtualBox 4.4.50-040450-generic #201702181144 SMP Sat Feb 18 
 16:45:38 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux

Thanks for any idea

Answer 1

You need to write to the overlay mount. The command you pasted is trying to write directly to the lower layer, which is indeed mounted read-only...

Use this instead, it should work:

$ echo "hello world" >> /tmp/overlay/lower-file.txt

Note that you'll see the contents of the lower layer in your /tmp/overlay mount, but any modifications you make will go to the upper layer (under the hood), while you'll get a merged view of the layers under your overlay mount point, which is the one you're supposed to access.