O overlayFS pode suportar mais de duas camadas?

Question

Minha pergunta é muito simples. Tanto quanto eu li a documentação do overlayFS suporta duas camadas - superior e inferior. Mas existe possibilidade de aumentá-lo para três camadas?

    

Answer 1

resposta curta: sim

resposta longa: das seções "Upper and Lower" e "Várias camadas inferiores":

"...The lower filesystem can be any filesystem supported by Linux and does not need to be writable. The lower filesystem even be another overlayfs. ..."

    

Answer 2

Exemplo mínimo executável

Testado no Ubuntu 18.04, kernel Linux v4.15:

dd if=/dev/zero of=lower.ext4 bs=1024 count=102400
mkfs -t ext4 lower.ext4
cp lower.ext4 upper1.ext4
cp lower.ext4 upper2.ext4
cp lower.ext4 upper3.ext4
sudo mount lower.ext4 lower
sudo mount upper1.ext4 upper1
sudo mount upper2.ext4 upper2
sudo mount upper3.ext4 upper3
sudo chown "$USER:$USER" \
  lower \
  upper1 \
  upper2 \
  upper3 \
;
mkdir \
  upper1/upper1 \
  upper1/work1 \
  upper2/upper2 \
  upper2/work2 \
  upper3/upper3 \
  upper3/work3 \
;
printf 'lower-content' > lower/lower-file
printf 'upper-content-1' > upper1/upper1/upper-file-1
printf 'upper-content-2' > upper2/upper2/upper-file-2
printf 'upper-content-3' > upper3/upper3/upper-file-3
sudo mount \
  -t overlay \
  -o lowerdir=lower,upperdir=upper1/upper1,workdir=upper1/work1 \
  none \
  overlay1 \
;
sudo mount \
  -t overlay \
  -o lowerdir=overlay1,upperdir=upper2/upper2,workdir=upper2/work2 \
  none \
  overlay2 \
;
if "$layer3"; then
  sudo mount \
    -t overlay \
    -o lowerdir=overlay2,upperdir=upper3/upper3,workdir=upper3/work3 \
    none \
    overlay3 \
  ;
fi
printf 'overlay-content' > overlay1/overlay-file-1
printf 'overlay-content' > overlay2/overlay-file-2
printf 'overlay-content' > overlay3/overlay-file-3
ls \
  lower \
  upper1/upper1 \
  upper1/work1 \
  upper2/upper2 \
  upper2/work2 \
  overlay1 \
  overlay2 \
  overlay3 \
;
if "$layer3"; then
  sudo umount overlay3
fi
sudo umount overlay2 overlay1
ls \
  lower \
  upper1/upper1 \
  upper1/work1 \
  upper2/upper2 \
  upper2/work2 \
  upper3/upper3 \
  upper3/work3 \
;
sudo umount upper3 upper2 upper1 lower

GitHub upstream .

Saída do primeiro ls com o mount:

lower1:
lost+found  lower-file-1

lower2:
lost+found  lower-file-2

lower3:
lost+found  lower-file-3

overlay:
lost+found  lower-file-1  lower-file-2  lower-file-3  upper-file

upper/upper:
upper-file

upper/work:
work

Saída do segundo ls sem o mount:

lower1:
lost+found  lower-file-1

lower2:
lost+found  lower-file-2

lower3:
lost+found  lower-file-3

upper/upper:
upper-file

upper/work:
work

Se você tentar encadear 3 uppers em vez de como neste exemplo , então mount falha com:

wrong fs type, bad option, bad superblock on none, missing codepage or helper program, or other error. 

e dmesg contém:

overlayfs: maximum fs stacking depth exceeded

Testado no Ubuntu 18.04, kernel do Linux 4.15.0.