Lab 3: OverlayFS — observing copy-up in real time

Objective

Watch OverlayFS copy a file from the image’s lower layer to the container’s upper layer on first write. This is the “copy-up” behaviour that affects write performance and disk usage.

Requirements

• A Linux host.
• Docker installed.
• An image with a known file (e.g. nginx:1.27).

Tasks

Task 1: Start a container

docker run --rm -d --name overlaytest nginx:1.27 sleep 3600
PID=$(docker inspect overlaytest --format '{{.State.Pid}}')
echo "Container PID: $PID"

Task 2: Find the OverlayFS directories

CGROUP=$(cat /proc/$PID/cgroup | awk -F: '{print $3}')
BASE=/sys/fs/cgroup$CGROUP
LOWER=$(cat $BASE/lower)
UPPER=$(cat $BASE/upper)
WORK=$(cat $BASE/work)
echo "Lower: $LOWER"
echo "Upper: $UPPER"
echo "Work: $WORK"

Task 3: Verify the file is in the lower layer

ls -la /var/lib/docker/overlay2/$LOWER/diff/usr/share/nginx/html/index.html

The file exists in the lower layer (image).

Task 4: Modify the file inside the container

docker exec overlaytest sh -c "echo modified > /usr/share/nginx/html/index.html"

Task 5: Check for the whiteout and upper file

ls -la /var/lib/docker/overlay2/$UPPER/diff/usr/share/nginx/html/
md5sum /var/lib/docker/overlay2/$LOWER/diff/usr/share/nginx/html/index.html

The lower layer’s file is unchanged. The upper layer has the modification.