Squashing layers in Docker - Step-by-Step Execution

Process Flow - Squashing layers

Start Dockerfile Build

↓

Create Layer 1

↓

Create Layer 2

↓

Create Layer 3

↓

Squash Layers

↓

Single Layer Image

↓

Finish Build

Docker builds images in layers for each command, squashing combines multiple layers into one to reduce image size.

Execution Sample

Docker

FROM alpine
RUN apk add --no-cache curl
RUN echo 'Hello World' > /hello.txt

Builds an image with Alpine, installs curl, and adds a hello.txt file in separate layers.

Process Table

Step	Dockerfile Command	Layer Created	Layer Size	Image Size After Step
1	FROM alpine	Base layer	5MB	5MB
2	RUN apk add --no-cache curl	Layer 2	2MB	7MB
3	RUN echo 'Hello World' > /hello.txt	Layer 3	1KB	7.001MB
4	Squash layers 2 and 3	Single combined layer	2.001MB	7.001MB

💡 Layers 2 and 3 are combined into one to reduce image size and number of layers.

Status Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Squash
Image Size	0MB	5MB	7MB	7.001MB	7.001MB
Number of Layers	0	1	2	3	2

Key Moments - 3 Insights

Why does the image size not reduce after adding the last small file layer before squashing?

Because each RUN command creates a new layer that adds to the image size cumulatively, as shown in step 3 of the execution_table.

What exactly does squashing do to the layers?

Squashing merges multiple layers into one, reducing the number of layers and sometimes the total image size, as seen in step 4 of the execution_table.

Does squashing remove the base layer?

No, the base layer remains unchanged; squashing only combines the subsequent layers, as shown in the concept_flow and execution_table.

Visual Quiz - 3 Questions

Look at the execution_table, what is the image size after step 2?

A 7MB

B 5MB

C 2MB

D 7.001MB

💡 Hint

Check the 'Image Size After Step' column for step 2 in the execution_table.

At which step does the number of layers reduce due to squashing?

A Step 3

B Step 1

C Step 4

D Step 2

💡 Hint

Look at the 'Number of Layers' row in variable_tracker after each step.

If the last RUN command added a 5MB file instead of 1KB, how would the image size after step 3 change?

A It would stay 7.001MB

B It would be about 12MB

C It would be 5MB

D It would be 2MB

💡 Hint

Add the new file size to the image size after step 2 from variable_tracker.

Concept Snapshot

Docker images build in layers, each RUN or command adds a layer.
Squashing merges multiple layers into one to reduce image size and layer count.
Base layer stays intact; only subsequent layers are squashed.
Use squashing to optimize image size and speed up deployment.

Full Transcript

Docker builds images by creating layers for each command in the Dockerfile. Each RUN command adds a new layer, increasing the image size. Squashing layers means combining multiple layers into a single one to reduce the total image size and number of layers. This helps make images smaller and faster to deploy. The base layer from the FROM command remains unchanged. In the example, installing curl and adding a file create separate layers, which are then squashed into one combined layer. This process reduces overhead and optimizes the image.