Container resource usage stats in Docker - Step-by-Step Execution

Process Flow - Container resource usage stats

Start docker stats command

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Docker daemon collects stats

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Retrieve CPU, Memory, Network, I/O data

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Display stats in real-time

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User views resource usage

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Stop command to end stats

The docker stats command starts, the daemon collects container resource data, then displays it live until stopped.

Execution Sample

Docker

docker stats --no-stream
CONTAINER ID   NAME       CPU %   MEM USAGE / LIMIT   MEM %   NET I/O     BLOCK I/O   PIDS
abc123def456   my_app     2.34%   50MiB / 1GiB       4.88%   1.2MB / 0.8MB  10MB / 5MB  12

This command shows a snapshot of resource usage for running containers without continuous updates.

Process Table

Step	Action	Data Collected	Output Displayed
1	Run 'docker stats --no-stream'	Start stats collection	Header row with column names
2	Docker daemon collects CPU usage	CPU % for each container	CPU % column shows e.g. 2.34%
3	Docker daemon collects Memory usage	Memory used and limit	MEM USAGE / LIMIT and MEM % columns filled
4	Docker daemon collects Network I/O	Bytes sent and received	NET I/O column shows e.g. 1.2MB / 0.8MB
5	Docker daemon collects Block I/O	Disk read/write bytes	BLOCK I/O column shows e.g. 10MB / 5MB
6	Docker daemon collects PIDs	Number of processes	PIDS column shows e.g. 12
7	Display all collected data in table	All stats combined	Full stats row for container displayed
8	Command ends (no streaming)	Stop collecting	Stats output ends after one snapshot

💡 Command ends after one snapshot because of --no-stream option

Status Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	After Step 5	After Step 6	Final
CPU %	N/A	2.34%	2.34%	2.34%	2.34%	2.34%	2.34%
Memory Usage	N/A	N/A	50MiB / 1GiB	50MiB / 1GiB	50MiB / 1GiB	50MiB / 1GiB	50MiB / 1GiB
Memory %	N/A	N/A	4.88%	4.88%	4.88%	4.88%	4.88%
Network I/O	N/A	N/A	N/A	1.2MB / 0.8MB	1.2MB / 0.8MB	1.2MB / 0.8MB	1.2MB / 0.8MB
Block I/O	N/A	N/A	N/A	N/A	10MB / 5MB	10MB / 5MB	10MB / 5MB
PIDs	N/A	N/A	N/A	N/A	N/A	12	12

Key Moments - 3 Insights

Why does the 'docker stats' command keep updating the output by default?

By default, 'docker stats' streams live data continuously. In the execution_table, step 8 shows the command ends only because of the '--no-stream' option.

What does the 'MEM USAGE / LIMIT' column represent?

It shows how much memory the container uses versus its maximum allowed memory. Step 3 in the execution_table shows this data is collected and displayed.

Why might CPU % be low even if the container is busy?

CPU % is a snapshot of usage at that moment. If the container is busy intermittently, the snapshot might catch it at a low usage time, as shown in step 2.

Visual Quiz - 3 Questions

Look at the execution_table at step 4, what resource data is collected?

A Memory usage

B Network I/O

C CPU usage

D Block I/O

💡 Hint

Check the 'Data Collected' column at step 4 in execution_table

At which step does the command stop collecting stats due to the '--no-stream' option?

A Step 6

B Step 7

C Step 8

D Step 5

💡 Hint

Look at the 'exit_note' and step 8 in execution_table

According to variable_tracker, what is the value of 'PIDs' after step 6?

A N/A

B 12

C 0

D Unknown

💡 Hint

Check the 'PIDs' row in variable_tracker under 'After Step 6'

Concept Snapshot

docker stats command shows live resource usage of containers
By default, it streams continuously; '--no-stream' shows one snapshot
Displays CPU %, memory usage and limit, network I/O, block I/O, and PIDs
Useful for monitoring container health and performance
Stops when user interrupts or with '--no-stream' option

Full Transcript

The docker stats command starts by sending a request to the Docker daemon to collect resource usage data for running containers. The daemon gathers CPU usage, memory usage and limits, network input/output, block device input/output, and the number of processes (PIDs) for each container. This data is displayed in a table format with columns for each metric. By default, docker stats streams this data live, updating continuously until the user stops it. Using the '--no-stream' option makes the command show only one snapshot and then exit. This helps users monitor container resource consumption in real time or as a single report.

Practice

1. What does the docker stats command show you?

easy

A. Docker network settings

B. Real-time resource usage like CPU and memory of containers

C. Logs of a running container

D. List of all Docker images on the system

Solution

1. Step 1: Understand the purpose of docker stats

   This command is designed to show resource usage statistics of running containers.

2. Step 2: Identify what resource stats include

   It shows CPU, memory, network IO, and block IO usage in real time.

3. Final Answer:

   Real-time resource usage like CPU and memory of containers -> Option B

4. Quick Check:

   docker stats = resource usage [OK]

Hint: Remember: stats = resource usage snapshot [OK]

Common Mistakes:

• Confusing stats with logs
• Thinking it lists images
• Assuming it shows network config

2. Which of these is the correct syntax to get a one-time snapshot of container stats for container named webapp?

easy

A. docker stats --no-stream webapp

B. docker stats webapp

C. docker stats --once webapp

D. docker stats webapp --snapshot

Solution

1. Step 1: Recall the correct option for one-time snapshot

   The flag --no-stream disables continuous updates and shows a single snapshot.

2. Step 2: Check correct command order

   The flag --no-stream should come before the container name webapp.

3. Final Answer:

   docker stats --no-stream webapp -> Option A

4. Quick Check:

   One-time stats = --no-stream before container [OK]

Hint: Use --no-stream before container name for snapshot [OK]

Common Mistakes:

• Using wrong flag like --once or --snapshot
• Omitting the --no-stream flag
• Omitting container name

3. What output will you get when running docker stats --no-stream with two running containers named db and cache?

medium

A. A single snapshot table showing CPU, memory, and IO stats for db and cache

B. An error saying container names are missing

C. A live updating table of resource usage for db and cache containers

D. Only stats for the first container found

Solution

1. Step 1: Understand the effect of --no-stream flag

   This flag disables live updates and shows only one snapshot of stats.

2. Step 2: Check default behavior without container names

   Without specifying containers, stats shows all running containers, here db and cache.

3. Final Answer:

   A single snapshot table showing CPU, memory, and IO stats for db and cache -> Option A

4. Quick Check:

   --no-stream + multiple containers = single snapshot for all [OK]

Hint: No container names + --no-stream = snapshot for all running [OK]

Common Mistakes:

• Expecting live updates with --no-stream
• Thinking container names are required
• Assuming only one container stats shown

4. You run docker stats --no-stream mycontainer but get an error: no such container: mycontainer. What is the likely cause?

medium

A. Docker daemon is not running

B. The --no-stream flag is not supported

C. The container name is misspelled or does not exist

D. You need to add --all flag to see stopped containers

Solution

1. Step 1: Analyze the error message

   The error says container not found, so the name is likely wrong or container is not running.

2. Step 2: Check other options

   --no-stream is valid, Docker daemon error would be different, and --all is not for stats command.

3. Final Answer:

   The container name is misspelled or does not exist -> Option C

4. Quick Check:

   Container not found error = wrong name [OK]

Hint: Check container name spelling if 'no such container' error [OK]

Common Mistakes:

• Assuming --no-stream causes error
• Ignoring Docker daemon status
• Thinking --all shows stats for stopped containers

5. You want to monitor CPU and memory usage of a container named api-server but only want a single snapshot, not continuous updates. Which command should you use and why?

hard

A. docker stats api-server --interval=1 to update every second

B. docker stats api-server because it shows live updates by default

C. docker stats --all api-server to include stopped containers

D. docker stats --no-stream api-server because it shows one-time resource usage snapshot

Solution

1. Step 1: Identify the need for a single snapshot

   Using --no-stream disables continuous updates and shows one snapshot.

2. Step 2: Confirm container name usage

   Specify the container name api-server to get stats only for it.

3. Step 3: Evaluate other options

   Default docker stats streams live updates, --all is invalid here, and --interval is not a valid flag.

4. Final Answer:

   docker stats --no-stream api-server because it shows one-time resource usage snapshot -> Option D

5. Quick Check:

   Single snapshot = --no-stream with container name [OK]

Hint: Use --no-stream with container name for single snapshot [OK]

Common Mistakes:

• Omitting --no-stream for single snapshot
• Using invalid flags like --all or --interval
• Expecting live updates when not wanted