Per-Container Storage and Copy-on-Write Benefits

3ed3c1a10430 acme/my-final-image:1.0 my_container_3 0B (virtual 7.75MB) a5ff32e2b551 acme/my-final-image:1.0 my_container_2 0B (virtual 7.75MB) 40ebdd763416 acme/my-final-image:1.0 my_container_1 0B (virtual 7.75MB) ``` The output above shows that all containers share the image's read-only layers (7.75MB), but no data was written to the container's filesystem, so no additional storage is used for the containers. {{< accordion title="Advanced: metadata and logs storage used for containers" >}} > [!NOTE] > > This step requires a Linux machine, and doesn't work on Docker Desktop, as > it requires access to the Docker Daemon's file storage. While the output of `docker ps` provides you information about disk space consumed by a container's writable layer, it doesn't include information about metadata and log-files stored for each container. More details can be obtained by exploring the Docker Daemon's storage location (`/var/lib/docker` by default). ```console $ sudo du -sh /var/lib/docker/containers/* 36K /var/lib/docker/containers/3ed3c1a10430e09f253704116965b01ca920202d52f3bf381fbb833b8ae356bc 36K /var/lib/docker/containers/40ebdd7634162eb42bdb1ba76a395095527e9c0aa40348e6c325bd0aa289423c 36K /var/lib/docker/containers/939b3bf9e7ece24bcffec57d974c939da2bdcc6a5077b5459c897c1e2fa37a39 36K /var/lib/docker/containers/a5ff32e2b551168b9498870faf16c9cd0af820edf8a5c157f7b80da59d01a107 36K /var/lib/docker/containers/cddae31c314fbab3f7eabeb9b26733838187abc9a2ed53f97bd5b04cd7984a5a ``` Each of these containers only takes up 36k of space on the filesystem. {{< /accordion >}} 3. Per-container storage To demonstrate this, run the following command to write the word 'hello' to a file on the container's writable layer in containers `my_container_1`, `my_container_2`, and `my_container_3`: ```console $ for i in {1..3}; do docker exec my_container_$i sh -c 'printf hello > /out.txt'; done ``` Running the `docker ps` command again afterward shows that those containers now consume 5 bytes each. This data is unique to each container, and not shared. The read-only layers of the containers aren't affected, and are still shared by all containers. ```console $ docker ps --size --format "table {{.ID}}\t{{.Image}}\t{{.Names}}\t{{.Size}}" CONTAINER ID IMAGE NAMES SIZE cddae31c314f acme/my-final-image:1.0 my_container_5 0B (virtual 7.75MB) 939b3bf9e7ec acme/my-final-image:1.0 my_container_4 0B (virtual 7.75MB) 3ed3c1a10430 acme/my-final-image:1.0 my_container_3 5B (virtual 7.75MB) a5ff32e2b551 acme/my-final-image:1.0 my_container_2 5B (virtual 7.75MB) 40ebdd763416 acme/my-final-image:1.0 my_container_1 5B (virtual 7.75MB) ``` The previous examples illustrate how copy-on-write filesystems help make containers efficient. Not only does copy-on-write save space, but it also reduces container start-up time. When you create a container (or multiple containers from the same image), Docker only needs to create the thin writable container layer. If Docker had to make an entire copy of the underlying image stack each time it created a new container, container creation times and disk space used would be significantly increased. This would be similar to the way that virtual machines work, with one or more virtual disks per virtual machine. The [`vfs` storage](vfs-driver.md) doesn't provide a CoW filesystem or other optimizations. When using this storage driver, a full copy of the image's data is created for each container. ## Related information * [Volumes](../volumes.md) * [Select a storage driver](select-storage-driver.md)

This section explains how Docker containers utilize copy-on-write filesystems for efficiency. It demonstrates that initially, containers share the base image layers without using additional storage. Writing data to a container's writable layer results in the container consuming additional space only for the modified data, with the read-only layers remaining shared. This copy-on-write mechanism saves space and reduces container start-up time, as Docker only needs to create a thin writable layer instead of making a full copy of the image. In contrast, storage drivers like `vfs` do not offer copy-on-write, leading to increased disk usage and longer container creation times.