CODEWITHSUNDEEP
docker
user9623401
user9623401

Reputation:

container has its own disk but shared memory?

I am new to docker, just a question on contain basis, below is a picture from a book: enter image description here

It says that containers share the the host computer's CPU, OS and memory but each container has its own computer name, ip address and disk.

I am a little bit confused about the disk, isn't that disk is just like memory as resource? if a container has 1gb data inside, it must get allocated 1gb disk space by the host computer from its own disk just like memory? so container's disk is also shared?

Upvotes: 2

Views: 940

Answers (1)

David Maze
David Maze

Reputation: 159732

You can make that diagram more precise by saying that each container has its own filesystem. /usr in a container is separate from /usr on other containers or on the host, even if they share the same underlying storage.

By way of analogy to ordinary processes, each process has its own address space and processes can't write to each other's memory, even though they share the same underlying memory hardware. The kernel assigns specific blocks (pages) of physical memory to specific process address spaces. If you go out of your way, there are actually a couple of ways to cause blocks of memory to be shared between processes. The same basic properties apply to container filesystems.

On older Docker installations (docker info will say devicemapper) Docker uses a reserved fixed-size disk area. On newer Docker installations (docker info will say overlay2) Docker can use the entire host disk. The Linux kernel is heavily involved in mapping parts of the host disk (or possibly host filesystem) into the per-container filesystem spaces.

Upvotes: 1

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