What happens when docker fails to share hard disk?

In the process of using Docker, sometimes we need to share Docker images and containers between different machines. A simple way is to package these images and containers into tar files, then transfer them to the target machine through the network, and then decompress and load them into Docker. However, even if we successfully transfer these images and containers to the target machine, we often encounter such a problem: unable to load the image or start the container, prompting "no space left on device", that is, the partition disk space is insufficient, especially when This problem is more likely to be encountered when using storage driver What happens when docker fails to share hard disk?.

What the hell is going on? I once encountered a similar problem. After some investigation and research, I found the reasons and solutions:

1. The working principle of the What happens when docker fails to share hard disk? storage driver

Docker storage Drivers include AUFS, DeviceMapper and OverlayFS, among which What happens when docker fails to share hard disk? is the more popular one. It is based on the overlayFS file system, superimposing multiple mirrored file systems to form a joint mount point, making it appear to be a complete file system, as shown in the following figure:

As shown in the figure, the blue part is the file system of the basic image, the green part is the file system of the container layer, and the red part is the file system of the read-only layer. The read-only layer contains the file system common to all images. The container layer is the file system of each container. It creates a read-only layer for each container and adds a writable layer on top of it so that each container can see It is an independent file system and does not interfere with each other.

2. Cause of insufficient partition space

When we create a Docker container, the What happens when docker fails to share hard disk? storage driver will be created for each container in the /var/lib/docker/What happens when docker fails to share hard disk? directory A separate subdirectory for the container's file system. The file data in these subdirectories are all stored in the base image, so their size does not affect the performance of the storage driver. However, when we package the Docker image and container from one machine and send it to another machine, when the What happens when docker fails to share hard disk? storage driver unpacks the data, it decompresses it to the /var/lib/docker directory, resulting in this directory The file space occupied is too large, and the disk space of the partition where /var/lib/docker is located has also become smaller accordingly. The following figure takes the /var/lib/docker directory as an example to reflect this problem:

In this picture, the /dev/vda1 partition disk size is 50GB and the /var/lib/docker partition disk size is 21GB. However, because docker creates too many containers, /var/ There is insufficient space in the lib/docker directory, only 30.72MB is left. Therefore, when we want to start the container from such a machine with a shortage of disk space, there will be a problem of startup failure.

3. What are the related solutions?
   In response to the above problems, I have provided some solutions:

3.1 Expand the partition disk space

This is the most common method. For virtual machines, we You can expand the disk partition size in the virtual machine management tool and restart the virtual machine. For cloud servers, most cloud platforms provide the function of online disk expansion, but the operation process is relatively complicated and requires caution to avoid data loss.

3.2 Mount the /var/lib/docker directory to a larger data disk

Prepare a disk larger than 21GB (such as 20T) in advance and format it into ext4 format, and mount it on /data directory, and then migrate the /var/lib/docker directory to the data disk for storage. The specific operation instructions are as follows:

# 制作文件系统格式
mkfs.ext4 /dev/vdb

# 挂载
mount /dev/vdb /data

# 备份原/var/lib/docker目录下所有数据
cp -au /var/lib/docker/* /data/

# 卸载/var/lib/docker目录
umount /var/lib/docker

# 将/var/lib/docker目录迁移到新的数据盘中
echo '/dev/vdb /var/lib/docker ext4 defaults 0 0' >> /etc/fstab
mount -a

3.3 Delete Docker images and containers that are no longer used

We can use the following command to clean up disk space:

# 清理所有停止的容器
docker container prune

# 清理所有未被标记的镜像
docker image prune -a

# 删除所有没有容器使用的镜像
docker image prune -a --filter "dangling=true"

Summary
In use When Docker, we need to always pay attention to whether the space used by the storage driver is enough, otherwise it may cause startup failure. To do this, you can take one of the above three solutions. I personally recommend the second method, which can quickly solve the problem without destroying the original file system. I hope my sharing can be helpful to you, thank you for reading.

The above is the detailed content of What happens when docker fails to share hard disk?. For more information, please follow other related articles on the PHP Chinese website!