Resizing /dev/shm

Scenario

/dev/shm is a temporary file system (tmpfs), which is a memory-based file system implemented in Linux or Unix and has high read/write efficiency.

If you use /dev/shm for data interaction between processes or for temporary data storage, the default size of /dev/shm (64 MB) in CCI cannot meet your requirements. CCI allows you to modify the size.

This practice shows how to resize /dev/shm by setting memory-backed emptyDir or running securityContext and mount commands.

Constraints

/dev/shm uses a memory-based tmpfs to temporarily store data. Data is not retained after the container is restarted.
You can use either of the following methods to modify the size of /dev/shm. However, do not use both methods in one pod.
The emptyDir uses the memory requested by the pod and does not occupy extra resources.
Writing data to /dev/shm is to request memory. In this scenario, you need to evaluate the memory usage of processes. When the sum of the memory requested by processes in the container plus the data volume in the emptyDir exceeds the memory limit of the container, memory overflow occurs.
When resizing /dev/shm, set the size to 50% of the pod's memory request.

Resizing /dev/shm Using Memory-backed emptyDir

emptyDir is applicable to temporary data storage, disaster recovery, and runtime data sharing. It will be deleted upon deletion or transfer of workload pods.

CCI supports the mounting of memory-backed emptyDir. You can specify the memory size allocated to the emptyDir and mount it to the /dev/shm directory in the container to resize /dev/shm.

apiVersion: v1
kind: Pod
metadata:  
 name: pod-emptydir-name
spec:  
 containers:   
  - image: 'library/ubuntu:latest'     
    volumeMounts:      
     - name: volume-emptydir1    
       mountPath: /dev/shm   
    name: container-0   
    resources:      
     limits:        
      cpu: '4'      
      memory: 8Gi  
     requests:   
      cpu: '4'    
      memory: 8Gi 
 volumes:   
  - emptyDir:     
     medium: Memory   
     sizeLimit: 4Gi  
    name: volume-emptydir1

After the pod is started, run the df -h command to go to the /dev/shm directory. If the following information is displayed, the size is successfully modified.

Figure 1 /dev/shm directory details
Click to enlarge

Resizing /dev/shm by Running securityContext and mount Commands

Grant the SYS_ADMIN permission to the container.

Linux provides the SYS_ADMIN permission. To apply this permission to the container, Kubernetes needs to add this information to pods by adding the description of the securityContext field to the pod's description file. For example:

 "securityContext": { 
                    "capabilities": {
                         "add": [
                             "SYS_ADMIN"
                                ]
                                     } 
                    }

Another description field CapAdd also needs to be added to the container description.

  "CapAdd": [
                 "SYS_ADMIN"
            ],

In this case, a parameter is added when the container is automatically started by kubelet.

docker run --cap-add=SYS_ADMIN

Insert the mount command in the startup command to resize /dev/shm.

apiVersion: v1
kind: Pod
metadata:
  name: pod-emptydir-name
spec:
  containers:
    - command:
        - /bin/sh
        - '-c'
        - mount -o size=4096M -o remount /dev/shm;bash
      securityContext:
        capabilities:
          add: ["SYS_ADMIN"]
      image: 'library/ubuntu:latest'
      name: container-0
      resources:
        limits:
          cpu: '4'
          memory: 8Gi
        requests:
          cpu: '4'
          memory: 8Gi

After the pod is started, run the df -h command to go to the /dev/shm directory. If the following information is displayed, the size is successfully modified.

Figure 2 /dev/shm directory details
Click to enlarge