Mount an emptyDir volume to increase shm size - Elastic Container Instance

ECI pods default to 64 MiB of shared memory, which is insufficient for workloads that use /dev/shm for inter-process communication or large in-memory buffers. Mount a memory-backed emptyDir volume to /dev/shm to increase the available shared memory beyond this limit.

How /dev/shm works in ECI pods

/dev/shm is a tmpfs-backed directory that provides fast, memory-based storage for inter-process communication. Deep learning frameworks, databases, and multi-process workloads use it to share large data buffers between processes.

Kubernetes sets /dev/shm to 64 MiB by default, and this limit cannot be changed through standard Kubernetes configurations. Run df -h inside a pod to confirm the default size:

:/$ df -h
Filesystem      Size  Used Avail Use% Mounted on
overlay          30G  4.1G   26G  14% /
tmpfs            64M     0   64M   0% /dev
tmpfs           2.0G     0  2.0G   0% /sys/fs/cgroup
/dev/vda4        30G  4.1G   26G  14% /etc/hosts
overlay         8.8G  5.0G  3.5G  59% /etc/hostname
shm              64M     0   64M   0% /dev/shm
tmpfs           4.0G   12K  4.0G   1% /run/secrets/kubernetes.io/serviceaccount
tmpfs           2.0G     0  2.0G   0% /proc/acpi
tmpfs           2.0G     0  2.0G   0% /sys/firmware

To raise this limit, mount a memory-backed emptyDir volume at /dev/shm. Setting medium: Memory and a sizeLimit on the volume replaces the default 64 MiB tmpfs with a tmpfs of your chosen size.

Mount an emptyDir volume at /dev/shm

Important

Keep the following in mind before configuring a larger /dev/shm:

Data is not persisted. /dev/shm stores data in memory. All data is lost when the container restarts.
The volume counts against the pod's memory limit. Writing to /dev/shm consumes the same memory pool as the container's processes. If the combined memory usage of the processes and the /dev/shm data exceeds the container's memory limit, an OOM error occurs.

Create a file named emptydir-shm.yaml with the following configuration.

vim emptydir-shm.yaml

The configuration sets medium: Memory and sizeLimit: 256Mi on the emptyDir volume and mounts it at /dev/shm, increasing the pod's shm size to 256 MiB.

apiVersion: apps/v1
kind: Deployment
metadata:
  name: test
  labels:
    app: test
spec:
  replicas: 1
  selector:
    matchLabels:
      app: nginx
  template:
    metadata:
      name: nginx-test
      labels:
        app: nginx
        alibabacloud.com/eci: "true" 
    spec:
      containers:
      - name: nginx
        image: registry.cn-shanghai.aliyuncs.com/eci_open/nginx:1.14.2
        ports:
        - containerPort: 80
        volumeMounts:
        - mountPath: /dev/shm
          name: cache-volume
      volumes:
      - emptyDir:
          medium: Memory
          sizeLimit: 256Mi
        name: cache-volume

Create the Deployment.
```
kubectl create -f emptydir-shm.yaml
```

Verify the result.

Connect to the container and run df -h. The /dev/shm row should show 256 MiB.

shell@Alicloud:~$ kubectl get pod
NAME                        READY   STATUS    RESTARTS   AGE
test-765f78d484-68wg9       1/1     Running   0          44m
shell@Alicloud:~$ kubectl exec -it test-765f78d484-68wg9 -- bash
root@test-765f78d484-68wg9:/# df -h
Filesystem      Size  Used Avail Use% Mounted on
overlay          30G  4.1G   26G  14% /
tmpfs            64M     0   64M   0% /dev
tmpfs           1.9G     0  1.9G   0% /sys/fs/cgroup
tmpfs           256M     0  256M   0% /dev/shm
/dev/vda4        30G  4.1G   26G  14% /etc/hosts
overlay         8.8G  4.9G  3.5G  58% /etc/hostname
tmpfs           4.0G   12K  4.0G   1% /run/secrets/kubernetes.io/serviceaccount
tmpfs           1.9G     0  1.9G   0% /proc/acpi
tmpfs           1.9G     0  1.9G   0% /sys/firmware