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Container Service for Kubernetes:CSI overview

Last Updated:Nov 01, 2023

The container storage capability of Container Service for Kubernetes (ACK) clusters is developed based on the Container Storage Interface (CSI) plug-in. This capability is deeply integrated with Elastic Block Storage (EBS), Apsara File Storage NAS (NAS) of Alibaba Cloud, Object Storage Service (OSS), and local disks of Alibaba Cloud, and is completely compatible with Kubernetes-native storage services, such as emptyDir volumes, hostPath volumes, Secrets, and ConfigMaps. This topic describes the overview, features, and limits of the CSI plug-in, and the permissions that are required to use the CSI plug-in.

ACK container storage architecture

ACK allows you to configure storage services to be automatically mounted to pods. The storage services include Alibaba Cloud disks, NAS, OSS, and local volumes. The following table describes the features and use scenarios of the storage services.


CSI supports dynamically provisioned OSS volumes. For more information about how to use dynamically provisioned OSS volumes, see Manage the lifecycle of OSS buckets.

Alibaba Cloud storage service

Statically provisioned volume

Dynamically provisioned volume

Default ACK storage

Key feature


Alibaba Cloud disks




Non-shared storage. A disk can be mounted only to one node.

  • High I/O and low latency

    Disks are block storage devices and are suitable for use in scenarios that require high I/O performance and low latency. For example, databases and middleware services.

  • Non-data sharing

    A disk can be provisioned only for one pod. You can use disk volumes in scenarios that do not require data sharing.

For more information, see Disk volume overview.





Shared storage that provides high performance and high throughput.

  • Data sharing

    NAS file systems allow multiple pods to access the same data. We recommend that you use NAS file systems if data needs to be shared.

  • Big data analysis

    NAS file systems provide high throughput and meet the requirement of shared storage access when large numbers of jobs are involved.

  • Web applications

    NAS file systems can provision storage for web applications and content management systems.

  • Log storage

    We recommend that you use NAS volumes to store log data.

For more information, see NAS volume overview.





Shared storage that supports file systems in user space.

  • Read-only media files such as video files and images

    You can use OSS volumes to read the preceding types of files.

  • Read-only configuration files of websites and applications

    ossfs provides limited network performance and can be used to read small files.


OSS volumes are mounted by using ossfs, which is implemented as a file system in user space (FUSE). The write performance is limited when you use OSS volumes. We recommend that you use other storage volumes in scenarios that require high write performance.

For more information, see OSS volume overview.


You can use the CSI plug-in to mount storage resources as statically provisioned volumes and as dynamically provisioned volumes in ACK clusters. To mount a storage resource as a statically provisioned volume, you must manually create a persistent volume (PV) and a persistent volume claim (PVC). When large numbers of PVs and PVCs are required, you can mount storage resources as dynamically provisioned volumes. Definitions of PV and PVC:

  • PV

    • A PV is a piece of storage in the cluster. The lifecycle of a PV is independent of the lifecycle of the pod that has the PV mounted. Different types of PVs can be provisioned by using different types of StorageClasses.

  • PVC

    • A PVC is a request for storage in the cluster. PVs are node resources consumed by pods. PVCs are claims that consume PVs. When PVs are insufficient, PVCs can dynamically provision PVs.

ACK container storage features

The following table describes the storage features supported by different types of ACK cluster.

Storage type


ACK cluster (Linux)

ACK Serverless cluster

Registered cluster (hybrid cloud or multi-cloud)

ACK Edge

ACK cluster (Windows)

Container Service cluster (Apsara Stack)

ACK cluster with sandboxed containers

Block storage

Mount and unmount disks


Online resizing




Container I/O monitoring


File system

XFS, ext4, and dBFS are supported.

XFS and ext4 are supported.


NTFS is supported.

XFS and ext4 are supported.

XFS and ext4 are supported.

Block devices and bare metal devices


Data restoration from snapshots


Disk queue settings


Customer managed key (CMK)-based encryption and Bring Your Own Key (BYOK) encryption


Multi-zone awareness


Custom labels


Cross-host migration


File storage

Create, mount, and unmount NAS file systems


Mount and unmount Samba file systems


Recycle bin (CNFS)


Subdirectories or shared directories of dynamically provisioned volumes (CNFS)


CMK-based encryption (CNFS and Extreme NAS file systems)


Quota limits (CNFS)

image.pngSupported only by ACK managed clusters


Capacity and I/O monitoring (CNFS)


Online resizing (CNFS)


Object storage

Mount and unmount OSS buckets


BYOK-based encryption


Local storage

Linux Volume Manager (LVM)-managed block storage


Automated volume groups


LVM-managed capacity-aware scheduling


PMEM Direct Mem


LVM-managed persistent memory (PMEM)


CSI deployment architectures

The CSI plug-in consists of two parts: CSI-Plugin and CSI-Provisioner. The following figure shows the deployment architectures of the CSI plug-in in an ACK managed cluster and an ACK dedicated cluster.


The CSI plug-in is automatically installed in ACK managed clusters and ACK dedicated clusters. If you use an ACK Serverless cluster or an ACK Edge cluster, you must manually install the CSI plug-in.

ACK managed clusters

ACK dedicated clusters

csi managed

In ACK managed clusters, CSI-Provisioner and CSI-Plugin are deployed on worker nodes.


In ACK dedicated clusters, CSI-Provisioner is deployed on master nodes. CSI-Plugin is automatically deployed as DaemonSets on master and worker nodes.

Permissions required to use CSI

Before you can use the CSI plug-in to mount, unmount, create, and delete volumes, you must grant the plug-in the permissions to access other cloud resources. You can use an AccessKey pair or a Resource Access Management (RAM) role to grant permissions to the CSI plug-in. The default method is to use a RAM role. The following table describes the two authorization methods.

Use an AccessKey pair

Use a RAM role

  • You can specify an AccessKey pair in the deployment template of the CSI plug-in.

  • You can also create a Secret to pass an AccessKey pair as environment variables.

The CSI plug-in uses the RAM role AliyunCSManagedCsiRole to access resources of other cloud services. For more information the RAM role, see ACK default roles. For more information about how to grant permissions to RAM roles, see Grant permissions to a RAM role.

  • Managed Kubernetes clusters

    The permission token of the RAM role used by the CSI plug-in is stored in a Secret named addon.csi.token. To grant permissions to the CSI plug-in and allow the plug-in to call API operations, you need only to mount the Secret to the plug-in.

  • ACK dedicated clusters

    The CSI plug-in assumes the RAM role assigned to the Elastic Compute Service (ECS) node that hosts the CSI pods.

Limits on CSI

When you use the CSI plug-in in ACK clusters, take note of the limits on the CSI plug-in and Alibaba Cloud storage services.

  • Limits on Alibaba Cloud storage services

    Alibaba Cloud storage service


    Alibaba Cloud disks

    • You can provision a disk as a volume only for one pod.

    • You cannot mount disks of all types to ECS instances of all types. For more information, see Overview of instance families.

    • You cannot mount or unmount subscription disks as volumes.

    • You can mount a disk only to an ECS instance in the same zone as the disk.

    • We recommend that you create StatefulSets instead of Deployments to use disk volumes.


      Deployments are used to create stateless applications. When a pod is restarted, the start time of the new pod may overlap with the end time of the old pod. If multiple pods are created for a Deployment, no dedicated volume is provisioned for each pod.

    • The minimum capacity of a disk volume is 20 GiB.


    • You can mount a NAS volume only to ECS instances in the same virtual private cloud (VPC) as the NAS file system.

    • You cannot use the CSI plug-in to mount Server Message Block (SMB) file systems.

    • The number of NAS file systems that you can create is limited. To request a quota increase, join the DingTalk group 35532895.


    We recommend that you do not perform data write operations on OSS volumes. Use other storage media for data write operations.


    OSS volumes are mounted by using ossfs, which is implemented as a file system in user space (FUSE). The write performance is limited when you use OSS volumes. We recommend that you use other storage volumes in scenarios that require high write performance.

  • Limits on the CSI plug-in

    The CSI plug-in is an open source plug-in for ACK clusters. In other types of clusters, such as clusters deployed in third-party clouds and self-managed clusters on Alibaba Cloud, you cannot directly use the CSI plug-in for reasons such as cluster configurations, permission management, and network differences. If you want to use the CSI plug-in in these types of clusters, you must modify the cluster configurations based on the source code. For more information, see alibaba-cloud-csi-driver.

Requirements on Kubernetes versions

To use the CSI plug-in in an ACK cluster, the Kubernetes version of the cluster must be 1.14 or later. In addition, the --enable-controller-attach-detach parameter must be set to true.

Installation and updates of the CSI plug-in

For more information about how to install and update the CSI plug-in, see Install and upgrade the CSI plug-in.

Differences between the CSI and FlexVolume plug-ins





FlexVolume is a traditional mechanism to extend the storage systems developed by the Kubernetes community. ACK supports FlexVolume. FlexVolume consists of the following parts:

  • FlexVolume: allows you to mount and unmount volumes. By default, ACK allows you to mount the following types of volumes by using csi-plugin: disk volumes, NAS volumes, and OSS volumes.

  • Disk-Controller: automatically creates disk volumes.

  • Nas-Controller: automatically creates NAS volumes.

For more information about FlexVolume, see FlexVolume overview.

For more information about how to update FlexVolume, see Manage components.


The Kubernetes community recommends the CSI plug-in. The CSI plug-in provided by ACK is compatible with the features of the community version. CSI consists of the following two parts:

  • CSI-Plugin: allows you to mount and unmount volumes. By default, ACK allows you to mount the following types of volumes by using csi-plugin: disk volumes, NAS volumes, and OSS volumes.

  • CSI-Provisioner: automatically creates disk volumes and NAS volumes.

For more information about CSI, see CSI overview and alibaba-cloud-csi-driver.

  • You must select a plug-in when you create an ACK cluster.

  • You cannot use CSI and FlexVolume in the same cluster.

  • You cannot change the plug-in from FlexVolume to CSI for a cluster.


  • For newly created ACK clusters, we recommend that you use CSI. The ACK technical team will continuously update CSI to support more features of the CSI community version.

  • For existing clusters, we recommend that you use the plug-in that is already installed. The ACK technical team will continue its support for FlexVolume.

How to check the storage plug-in used in a cluster

You can confirm the volume plug-in used by the cluster by checking node annotations in the ACK console or checking kubelet parameters in the CLI.

Check node annotations in the ACK console

  1. Log on to the ACK console. In the left-side navigation pane, click Clusters.

  2. On the Clusters page, click the name of the cluster that you want to manage and choose Nodes > Nodes in the left-side navigation pane.

  3. Select a node and click More > Details in the Actions column.

    On the Overview tab, check the annotations of the node.

    • If true is displayed, the cluster uses the CSI plug-in.

    • If true is not displayed, the cluster uses the FlexVolume plug-in.

Check kubelet parameters in the CLI

Run the following command to check kubelet parameters:

ps -ef | grep kubelet

Expected output:

  • If the value of --enable-controller-attach-detach is true, the cluster uses the CSI plug-in.

  • If the value of --enable-controller-attach-detach is false, the cluster uses the FlexVolume plug-in.