You can create volumes to mount Alibaba Cloud disks to a Container Service for Kubernetes (ACK) cluster. You can use the Contain Storage Interface (CSI) plug-in provided by Alibaba Cloud to mount disks by creating persistent volumes (PVs) and persistent volume claims (PVCs). A PV can be statically or dynamically provisioned. This topic describes the features, disk specifications, use scenarios, limits, and billing rules of disk volumes.

Overview

Alibaba Cloud disks are block-level data storage resources for Elastic Compute Service (ECS). Alibaba Cloud disks provide low latency, high performance, high durability, and high reliability. Alibaba Cloud disks use a distributed triplicate mechanism to ensure data reliability for ECS instances. If service disruptions occur within a zone due to hardware errors, data in the zone is automatically replicated to an unaffected disk in another zone to ensure data availability.

  • Enhanced SSDs (ESSDs): ESSDs are based on the next-generation distributed block storage architecture and use the 25 Gigabit Ethernet and remote direct memory access (RDMA) technologies. ESSDs provide low-latency input and output. Each ESSD can provide up to 1,000,000 random read/write IOPS. For more information, see ESSDs.

    We recommend that you use ESSDs for scenarios such as online transactional processing (OLTP) databases, NoSQL databases, and Elasticsearch, Logstash, and Kibana (ELK) distributed logs.

  • Standard SSDs: Standard SSDs are high-performance disks that provide consistent high random IOPS and high data reliability.

    We recommend that you use standard SSDs for scenarios such as I/O-intensive applications, small and medium-sized relational databases, and NoSQL databases.

  • Ultra disks: Ultra disks are cost-effective and provide medium random IOPS and high data reliability.

    We recommend that you use ultra disks as system disks for scenarios such as development and testing.

  • Basic disks: Basic disks are the previous generation of disks and are unavailable for purchase.

Disk specifications

Category ESSD Standard SSD Ultra disk Basic disk
PL3 PL2 PL1 PL0
Capacity range per disk (GiB) 1261~32768 461~32768 20~32768 40~32768 20~32768 20~32768 5~2000
Maximum IOPS 1000000 100000 50000 10000 25000 5000 Several hundreds
Maximum throughput (MB/s) 4000 750 350 180 300 140 30~40
Formula for calculating the IOPS per disk min{1800 + 50 × Capacity, 1000000} min{1800 + 50 × Capacity, 100000} min{1800 + 50 × Capacity, 50000} min{1800 + 12 × Capacity, 10000} min{1800 + 30 × Capacity, 25000} min{1800 + 8 × Capacity, 5000} N/A
Formula for calculating the throughput per disk (MB/s) min{120 + 0.5 × Capacity, 4000} min{120 + 0.5 × Capacity, 750} min{120 + 0.5 × Capacity, 350} min{100 + 0.25 × Capacity, 180} min{120 + 0.5 × Capacity, 300} min{100 + 0.15 × Capacity, 140} N/A
Average single-channel random write latency in milliseconds (block size = 4 K) 0.2 0.3~0.5 0.5~2 1~3 5~10
API parameter value cloud_essd cloud_ssd cloud_efficiency cloud

For more information about disk performance, see EBS performance.

Scenarios

The following table describes the operations that you can perform on disks to meet your business requirements.

Scenario References
Store application data
Resize a system disk or a data disk

For more information, see Overview.

Back up disk data For more information, see Use volume snapshots created from disks.
Encrypt disk data For more information, see Encryption overview.

For more information, see Encrypt disk volumes.

Precautions

  • We recommend that you mount a disk by using a StatefulSet. If you use a Deployment to mount a disk, you must set the number of replicated pods to 1. If the number of replicated pods is not set to 1, multiple nodes may use the disk at the same time when replicated pods are scheduled to different nodes. In this case, you cannot prioritize the node where you want to mount or unmount a disk. When Deployment pods are restarted, disk mounting may fail due to the upgrade policy. Therefore, we recommend that you do not use a Deployment to mount a disk.
  • The type of disk must match the ECS instance types that are used in your cluster before you can mount a disk. For more information about the matching rules between disk types and ECS instance types, see Instance family.
  • You can mount at most 16 disks to each node. The maximum capacity of each disk is 32 TiB.
  • When you mount a disk volume to an application, you must specify the type description in the securityContext.fsgroup parameter. Apsara File Storage NAS (NAS) and Object Storage Service (OSS) are supported. If the securityContext.fsgroup parameter is set in the application template, kubelet performs the chmod or chown operation after the volume is mounted. This increases the mounting time. For more information about how to reduce the mounting time, see Why does it require a long time to mount a NAS volume? and Why does it require a long time to mount an OSS volume?.

Billing

  • Only pay-as-you-go disks can be mounted. If you change the billing method of an ECS instance in the cluster from pay-as-you-go to subscription, you cannot change the billing method of its disks to subscription. Otherwise, the disks cannot be mounted to the cluster.
  • For more information, visit the ECS product page.
For more information, see Billing methods.

StorageClasses

If you create a large number of PVCs, you must also create a large number of PVs. This increases the workloads of O&M engineers. To solve this issue, Kubernetes provides the StorageClass object that you can use to enable automatic PV creation.

StorageClass

ACK clusters support the following types of StorageClass:
  • alicloud-disk-efficiency: ultra disk.
  • alicloud-disk-ssd: standard SSD.
  • alicloud-disk-essd: ESSD.
  • alicloud-disk-available: a high-availability mode. In this mode, the system first attempts to create a standard SSD. If SSD resources are exhausted, the system attempts to create an ultra disk.
    Notice
    • For alicloud-csi-provisioner versions earlier than 1.14.8.39-0d749258-aliyun, the system first attempts to create an ESSD.
    • If ESSDs are out of stock, the system attempts to create a standard SSD.
    • If standard SSDs are out of stock, the system attempts to create an ultra disk.
  • alicloud-disk-topology: creates a disk in WaitForFirstConsumer mode.

The first four types of StorageClass are suitable for clusters that are created in single zones. The last type of StorageClass is suitable for clusters that are created across zones.

When you configure a StorageClass, the following rules determine the zone where the disk is created:

  • If you set volumeBindingMode: WaitForFirstConsumer in the StorageClass configuration, a disk is created in the zone where the pod is deployed.
  • If you set volumeBindingMode: Immediate and specify only one zone in the zoneId parameter of the StorageClass configuration, a disk is created in the specified zone.
  • If you set volumeBindingMode: Immediate and specify multiple zones in the zoneId parameter of the StorageClass configuration, the system attempts to create a disk in the specified zones in a round robin manner.
  • If you set volumeBindingMode: Immediate and do not specify the zoneId parameter in the StorageClass configuration, a disk is created in the zone where the csi-provisioner is deployed.

If your cluster is deployed across zones, we recommend that you specify volumeBindingMode: WaitForFirstConsumer in the StorageClass configurations. You can create a StorageClass based on the type of disk that is required.

Default StorageClass

Kubernetes provides the default StorageClass feature. If a PVC does not specify a StorageClass, the default StorageClass is used to provision a PV for the PVC. For more information, see Default StorageClass.

Note
  • The default StorageClass takes effect on all PVCs. Proceed with caution if your cluster uses PVCs of different storage types. For example, the default StorageClass may create a disk PV for a PVC that defines a NAS file system. Therefore, ACK clusters do not provide default StorageClasses. If you want to configure a default StorageClass, perform the following steps.
  • You can configure only one default StorageClass for each cluster. If you configure more than one default StorageClass for a cluster, all default StorageClasses become invalid.
  1. Configure the default StorageClass.
    Run the following command to set alicloud-disk-ssd as the default StorageClass:
    kubectl patch storageclass alicloud-disk-ssd -p '{"metadata": {"annotations":{"storageclass.kubernetes.io/is-default-class":"true"}}}'
    After the default StorageClass is configured, alicloud-disk-ssd is marked as (default)
    kubectl get sc

    Expected output:

    NAME                          PROVISIONER                       AGE
    alicloud-disk-ssd (default)   diskplugin.csi.alibabacloud.com   96m
  2. Use the default StroageClass.
    1. Use the following template to create a PVC without specifying a StorageClass:
      apiVersion: v1
      kind: PersistentVolumeClaim
      metadata:
        name: disk-pvc
      spec:
        accessModes:
        - ReadWriteOnce
        resources:
          requests:
            storage: 20Gi
      The cluster automatically creates a disk PV based on the default StorageClass alicloud-disk-ssd.
      kubectl get pvc

      Expected output:

      NAME       STATUS   VOLUME                   CAPACITY   ACCESS MODES   STORAGECLASS        AGE
      disk-pvc   Bound    d-bp18pbai447qverm3ttq   20Gi       RWO            alicloud-disk-ssd   49s
You can also run the following command to disable the default StorageClass:
kubectl patch storageclass alicloud-disk-ssd -p '{"metadata": {"annotations":{"storageclass.kubernetes.io/is-default-class":"false"}}}'