Create and manage synchronously replicated disks

This document explains how to do the following tasks for replicated disks:

• Create replicated disks.
• Attach a replicated disk to your VM.
• Change a zonal disk to a replicated disk.
• Create a new VM with replicated disks.
• Use a replicated disk as a boot disk for a new VM.
• Attach a replicated boot disk to a VM.
• List and describe your replicated disks.
• Resize a replicated disk.

Before you begin

• Review the differences between different types of disk storage options.
• Review the basics of synchronous disk replication.
• Read about replicated disk failover.
• If you haven't already, then set up authentication. Authentication is the process by which your identity is verified for access to Google Cloud services and APIs. To run code or samples from a local development environment, you can authenticate to Compute Engine by selecting one of the following options:
  

  Select the tab for how you plan to use the samples on this page:

  Console

  When you use the Google Cloud console to access Google Cloud services and APIs, you don't need to set up authentication.

  gcloud

  1. Install the Google Cloud CLI, then initialize it by running the following command:

     gcloud init

  2. Set a default region and zone.

  Terraform

  To use the Terraform samples on this page in a local development environment, install and initialize the gcloud CLI, and then set up Application Default Credentials with your user credentials.

  1. Install the Google Cloud CLI.

  2. To initialize the gcloud CLI, run the following command:

     gcloud init

  3. If you're using a local shell, then create local authentication credentials for your user account:

     gcloud auth application-default login

     You don't need to do this if you're using Cloud Shell.

  For more information, see Set up authentication for a local development environment.

  REST

  To use the REST API samples on this page in a local development environment, you use the credentials you provide to the gcloud CLI.

  Install the Google Cloud CLI, then initialize it by running the following command:

  gcloud init

  For more information, see Authenticate for using REST in the Google Cloud authentication documentation.

Required roles and permissions

To get the permissions that you need to create a synchronously replicated disk, ask your administrator to grant you the following IAM roles on the project:

• Compute Instance Admin (v1) (roles/compute.instanceAdmin.v1)
• To connect to a VM that can run as a service account: Service Account User (v1) (roles/iam.serviceAccountUser)

For more information about granting roles, see Manage access to projects, folders, and organizations.

These predefined roles contain the permissions required to create a synchronously replicated disk. To see the exact permissions that are required, expand the Required permissions section:

You might also be able to get these permissions with custom roles or other predefined roles.

Limitations

• You can attach regional Persistent Disk only to VMs that use E2, N1, N2, and N2D machine types.
• You can attach Hyperdisk Balanced High Availability only to supported machine types.
• You cannot create a regional Persistent Disk from an image, or from a disk that was created from an image.
• When using read-only mode, you can attach a regional balanced Persistent Disk to a maximum of 10 VM instances.
• The minimum size of a regional standard Persistent Disk is 200 GiB.
• You can only increase the size of a regional Persistent Disk or Hyperdisk Balanced High Availability volume; you can't decrease its size.
• Regional Persistent Disk and Hyperdisk Balanced High Availability volumes have different performance characteristics than their corresponding zonal disks. For more information, see Block storage performance.
• You can't use a Hyperdisk Balanced High Availability volume that's in multi-writer mode as a boot disk.
• If you create a replicated disk by cloning a zonal disk, then the two zonal replicas aren't fully in sync at the time of creation. After creation, you can use the regional disk clone within 3 minutes, on average. However, you might need to wait for tens of minutes before the disk reaches a fully replicated state and the recovery point objective (RPO) is close to zero. Learn how to check if your replicated disk is fully replicated.

About using a replicated disk as a boot disk for a VM

You can attach a regional Persistent Disk or Hyperdisk Balanced High Availability (Preview) disk as a boot disk for stateful workloads that are provisioned ahead of time, before you provision a production workload. Replicated boot disks are not intended for hot standbys, because the replicated boot disks cannot be attached simultaneously to two VMs.

You can only create regional Persistent Disk or Hyperdisk Balanced High Availability volumes from snapshots; it isn't possible to create a replicated disk from an image.

To use a replicated disk as a VM boot disk, use either of the following methods:

1. Create a new VM with a replicated boot disk.
2. Create a replicated boot disk, and then attach it to a VM:
   1. Create a replicated disk from a snapshot of a boot disk.
   2. Attach a replicated boot disk to a VM.

If you need to failover a replicated boot disk to a running standby VM in the replica zone, use the steps described in Attach a replicated boot disk to a VM.

Create a synchronously replicated disk

Create a regional Persistent Disk or Hyperdisk Balanced High Availability (Preview) volume. The disk must be in the same region as the VM that you plan to attach it to.

If you create a Hyperdisk Balanced High Availability volume, you can also allow different VMs to simultaneously access the disk by setting the disk access mode. For more information, see Share a disk between VMs.

gcloud compute disks create DISK_NAME \
   --size=DISK_SIZE \
   --type=DISK_TYPE \
   --region=REGION \
   --replica-zones=ZONE1,ZONE2
   --access-mode=DISK_ACCESS_MODE

resource "google_compute_region_disk" "regiondisk" {
  name                      = "region-disk-name"
  snapshot                  = google_compute_snapshot.snapdisk.id
  type                      = "pd-ssd"
  region                    = "us-central1"
  physical_block_size_bytes = 4096
  size                      = 11

  replica_zones = ["us-central1-a", "us-central1-f"]
}

POST https://compute.googleapis.com/compute/v1/projects/PROJECT_ID/regions/REGION/disks
{
  "name": "DISK_NAME",
  "region": "projects/PROJECT_ID/regions/REGION",
  "replicaZones": [
    "projects/PROJECT_ID/zones/ZONE1",
    "projects/PROJECT_ID/zones/ZONE2"
  ],
  "sizeGb": "DISK_SIZE",
  "type": "projects/PROJECT_ID/regions/REGION/diskTypes/DISK_TYPE",
  "accessMode": "DISK_ACCESS_MODE"
}

Attach a replicated disk to your VM

For disks that are not boot disks, after you create a regional Persistent Disk or Hyperdisk Balanced High Availability (Preview) volume, you can attach it to a VM. The VM must be in the same region as the disk.

To attach a replicated boot disk to a VM, see Attach a replicated boot disk to a VM.

To attach a disk to multiple VMs, repeat the procedure in this section for each VM.

gcloud compute instances attach-disk VM_NAME \
    --disk=DISK_NAME \
    --disk-scope=regional

resource "google_compute_instance" "test_node" {
  name         = "test-node"
  machine_type = "f1-micro"
  zone         = "us-west1-a"

  boot_disk {
    initialize_params {
      image = "debian-cloud/debian-11"
    }
  }
  attached_disk {
    source      = google_compute_disk.default.id
    device_name = google_compute_disk.default.name
  }

  network_interface {
    network = "default"
    access_config {
      # Ephemeral IP
    }
  }

  # Ignore changes for persistent disk attachments
  lifecycle {
    ignore_changes = [attached_disk]
  }


}

POST https://compute.googleapis.com/compute/v1/projects/PROJECT_ID/zones/ZONE/instances/VM_NAME/attachDisk
{
  "source": "/projects/PROJECT_ID/regions/REGION/disks/DISK_NAME"
}

For non-boot disks, after you create and attach a blank replicated disk to a VM, you must format and mount the disk, so that the operating system can use the available storage space.

Change a zonal disk to a replicated disk

To convert a Hyperdisk to a synchronously replicated disk, create a new Hyperdisk Balanced High Availability disk from a snapshot of the existing disk, as described in Change the disk type.

Create a new VM with replicated disks

When creating a VM, you can optionally include regional Persistent Disk or Hyperdisk Balanced High Availability (Preview) volumes as additional disks.

To create and attach a regional Persistent Disk or Hyperdisk Balanced High Availability volume to a VM during VM creation, see either of the following:

• Create a VM instance with additional non-boot disks
• Create a new instance template

Create a new VM with a replicated boot disk

When setting up a highly available VM instance, you can create the primary VM with a replicated boot disk. If a zonal outage occurs, this lets you restart the VM in the secondary zone instead of creating a new VM.

In a high availability setup, where the boot device is a replicated disk, Google recommends that you don't pre-create and start the standby instance. Instead, at the failover stage, attach the existing replicated disk when you create the standby instance by using the forceAttach option.

To create a VM with a boot disk that is a replicated disk, use either of the following methods:

gcloud compute instances create PRIMARY_INSTANCE_NAME  \
 --zone=ZONE  \
 --create-disk=^:^name=REPLICATED_DISK_NAME:scope=regional:boot=true:type=DISK_TYPE:source-snapshot=SNAPSHOT_NAME:replica-zones=ZONE,REMOTE_ZONE

POST https://compute.googleapis.com/compute/v1/projects/PROJECT_ID/zones/ZONE/instances
{
 "name": "VM_NAME",
 "disks": [{
    "boot": true,
    "initializeParams": {
       "sourceSnapshot": "global/snapshots/BOOT_SNAPSHOT_NAME",
       "replicaZones": [
           "projects/PROJECT_ID/zones/ZONE",
           "projects/PROJECT_ID/zones/REMOTE_ZONE"
       ],
       "diskType": "projects/PROJECT_ID/zones/ZONE/diskTypes/DISK_TYPE"
    }
  }],
 "networkInterfaces": [
    {
      "network": "global/networks/default"
    }
  ]
}

Attach a replicated boot disk to a VM

Use the following steps to:

• Replace the boot disk of an existing VM with a replicated boot disk.
• Failover a replicated boot disk to a hot standby VM that is running in the backup zone. You do this by attaching the replicated disk to the VM as the boot disk.

These steps assume that the replicated disk and VM already exist.

List and describe your replicated disks

You can view a list of all your configured replicated disks, and information about their properties, including the following:

• Disk ID
• Disk name
• Size
• Disk type
• Region
• Zonal replicas

To view detailed information about your replicated disks, use the following:

• To view the details of all replicated disks in a specific region and project:
  • Construct a GET request to the compute.regionDisks.list method.
  • Use the gcloud compute disks list command and filter the results by region.
• To view the details of a specific replicated disk:
  • Run the gcloud compute disks describe command with the --region flag, and specify the name of the disk and its region.
  • Construct a GET request to the compute.regionDisks.get method.

Resize a replicated disk

If VMs with synchronously replicated disks require additional storage space, you can resize the disks. You can resize disks at any time, regardless of whether the disk is attached to a running VM. If you need to separate your data into unique volumes, create multiple secondary disks for the VM. For Hyperdisk Balanced High Availability you can also increase the IOPS and throughput limits for the disk.

The command for resizing a replicated disk is very similar to that for resizing a non-replicated disk. However, you must specify a region instead of a zone for the disk location.

You can only increase, and not decrease, the size of a disk. To decrease the disk size, you must create a new disk with a smaller size. Until you delete the original, larger disk, you are charged for both disks.

For instructions on how to modify a replicated disk, see the following:

• Regional Persistent Disk: Increase the size of a persistent disk
• Hyperdisk Balanced High Availability: Modify a Hyperdisk volume

What's next

• Learn about disk pricing.
• Learn how to monitor the replica states of replicated disks.
• Learn how to determine the replication state of a replicated disk.
• Review Share Persistent Disk volumes between VMs as an alternative to Regional Persistent Disks for read-only data.
• Create a snapshot of a disk.
• Learn about instance groups for VM instances.
• Learn how to build scalable and resilient web applications on Google Cloud.
• See the Google Cloud disaster recovery planning guide.