Doc about Persistent disk added

docs/book/src/topics/persistent-disks.md

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+# Persistent Disks
+---
+
+This document describes how persistent disks are to be provisioned and attached to VMs in Google Cloud Platform.
+
+## Storage Disks
+---
+
+See [Storage Options](https://cloud.google.com/compute/docs/disks) for more information.
+
+### Disk Types
+
+We can either configure a zonal or regional persistent disk, we can choose following disk types: 
+- Standard persistent disks (`pd-standard`)
+- Balanced persistent disks (`pd-balanced`)
+- SSD persistent disks (`pd-ssd`)
+- Extreme persistent disks (`pd-extreme`)
+
+If you create a disk in the Cloud Console, the default disk type is `pd-balanced`. If you create a disk using the gcloud tool the default disk type is `pd-standard`.
+
+## Disk Specification
+---
+
+- `DISK_NAME`: the name of the new disk
+- `DISK_SIZE`: the size, in gigabytes, of the new disk. Acceptable sizes range, in 1 GB increments, from 10 GB to 65,536 GB inclusive.
+- `DISK_TYPE`: ull or partial URL for the type of the persistent disk. Example: `https://www.googleapis.com/compute/v1/projects/PROJECT_ID/zones/ZONE/diskTypes/pd-ssd`
+
+### Creating disk
+
+```sh
+gcloud compute disks create *(DISK_NAME)* \
+--size *(DISK_SIZE)* \
+--zone *(ZONE)*\
+--type *(DISK_TYPE)*
+```
+
+### Attaching disk to running or stopped VM instance
+
+```sh
+gcloud compute instance attach-disk *(INSTANCE_NAME)* \
+--disk *(DISK_NAME)*
+```
+
+After this, use `gcloud compute disks describe` to see the description of the disks.
+
+After you create and attach the new disk to a VM, you must format and mount the disk, so that the operating system can use the available storage space.
+
+## Restrictions
+---
+- You can attach up to 127 secondary non-boot zonal persistent disks.
+- You can have a total attached capacity of 257 TB per instance.
+
+## EXAMPLE
+---
+The below example shows how to create and attach a custom disk "my_disk" at vm-instance-1 for every control plane machine, in addition to the etcd disk. NOTE: the same can be applied to the worker machine.
+
+```yaml
+kind: KubeadmControlPlane
+apiVersion: controlplane.cluster.x-k8s.io/v1alpha4
+metadata:
+  name: "${CLUSTER_NAME}-control-plane"
+spec:
+    [...]
+    diskSetup:
+      partitions:
+        - device: gcloud compute --project \
+                  "project-1" ssh \
+                  --zone us-central-a \
+                  vm-instance-1
+          type: bq
+          layout: true
+          overwrite: false
+        - device: gcloud compute --project \
+                  "project-1" ssh \
+                  --zone us-central-b \
+                  vm-instance-2
+          type: bq
+          layout: true 
+          overwrite: false
+      filesystems:
+        - label: etcd_disk
+          filesystem: ext4
+          device: gcloud compute --project \
+                  "project-1" ssh \
+                  --zone us-central-b \
+                  vm-instance-2
+        - label: my_disk
+          filesystem: ext4
+          device: gcloud compute --project \
+                  "project-1" ssh \
+                  --zone us-central-a \
+                  vm-instance-1
+    mounts:
+      - - LABEL=etcd_disk
+        - /var/lib/etcddisk
+      - - LABEL=my_disk
+        - /var/lib/mydir
+---
+kind: GCPMachineTemplate
+apiVersion: infrastructure.cluster.x-k8s.io/v1alpha4
+metadata:
+  name: "${CLUSTER_NAME}-control-plane"
+spec:
+  template:
+    spec:
+      [...]
+      dataDisks:
+        - nameSuffix: etcddisk
+          diskSizeGB: 256
+        - nameSuffix: my_disk
+          diskSizeGB: 128
+```