Add introduction overview for installation on Linux

content/en/installation/intro.md

@@ -1,73 +1,70 @@
 ---
 title: "Introduction"
-keywords: 'kubernetes, docker, helm, jenkins, istio, prometheus'
-description: 'KubeSphere Installation Overview'
+keywords: 'Kubernetes, KubeSphere, Installation on Linux'
+description: 'The Overview of KubeSphere Installation on Linux'
 ---
 
-[KubeSphere](https://kubesphere.io/) is an enterprise-grade multi-tenant container platform built on [Kubernetes](https://kubernetes.io). It provides an easy-to-use UI for users to manage application workloads and computing resources with a few clicks, which greatly reduces the learning curve and the complexity of daily work such as development, testing, operation and maintenance. KubeSphere aims to alleviate the pain points of Kubernetes including storage, network, security and ease of use, etc.
+For the installation on Linux, KubeSphere can be installed both in clouds and in on-premises environments, such as AWS EC2, Azure VM and bare metal. Users can install KubeSphere on Linux hosts as they provision fresh Kubernetes clusters. The installation process is easy and friendly. Meanwhile, KubeSphere offers not only the online installer, or [KubeKey](https://github.com/kubesphere/kubekey), but also an air-gapped installer for the environment with no Internet access.
 
-KubeSphere supports installing on cloud-hosted and on-premises Kubernetes cluster, e.g. native K8s, GKE, EKS, RKE, etc. It also supports installing on Linux host including virtual machine and bare metal with provisioning fresh Kubernetes cluster. Both of the two methods are easy and friendly to install KubeSphere. Meanwhile, KubeSphere offers not only online installer, but air-gapped installer for such environment with no access to the internet.
+As an open-source project on [GitHub](https://github.com/kubesphere), KubeSphere is home to a community with thousands of users. Many of them are running KubeSphere for their production workloads.
 
-KubeSphere is open source project on [GitHub](https://github.com/kubesphere). There are thousands of users are using KunbeSphere, and many of them are running KubeSphere for their production workloads.
+Users are provided with multiple installation options. Please note not all options are mutually exclusive. For instance, you can deploy KubeSphere with minimal packages on multiple nodes in an air-gapped environment.
 
-In summary, there are several installation options you can choose. Please note not all options are mutually exclusive. For instance, you can deploy KubeSphere with minimal packages on existing K8s cluster on multiple nodes in air-gapped environment. Here is the decision tree shown in the following graph you may reference for your own situation.
-
-- [All-in-One](../all-in-one): Intall KubeSphere on a singe node. It is only for users to quickly get familar with KubeSphere.
+- [All-in-One](../all-in-one): Install KubeSphere on a single node. It is only for users to quickly get familiar with KubeSphere.
 - [Multi-Node](../multi-node): Install KubeSphere on multiple nodes. It is for testing or development.
-- [Install KubeSphere on Air Gapped Linux](../install-ks-on-linux-airgapped): All images of KubeSphere have been encapsulated into a package, it is convenient for air gapped installation on Linux machines.
-- [High Availability Multi-Node](../master-ha): Install high availability KubeSphere on multiple nodes which is used for production environment.
-- [KubeSphere on Existing K8s](../install-on-k8s): Deploy KubeSphere on your Kubernetes cluster including cloud-hosted services such as GKE, EKS, etc.
-- [KubeSphere on Air-Gapped K8s](../install-on-k8s-airgapped): Install KubeSphere on a disconnected Kubernetes cluster.
-- Minimal Packages: Only install minimal required system components of KubeSphere. The minimum of resource requirement is down to 1 core and 2G memory.
-- [Full Packages](../complete-installation): Install all available system components of KubeSphere including DevOps, service mesh, application store, etc.
+- [Install KubeSphere on Air-gapped Linux](../install-ks-on-linux-airgapped): All images of KubeSphere have been encapsulated into a package. It is convenient for air-gapped installation on Linux machines.
+- [High Availability Installation](../master-ha): Install high availability KubeSphere on multiple nodes which is used for the production environment.
+- Minimal Packages: Only install the minimum required system components of KubeSphere. Here is the minimum resource requirement:
+  - 2vCPUs
+  - 4GB RAM
+  - 20GB Storage
+- [Full Packages](../complete-installation): Install all available system components of KubeSphere such as DevOps, service mesh, and alerting.
 
-![Installer Options](https://pek3b.qingstor.com/kubesphere-docs/png/20200305093158.png)
+For the installation on Kubernetes, see Overview of Installing on Kubernetes.
 
 ## Before Installation
 
-- As the installation will pull images and update operating system from the internet, your environment must have the internet access. If not, then you need to use the air-gapped installer instead.
+- As images will be pulled and operating systems will be downloaded from the Internet, your environment must have Internet access. Otherwise, you need to use the air-gapped installer instead.
 - For all-in-one installation, the only one node is both the master and the worker.
-- For multi-node installation, you are asked to specify the node roles in the configuration file before installation.
+- For multi-node installation, you need to specify the node roles in the configuration file before installation.
 - Your linux host must have OpenSSH Server installed.
 - Please check the [ports requirements](../port-firewall) before installation.
 
-## Quick Install For Development and Testing
-
-KubeSphere has decoupled some components since v2.1.0. The installer only installs required components by default which brings the benefits of fast installation and minimal resource consumption. If you want to install any optional component, please check the following section [Pluggable Components Overview](../intro#pluggable-components-overview) for details.
-
-The quick install of KubeSphere is only for development or testing since it uses local volume for storage by default. If you want a production install please refer to the section [High Availability Installation for Production Environment](../intro#high-availability-installation-for-production-environment).
+## KubeKey
 
-### 1. Install KubeSphere on Linux
+Developed in Go language, KubeKey represents a brand-new installation tool as a replacement for the ansible-based installer used before. KubeKey provides users with flexible installation choices, as they can install KubeSphere and Kubernetes separately or install them at one time, which is convenient and efficient.
 
-- [All-in-One](../all-in-one): It means a single-node hassle-free configuration installation with one-click.
-- [Multi-Node](../multi-node): It allows you to install KubeSphere on multiple instances using local volume, which means it is not required to install storage server such as Ceph, GlusterFS.
+Three scenarios to use KubeKey:
 
-> Note：With regard to air-gapped installation please refer to [Install KubeSphere on Air Gapped Linux Machines](../install-ks-on-linux-airgapped).
+- Install Kubernetes only;
+- Install Kubernetes and KubeSphere together in one command;
+- Install Kubernetes first, and deploy KubeSphere on it using [ks-installer](https://github.com/kubesphere/ks-installer).
 
-### 2. Install KubeSphere on Existing Kubernetes
+> Important: If you have existing Kubernetes clusters, please refer to [ks-installer](https://github.com/kubesphere/ks-installer).
 
-You can install KubeSphere on your existing Kubernetes cluster. Please refer [Install KubeSphere on Kubernetes](../install-on-k8s) for instructions.
+## Quick Installation for Development and Testing
 
-## High Availability Installation for Production Environment
+KubeSphere has decoupled some components since v2.1.0. KubeKey only installs necessary components by default as this way features fast installation and minimal resource consumption. If you want to install any optional component, see [Overview of Pluggable Components](../intro#pluggable-components-overview) for details.
 
-### 1. Install HA KubeSphere on Linux
+The quick installation of KubeSphere is only for development or testing since it uses local volume for storage by default. If you want a production installation, see HA Cluster Configuration.
 
-KubeSphere installer supports installing a highly available cluster for production with the prerequisites being a load balancer and persistent storage service set up in advance.
+- **All-in-one**. It means a single-node hassle-free installation with just one command.
+- **Multi-node**. It allows you to install KubeSphere on multiple instances using the default storage class (local volume), which means it is not required to install storage server such as Ceph and GlusterFS.
 
-- [Persistent Service Configuration](../storage-configuration): By default, KubeSphere Installer uses [Local Volume](https://kubernetes.io/docs/concepts/storage/volumes/#local) based on [openEBS](https://openebs.io/) to provide storage service with dynamic provisioning in Kubernetes cluster. It is convenient for quick install of testing environment. In production environment, it must have a storage server set up. Please refer [Persistent Service Configuration](../storage-configuration) for details.
-- [Load Balancer Configuration for HA install](../master-ha): Before you get started with multi-node installation in production environment, you need to configure a load balancer. Either cloud LB or `HAproxy + keepalived` works for the installation.
+> Important: For air-gapped installation, please refer to [Install KubeSphere on Air Gapped Linux Machines](../install-ks-on-linux-airgapped).
 
-### 2. Install HA KubeSphere on Existing Kubernetes
+## Install HA KubeSphere on Linux
 
-Before you install KubeSphere on existing Kubernetes, please check the prerequisites of the installation on Linux described above, and verify the existing Kubernetes to see if it satisfies these prerequisites or not, i.e., a load balancer and persistent storage service.  
+KubeKey allows users to install a highly available cluster for production. Users need to configure load balancers and persistent storage services in advance.
 
-If your Kubernetes is ready, please refer [Install KubeSphere on Kubernetes](../install-on-k8s) for instructions.
+- [Persistent Service Configuration](../storage-configuration): By default, KubeKey uses [Local Volume](https://kubernetes.io/docs/concepts/storage/volumes/#local) based on [openEBS](https://openebs.io/) to provide storage services with dynamic provisioning in Kubernetes clusters. It is convenient for the quick installation of a testing environment. In a production environment, it must have a storage server set up. Please refer to [Persistent Service Configuration](../storage-configuration) for details.
+- [Load Balancer Configuration for HA installation](../master-ha): Before you get started with multi-node installation in a production environment, you need to configure load balancers. Either cloud LB or `HAproxy + keepalived` works for the installation.
 
-> You can install KubeSphere on cloud Kubernetes service such as [Installing KubeSphere on GKE cluster](../install-on-gke)
+For more information, see HA Cluster Configuration. You can also see the specific step of HA installations across major cloud providers in Installing on Public Cloud.
 
-## Pluggable Components Overview
+## Overview of Pluggable Components
 
-KubeSphere has decoupled some core feature components since v2.1.0. These components are designed to be pluggable, which means you can enable any of them before or after installation. The installer by default does not install the pluggable components. Please check the guide [Enable Pluggable Components Installation](../pluggable-components) for your requirement.
+KubeSphere has decoupled some core feature components since v2.1.0. These components are designed to be pluggable, which means you can enable any of them both before and after the installation. By default, KubeKey does not install these pluggable components. For more information, see Enable Pluggable Components.
 
 ![Pluggable Components](https://pek3b.qingstor.com/kubesphere-docs/png/20191207140846.png)
 
@@ -81,10 +78,24 @@ The following links explain how to configure different types of persistent stora
 - [QingCloud Block Storage](https://docs.qingcloud.com/product/storage/volume/)
 - [QingStor NeonSAN](https://docs.qingcloud.com/product/storage/volume/super_high_performance_shared_volume/)
 
-## Add New Nodes
+## Cluster Operation and Maintenance
+
+### Add New Nodes
+
+With KubeKey, you can scale the number of nodes to meet higher resource needs after the installation, especially in a production environment. For more information, see [Add New Nodes](../add-nodes).
 
-KubeSphere Installer allows you to scale the number of nodes, see [Add New Nodes](../add-nodes).
+### Remove Nodes
+
+You need to drain a node before you remove. For more information, see Remove Nodes.
+
+### Add New Storage Classes
+
+KubeKey allows you to set a new storage class after the installation. By default, the local volume of OpenEBS is adopted for persistent storage services. However, this is for development or testing purpose. It is recommended to change the default storage class in a production environment.
+
+For more information, see Add New Storage Classes.
 
 ## Uninstall
 
-Uninstall will remove KubeSphere from the machines. This operation is irreversible and dangerous. Please check [Uninstall](../uninstall).
+Uninstalling KubeSphere means it will be removed from the machines, which is irreversible. Please be cautious with the operation.
+
+For more information, see [Uninstall](../uninstall).