Add cluster management guide

Signed-off-by: Gao Hongtao <hanahmily@gmail.com>

CHANGES.md

@@ -45,6 +45,7 @@ Release Notes.
 - Add quick-start guide.
 - Add web-ui interacting guide.
 - Add bydbctl interacting guide.
+- Add cluster management guide.
 
 ### Chores
 

docs/operation/cluster.md

@@ -0,0 +1,57 @@
+# Cluster Maintenance
+
+## Introduction
+Properly maintaining and scaling a cluster is crucial for ensuring its reliable and efficient operation. This document provides guidance on setting up a cluster, planning its capacity, and scaling it to meet evolving requirements.
+
+## Cluster Setup
+Before deploying or maintaining a cluster, it is recommended to familiarize oneself with the basic clustering concepts by reviewing the [clustering documentation](../concept/clustering.md).
+
+To set up a cluster, one can refer to the [cluster installation guide](../installation/cluster.md), which describes the process in detail. A minimal cluster should consist of the following nodes:
+
+- 3 etcd nodes
+- 2 liaison nodes
+- 2 data nodes
+
+This configuration is recommended for high availability, ensuring that the cluster can continue operating even if a single node becomes temporarily unavailable, as the remaining nodes can handle the increased workload.
+
+It is generally preferable to deploy multiple smaller data nodes rather than a few larger ones, as this approach reduces the workload increase on the remaining data nodes when some nodes become temporarily unavailable.
+
+To balance the write and query traffic to the liaison nodes, the use of an gRPC load balancer is recommended. The gRPC port defaults to `17912`, but the gRPC host and port can be altered using the `grpc-host` and `grpc-port` configuration options.
+
+For those seeking to set up a cluster in a Kubernetes environment, a [dedicated guide](../installation/kubernetes.md) is available to assist with the process.
+
+## Capacity Planning
+Each node role can be provisioned with the most suitable hardware resources. The cluster's capacity scales linearly with the available resources. The required amounts of CPU and RAM per node role depend highly on the workload, such as the number of time series, query types, and write/query QPS. It is recommended to set up a test cluster mirroring the production workload and iteratively scale the per-node resources and the number of nodes per role until the cluster becomes stable. Additionally, the use of observability tools is advised, as they can help identify bottlenecks in the cluster setup.
+
+The necessary storage space can be estimated based on the disk space usage observed during a test run. For example, if the storage space usage is 10GB after a day-long test run on a production workload, then the cluster should have at least 10GB*7=70GB of disk space for a group with `ttl=7day`.
+
+To ensure the cluster's resilience and responsiveness, it is recommended to maintain the following spare resource levels:
+
+- 50% of free RAM across all the nodes to reduce the probability of OOM (out of memory) crashes and slowdowns during temporary spikes in workload.
+- 50% of spare CPU across all the nodes to reduce the probability of slowdowns during temporary spikes in workload.
+- At least 20% of free storage space at the directories pointed by `measure-root-path` and `stream-root-path`.
+
+## Scalability
+The cluster's performance and capacity can be scaled in two ways: vertical scalability and horizontal scalability.
+
+### Vertical Scalability
+Vertical scalability refers to adding more resources (CPU, RAM, disk I/O, disk space, network bandwidth) to existing nodes in the cluster.
+
+Increasing the CPU and RAM of existing liaison nodes can improve the performance for heavy queries that process a large number of time series with many data points.
+
+Increasing the CPU and RAM of existing data nodes can increase the number of time series the cluster can handle. However, it is generally preferred to add more data nodes rather than increasing the resources of existing data nodes, as a higher number of data nodes increases cluster stability and improves query performance over time series.
+
+Increasing the disk I/O and disk space of existing etcd nodes can improve the performance for heavy metadata queries that process a large number of metadata entries.
+
+### Horizontal Scalability
+Horizontal scalability refers to adding more nodes to the cluster.
+
+Increasing the number of liaison nodes can increase the maximum possible data ingestion speed, as the ingested data can be split among a larger number of liaison nodes. It can also increase the maximum possible query rate, as the incoming concurrent requests can be split among a larger number of liaison nodes.
+
+Increasing the number of data nodes can increase the number of time series the cluster can handle. This can also improve query performance, as each data node contains a lower number of time series when the number of data nodes increases.
+
+The new added data nodes can be automatically discovered by the existing liaison nodes. It is recommended to add data nodes one by one to avoid overloading the liaison nodes with the new data nodes' metadata.
+
+The cluster's availability is also improved by increasing the number of data nodes, as active data nodes need to handle a lower additional workload when some data nodes become unavailable. For example, if one node out of 2 nodes is unavailable, then 50% of the load is re-distributed across the remaining node, resulting in a 100% per-node workload increase. If one node out of 10 nodes is unavailable, then 10% of the load is re-distributed across the 9 remaining nodes, resulting in only an 11% per-node workload increase.
+
+Increasing the number of etcd nodes can increase the cluster's metadata capacity and improve the cluster's metadata query performance. It can also improve the cluster's metadata availability, as the metadata is replicated across all the etcd nodes. However, the cluster size should be odd to avoid split-brain situations.