add dashboard diagnostics docs (#2690)

dashboard/dashboard-diagnostics-access.md

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+---
+title: TiDB Dashboard Cluster Diagnostic Page
+summary: Learn how to use the cluster diagnostic page.
+category: how-to
+---
+
+# TiDB Dashboard Cluster Diagnostics Page
+
+The cluster diagnostics feature in TiDB Dashboard diagnoses the problems that might exist in a cluster within a specified time range, and summarizes the diagnostic results and the cluster-related load monitoring information into a diagnostic report. This diagnostic report is in the form of a web page. You can browse the page offline and circulate this page link after saving the page from a browser.
+
+> **Note:**
+>
+> The cluster diagnostics feature depends on Prometheus deployed in the cluster. For details about how to deploy this monitoring component, see [TiUP](/tiup/tiup-overview.md) or [TiDB Ansible](/online-deployment-using-ansible.md) deployment document. If no monitoring component is deployed in the cluster, the generated diagnostic report will indicate a failure.
+
+## Access the page
+
+You can use one of the following methods to access the cluster diagnostics page:
+
+* After logging into TiDB Dashboard, click **Cluster Diagnostics** on the left navigation menu:
+
+    ![Access Cluster Diagnostics page](/media/dashboard/dashboard-diagnostics-access.png)
+
+* Visit `http://127.0.0.1:2379/dashboard/#/diagnose` in your browser. Replace `127.0.0.1:2379` with the actual PD address and port number.
+
+## Generate diagnostic report
+
+To diagnose a cluster within a specified time range and check the cluster load, you can take the following steps to generate a diagnostic report:
+
+1. Set the **Range Start Time**, such as `2020-05-21 14:40:00`.
+2. Set the **Range Duration**, such as `10 min`.
+3. Click **Start**.
+
+![Generate diagnostic report](/media/dashboard/dashboard-diagnostics-gen-report.png)
+
+> **Note:**
+>
+> It is recommended that the **Range Duration** of the report is between 1 minute and 60 minutes. This **Range Duration** cannot exceed 60 minutes.
+
+The steps above generate a diagnostic report for the time range from `2020-05-21 14:40:00` to `2020-05-21 14:50:00`. After clicking **Start**, you can see the interface below. **Progress** is the progress bar of the diagnostic report. After the report is generated, click **View Full Report**.
+
+![Report progress](/media/dashboard/dashboard-diagnostics-gen-process.png)
+
+## Generate comparison report
+
+If a system exception occurs at a certain point, for example, QPS jitter or higher latency, a diagnostic report can be generated. Particularly, this report compares the system in the abnormal time range with the system in the normal time range. For example:
+
+* Abnormal time range: `2020-05-21 14:40:00`-`2020-05-21 14:45:00`. Within this time range, the system is abnormal.
+* Normal time range: `2020-05-21 14:30:00` - `2020-05-21 14:35:00`. Within this time range, the system is normal.
+
+You can take the following steps to generate a comparison report for the two time ranges above:
+
+1. Set the **Range Start Time**, which is the start time of the range in which the system becomes abnormal, such as `2020-05-21 14:40:00`.
+2. Set the **Range Duration**. Generally, this duration is the duration of system anomalies, such as 5 minutes.
+3. Enable **Compare by baseline**.
+4. Set the **Baseline Range Start Time**, which is the start time of the range (to be compared with) in which the system is normal, such as `2020-05-21 14:30:00`.
+5. Click **Start**.
+
+![Generate comparison report](/media/dashboard/dashboard-diagnostics-gen-compare-report.png)
+
+Then wait for the report to be generated and click **View Full Report**.
+
+In addition, the historical diagnostic report is displayed in the list on the main page of the diagnostic report. You can click to view these historical reports directly.