tiflash: add the alert-rules.md (#2191)

TOC.md

@@ -304,6 +304,7 @@
     - [Scale TiFlash](/reference/tiflash/scale.md)
     - [Upgrade TiFlash Nodes](/reference/tiflash/upgrade.md)
     - [Configure TiFlash](/reference/tiflash/configuration.md)
+    - [TiFlash Alert Rules](/reference/tiflash/alert-rules.md)
     - [Tune TiFlash Performance](/reference/tiflash/tune-performance.md)
     - [FAQ](/reference/tiflash/faq.md)
   + TiDB Binlog

reference/tiflash/alert-rules.md

@@ -0,0 +1,69 @@
+---
+title: TiFlash Alert Rules
+summary: Learn the alert rules of the TiFlash cluster.
+category: reference
+---
+
+# TiFlash Alert Rules
+
+This document introduces the alert rules of the TiFlash cluster.
+
+## `TiFlash_schema_error`
+
+- Alert rule:
+
+    `increase(tiflash_schema_apply_count{type="failed"}[15m]) > 0`
+
+- Description:
+
+    When the schema apply error occurs, an alert is triggered.
+
+- Solution:
+
+    The error might be caused by some wrong logic. Contact [TiFlash R&D](mailto:support@pingcap.com) for support.
+
+## `TiFlash_schema_apply_duration`
+
+- Alert rule:
+
+    `histogram_quantile(0.99, sum(rate(tiflash_schema_apply_duration_seconds_bucket[1m])) BY (le, instance)) > 20`
+
+- Description:
+
+    When the probability that the apply duration exceeds 20 seconds is over 99%, an alert is triggered.
+
+- Solution:
+
+    It might be caused by the internal problems of the TiFlash TMT engine. Contact [TiFlash R&D](mailto:support@pingcap.com) for support.
+
+## `TiFlash_raft_read_index_duration`
+
+- Alert rule:
+
+    `histogram_quantile(0.99, sum(rate(tiflash_raft_read_index_duration_seconds_bucket[1m])) BY (le, instance)) > 3`
+
+- Description:
+
+    When the probability that the read index duration exceeds 3 seconds is over 99%, an alert is triggered.
+
+    > **Note:**
+    >
+    > `read index` is the kvproto request sent to the TiKV leader. TiKV region retries, busy store, or network problems might lead to long request time of `read index`.
+
+- Solution:
+
+    The frequent retries might be caused by frequent splitting or migration of the TiKV cluster. You can check the TiKV cluster status to identify the retry reason.
+
+## `TiFlash_raft_wait_index_duration`
+
+- Alert rule:
+
+    `histogram_quantile(0.99, sum(rate(tiflash_raft_wait_index_duration_seconds_bucket[1m])) BY (le, instance)) > 2`
+
+- Description:
+
+    When the probability that the waiting time for Region Raft Index in TiFlash exceeds 2 seconds is over 99%, an alert is triggered.
+
+- Solution:
+
+    It might be caused by a communication error between TiKV and the proxy. Contact [TiFlash R&D](mailto:support@pingcap.com) for support.