📝 update troubleshooting docs

Signed-off-by: vankichi <kyukawa315@gmail.com>

docs/troubleshooting/chart.md

@@ -12,5 +12,5 @@ If any components output the `INTERNAL` error log, let us know the detail.
 
 ## Related Document
 
-- [Troubleshooting](../troubleshooting/troubleshooting.md)
+- [Operation Troubleshooting](../troubleshooting/operation-troubleshooting.md)
 

docs/troubleshooting/operation-troubleshooting.md

@@ -0,0 +1,57 @@
+# Operation troubleshooting
+
+This page introduces the popular troubleshooting for operating a Vald cluster.
+
+The [Flow chart](../troubleshooting/chart.md) helps you to find the root reason for your problem.
+Additionally, if you encounter some errors when using API, [API status code](../api/status.md) helps you, too.
+
+## Insert Operation
+
+### Vald Agent NGT crashed at the insert process.
+
+Let’s check your container limit of memory at first.
+Vald Agent requires memory for keeping indexing on memory.
+
+```bash
+kubectl describe statefulset vald-agent-ngt
+```
+
+If the limit memory is set, please remove it or update value larger.
+
+## Search Operation
+
+### Vald returns no search result.
+
+There are two possible reasons.
+
+1. Indexing has not finished in Vald Agent
+
+    Vald will search the nearest vectors of query from the indexing in Vald Agent.
+    If the indexing process is running, Vald Agent returns no search result.
+
+    It will resolve when completed indexing instruction, like as `CreateIndex`.
+
+1. Too short timeout for searching
+
+    When the search timeout configuration is too short, Vald LB Gateway stops the searching process before getting search result from Vald Agent.
+
+In the sense of search operation, you can modify search timeout by [payload config](../api/search.md).
+
+<div class="notice">
+It is easy to find out which problem occurs by inspections the log of each Pod, like <a href="https://github.com/wercker/stern">stern</a>.
+</div>
+
+## Others
+
+### Vald Agent NGT crashed when initContainer.
+
+Vald Agent NGT requires an AVX2 processor for running.
+Please check your CPU information.
+
+---
+
+## Related Document
+
+- [Flow Chart](../troubleshooting/chart.md)
+- [API Status](../api/status.md)
+- [FAQ](../support/faq.md)