doc: Debugging Gitaly

Why is this change being made?

Primarily aimed at Create oncall training for Gitaly (gitlab-org/core-platform-section/discussions#178), also serve as a basis for debugging customer issues.

This is NOT oncall training, but a supporting document for that, as well as for customer issues or just simply debugging while developing.

The main goal is to bring everyone on the same page about what tools are available, and lower their barrier of entry. It's intended both for newcomers and as reference material in day-to-day.

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Commits

• doc: Debugging Gitaly

---

content/handbook/engineering/infrastructure/core-platform/systems/gitaly/debug.md

@@ -42,7 +42,7 @@ A Gitaly dashboard could be either auto-generated or manually drafted. We use Js
 
 A standardized dashboard should have a top-level section containing environment filters, node filters, and useful annotations such as feature flag activities, deployments, etc. Some dashboards have an interlinked system that connects Grafana and Kibana with a single click.
 
-Such dashboards usually include two parts. The second half contains panels of custom metrics collected from Gitaly. The first half is more complicated. It contains GitLab-wide indicators telling if Gitaly is "healthy" and node-level resource metrics. The aggregation and calculation are sophisticated. In summary, those dashboards tell us if Gitaly performs well according to predefined [thresholds](https://gitlab.com/gitlab-com/runbooks/-/blob/d0a5ff2f0ae23984679e0cf6e3361c6d4e71550b/metrics-catalog/services/gitaly.jsonnet), . We could contact [Scalability:Observability Team](../../../infrastructure/team/scalability/observability/) for any questions.
+Such dashboards usually include two parts. The second half contains panels of custom metrics collected from Gitaly. The first half is more complicated. It contains GitLab-wide indicators telling if Gitaly is "healthy" and node-level resource metrics. The aggregation and calculation are sophisticated. In summary, those dashboards tell us if Gitaly performs well according to predefined [thresholds](https://gitlab.com/gitlab-com/runbooks/-/blob/master/metrics-catalog/services/gitaly.jsonnet), . We could contact [Scalability:Observability Team](../../../team/scalability/observability/) for any questions.
 
 ![Gitaly Debug Indicators](gitaly-debug-indicators.png)
 
@@ -54,7 +54,7 @@ Some examples of using built-in dashboards to investigate production issues, fro
 
 ## Gitaly's Prometheus metrics
 
-A panel in a dashboard is a visualization of the aggregated version of underlying metrics. We use [Prometheus](https://prometheus.io/docs/introduction/overview/) to collect metrics. To simplify, the Gitaly server exposes an HTTP server ([code](https://gitlab.com/gitlab-org/gitaly/-/blob/3b872218f78151d681011e5ef2bbc22b3721f6a2/internal/cli/gitaly/serve.go#L514)) that allows Prometheus instances to fetch metrics periodically.
+A panel in a dashboard is a visualization of the aggregated version of underlying metrics. We use [Prometheus](https://prometheus.io/docs/introduction/overview/) to collect metrics. To simplify, the Gitaly server exposes an HTTP server ([code](https://gitlab.com/gitlab-org/gitaly/-/blob/master/internal/cli/gitaly/serve.go#L514)) that allows Prometheus instances to fetch metrics periodically.
 
 In a dashboard, you can click on the top-right hamburger button and choose "Explore" to get access to the underlying metrics. Or you could use [the Explore page](https://dashboards.gitlab.net/explore) to play with metrics.
 
@@ -64,7 +64,7 @@ Unfortunately, we don't have a curated list of all Gitaly metrics as well as the
 
 - Node-level or environmental metrics. Those metrics are powered by other systems that host the Gitaly process. They are not exposed by Gitaly but are very useful, for example: CPU metrics, memory metrics, or cgroup metrics.
 - Gitaly-specific metrics. Those metrics are accounted for directly in the code. Typically, they have `gitaly_` prefixes.
-- Aggregated metrics, such as combining different metrics or downsizing metrics due to high cardinality issues. The list of Gitaly's aggregated metrics is listed [in this file](https://gitlab.com/gitlab-com/runbooks/-/blob/e1d0ad78d24d51c36ea7dea28765ba16fd588d42/mimir-rules/gitlab-gprd/gitaly/gitaly.yml).
+- Aggregated metrics, such as combining different metrics or downsizing metrics due to high cardinality issues. The list of Gitaly's aggregated metrics is listed [in this file](https://gitlab.com/gitlab-com/runbooks/-/blob/master/mimir-rules/gitlab-gprd/gitaly/gitaly.yml).
 
 ![Gitaly Debug Metric Lists](gitaly-debug-list-metrics.png)
 
@@ -103,6 +103,25 @@ The query uses [PromQL](https://prometheus.io/docs/prometheus/latest/querying/ba
 - [hyperfine](https://github.com/sharkdp/hyperfine): a performance tool that can benchmarks over time
   - hyperfine can be used together with grpcurl to check the response time of a gPRC call
 
+### strace
+
+`strace(1)` a gitaly process:
+
+```shell
+strace -fttTyyy -s 1024 -o /paht/filename -p $(pgrep -fd, gitaly)
+```
+
+Or wrap a process to make it easy to strace, especially if it then spawns more processes:
+
+```shell
+#!/bin/bash/sh
+echo $(date)" $PPID $@" >> /tmp/gitlab-shell.txt
+exec /opt/gitlab/embedded/service/gitlab-shell/bin/gitlab-shell-orig "$@"
+# strace -fttTyyy -s 1024 -o /tmp/sshd_trace-$PPID /opt/gitlab/embedded/service/gitlab-shell/bin/gitlab-shell-orig
+```
+
+https://gitlab.com/gitlab-com/support/toolbox/strace-parser is useful to make the results more readable.
+
 ## Log analysis
 
 Kibana (Elastic) Dashboards
@@ -112,4 +131,8 @@ Kibana (Elastic) Dashboards
 
 ## Capacity management
 
-TBD
+Gitaly team is responsible for maintaining reasonable serving capacity for gitlab.com.
+
+We get alerts from Tamland if capacity runs low, see [this issue comment](https://gitlab.com/gitlab-com/gl-infra/capacity-planning-trackers/gitlab-com/-/issues/1666#note_1786916965).
+
+[Capacity planning](../../../team/scalability/observability/capacity_planning/) documentation explains how this works in general.