Add in-memory kustomize builds advice to scaling docs

Signed-off-by: Stefan Prodan <[email protected]>

content/en/flux/installation/configuration/vertical-scaling.md

@@ -9,6 +9,7 @@ When Flux is managing hundreds of applications that are deployed multiple times
 can fine tune the Flux controller at [bootstrap time](boostrap-customization.md) to run at scale by:
 
 - [Increasing the number of workers and resource limits](#increase-the-number-of-workers-and-limits)
+- [Enabling in-memory kustomize builds](#enable-in-memory-kustomize-builds)
 - [Enabling Helm repository caching to reduce memory usage](#enable-helm-repositories-caching)
 - [Enabling persistent storage for internal artifacts](#persistent-storage-for-flux-internal-artifacts)
 - [Running the Flux controllers on dedicated nodes](#node-affinity-and-tolerations)
@@ -33,7 +34,7 @@ patches:
   - patch: |
       - op: add
         path: /spec/template/spec/containers/0/args/-
-        value: --concurrent=20
+        value: --concurrent=10
       - op: add
         path: /spec/template/spec/containers/0/args/-
         value: --requeue-dependency=5s
@@ -59,6 +60,39 @@ patches:
       name: "(kustomize-controller|helm-controller|source-controller)"
 ```
 
+## Enable in-memory kustomize builds
+
+To speed up the Flux kustomize build operations, it is advised to use tmpfs for the `/tmp` filesystem:
+
+```yaml
+apiVersion: kustomize.config.k8s.io/v1beta1
+kind: Kustomization
+resources:
+  - gotk-components.yaml
+  - gotk-sync.yaml
+patches:
+  - patch: |
+      - op: add
+        path: /spec/template/spec/containers/0/args/-
+        value: --concurrent=20
+      - op: replace
+        path: /spec/template/spec/volumes/0
+        value:
+          name: temp
+          emptyDir:
+            medium: Memory
+    target:
+      kind: Deployment
+      name: kustomize-controller
+```
+
+{{% alert color="info" title="“I/O Device Contention" %}}
+Note that increase the number of reconciliations for kustomize-controller, without using a
+RAM-backed filesystem, may not have the desired effect due to IO contention
+on the Kubernetes node disk.
+{{% /alert %}}
+
+
 ## Enable Helm repositories caching
 
 If Flux connects to Helm repositories hosting hundreds of Helm charts,
@@ -90,7 +124,6 @@ When `helm-cache-max-size` is reached, an error is logged and the index is inste
 read from file. Cache hits are exposed via the `gotk_cache_events_total` Prometheus
 metrics. Use this data to fine-tune the configuration flags.
 
-
 ## Persistent storage for Flux internal artifacts
 
 Flux maintains a local cache of artifacts acquired from external sources.