[Docs] add dqlite configuration to troubleshooting page (#1022)

* add dqlite configuration to troubleshooting page

Co-authored-by: Louise K. Schmidtgen <[email protected]>

docs/canonicalk8s/styles/config/vocabularies/Canonical/accept.txt

@@ -376,3 +376,4 @@ DTMs
 cybersecurity
 uid
 runc
+unoptimized

docs/src/snap/reference/troubleshooting.md

@@ -112,6 +112,47 @@ containerd-base-dir: $containerdBaseDir
 EOF
 ```
 
+## Increased memory usage in Dqlite
+
+### Problem
+
+The datastore used for {{product}} Dqlite, reported an [issue #196] of increased
+memory usage over time. This was particularly evident in smaller clusters.
+
+### Explanation
+
+This issue was caused due to an inefficient resource configuration of 
+Dqlite for smaller clusters. The threshold and trailing parameters are 
+related to Dqlite transactions and must be adjusted. The threshold is 
+the number of transactions we allow before a snapshot is taken of the 
+leader. The trailing is the number of transactions we allow the follower 
+node to lag behind the leader before it consumes the updated snapshot of the 
+leader. Currently, the default snapshot configuration is 1024 for the
+threshold and 8192 for trailing which is too large for small clusters. Only
+setting the trailing parameter in a configuration yaml automatically sets the
+threshold to 0. This leads to a snapshot being taken every transaction and 
+increased CPU usage.
+
+### Solution
+
+Apply a tuning.yaml custom configuration to the Dqlite datastore in order to
+adjust the trailing and threshold snapshot values. The trailing parameter
+should be twice the threshold value. Create the tuning.yaml
+file and place it in the Dqlite directory
+`/var/snap/k8s/common/var/lib/k8s-dqlite/tuning.yaml`:
+
+```
+snapshot:
+  trailing: 1024
+  threshold: 512
+```
+
+Restart Dqlite:
+
+```
+sudo snap restart snap.k8s.k8s-dqlite
+```
+
 <!-- LINKS -->
 
 [lxd-install]: ../howto/install/lxd.md
@@ -120,3 +161,4 @@ EOF
 [kubernetes-125923]: https://github.com/kubernetes/kubernetes/pull/125923
 [kubernetes-122955-2020403422]: https://github.com/kubernetes/kubernetes/issues/122955#issuecomment-2020403422
 [@haircommander]: https://github.com/haircommander
+[issue #196]: https://github.com/canonical/k8s-dqlite/issues/196#issuecomment-2621527026