doc/support: detail the 3 levels of LTS support (#14562)

.github/workflows/tests.yml

@@ -602,7 +602,7 @@ jobs:
 
   documentation:
     name: Documentation
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-24.04
     steps:
       - name: Checkout
         uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
@@ -658,7 +658,7 @@ jobs:
 
   snap:
     name: Trigger snap edge build
-    runs-on: ubuntu-22.04
+    runs-on: ubuntu-24.04
     needs: [code-tests, system-tests, client, documentation, ui-e2e-tests]
     if: ${{ github.repository == 'canonical/lxd' && github.event_name == 'push' && github.actor != 'dependabot[bot]' }}
     steps:

doc/howto/snap.md

@@ -83,6 +83,7 @@ Specify a refresh window
 
   See [Control updates with system options](https://snapcraft.io/docs/managing-updates#heading--refresh-hold) in the snap documentation for detailed information.
 
+(howto-snap-cohort)=
 #### Keep cluster members in sync
 
 The cluster members that are part of the LXD deployment must always run the same version of the LXD snap.

doc/installing.md

@@ -64,6 +64,20 @@ Complete the following steps to install the snap:
 
         sudo snap install lxd --channel=5.0/stable
 
+```{note}
+LXD cluster members all need to use the exact same version of LXD and this requires special consideration due to how snaps are deployed as explained in {ref}`Keep cluster members in sync<howto-snap-cohort>`. To ensure all the cluster members are offered the same snap revision, it is possible to specify a cohort parameter which will bypass the progressive deployment:
+
+        snap install lxd --cohort="+"
+
+This can also be specified during refreshes (not required if done at install time):
+
+        snap refresh lxd --cohort="+"
+
+If for some reason, the cohort mechanism did not work as expected, it is also possible to install a specific snap revision that matches that used on all the cluster members:
+
+        snap install lxd --revision=<revision_number>
+```
+
 For more information about LXD snap packages (regarding more versions, update management etc.), see [Managing the LXD snap](https://discourse.ubuntu.com/t/managing-the-lxd-snap-package/37214).
 
 ```{note}

doc/support.md

@@ -8,23 +8,33 @@ LXD maintains different release branches in parallel.
 Long term support (LTS) releases
 : The current LTS releases are LXD 5.21.x (snap channel `5.21/stable` - this is the default channel), LXD 5.0.x (snap channel `5.0/stable`) and LXD 4.0.x (snap channel `4.0/stable`).
 
+  An LTS release starts its life with full support, then moves to maintenance support, and finally to extended support. OS vendors might provide additional support[^1].
+
+  - Full support: some new features, frequent bugfixes, and security updates are provided.
+  - Maintenance support: high impact bugfixes and security updates are provided.
+  - Extended support: only high and critical security updates are provided.
+
   The LTS releases follow the Ubuntu release schedule and are released every two years:
 
   - LXD 5.21 is supported until June 2029.
     It gets frequent bugfix and security updates, but does not receive any feature additions.
     Updates to this release happen approximately every six months, but this schedule should be seen as a rough estimation that can change based on priorities and discovered bugs.
+    Currently in full support phase.
   - LXD 5.0 is supported until June 2027.
+    Currently in maintenance support phase.
   - LXD 4.0 is supported until June 2025.
+    Currently in extended support phase.
 
 Feature releases
-: After LXD 5.21 is released, the next feature release will be LXD 6.x (starting with 6.1).
-  It is available through the snap channels `latest/stable`, `latest/candidate`, and `latest/edge`, in addition to channels for the most recent specific releases (for example, `6.1/stable`).
+: After LXD 5.21 is released, the next feature release will be LXD 6.x (starting with 6.1), available from the `6/stable` channel.
   See `snap info lxd` for a full list of available channels.
 
   Feature releases are pushed out about every month and contain new features as well as bugfixes.
   The normal support length for those releases is until the next release comes out.
   Some Linux distributions might offer longer support for particular feature releases that they decided to ship.
 
+[^1]: Canonical provides additional support through the [Ubuntu Pro](https://ubuntu.com/pro) offering.
+
 <!-- Include end release -->
 
 % Include content from [../README.md](../README.md)