DS-920+ 20 GB RAM + SSD-Cache: **Production-grade Docker-Swarm Lab Design**
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High-level goal  
• 6 VMs (5 Swarm nodes + 1 gateway)  
• All-in-one on the NAS today, but architected so that **any** node can be migrated to bare metal later.  
• Observability stack is **first-class** (Prometheus, Loki, Grafana).  
• After bake-in period we **down-size RAM** per VM and rely on SSD-cache + ballooning.

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1.  Physical resource envelope
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CPU      : 4 cores / 8 threads (J4125)  
RAM      : 20 GB (16 GB upgrade + 4 GB stock)  
Storage  : 4×HDD in SHR-1 + 2×NVMe read/write SSD cache (RAID-1)  
Network  : 1×1 GbE (bond later if you add a USB-NIC)

Hard limits  
• Max 4 vCPUs per VM to leave headroom for DSM.  
• Plan for **≤ 18 GB VM RAM total** so DSM + containers never swap to HDD.

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2.  VM map (initial “generous” sizing)
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| VM      | vCPU | RAM (GB) | Disk | Role / Notes |
|---------|------|----------|------|--------------|
| d12-gw  | 1    | 1        | 8 GB | Router, DNS, DHCP, jump box |
| d12-m1  | 2    | 4        | 16 GB| Swarm mgr-1 + Prometheus |
| d12-m2  | 2    | 4        | 16 GB| Swarm mgr-2 + Loki |
| d12-m3  | 2    | 4        | 16 GB| Swarm mgr-3 + Grafana |
| d12-w1  | 2    | 3        | 32 GB| Swarm worker-1 |
| d12-w2  | 2    | 3        | 32 GB| Swarm worker-2 |
TOTAL     11   19 GB   ≈ 120 GB thin-provisioned

Disk layout on NAS  
• All VMs on **SSD-cache-backed** volume (QoS = high).  
• Enable **“SSD cache advisor”** → pin VM disks (random I/O) into cache.

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3.  Network topology
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Virtual switches  
• **vs-lan** : 192.168.1.0/24 (eth0 on every VM) – upstream & mgmt.  
• **vs-swarm** : 10.10.10.0/24 (eth1 on every VM) – overlay & control-plane.  

Firewall rules on d12-gw  
• Forward/NAT only if you need Internet from vs-swarm.  
• SSH jump via d12-gw (port 2222 → internal 10.10.10.x:22).  

MTU tuning  
• vs-swarm MTU 1550 → Docker overlay MTU 1450 (leaves 100 bytes for VXLAN).

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4.  Storage layer
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Inside Swarm  
• Local-path-provisioner on each worker (fast SSD-cache) for stateless pods.  
• NFS share on d12-gw (`/srv/nfs`) exported to 10.10.10.0/24 for shared volumes (logs, Prometheus TSDB cold tier).  

Backup policy  
• DSM Snapshot Replication on VM folders nightly.  
• Off-site push via Hyper-Backup to cloud bucket.

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5.  Observability stack (first-class)
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Namespace: `monitoring`

| Service | Placement | Resource limits | Notes |
|---------|-----------|-----------------|-------|
| Prometheus | mgr-1 | 1 vCPU, 2 GB RAM | 15-day retention, 10 GiB PVC |
| Loki | mgr-2 | 1 vCPU, 1 GB RAM | 7-day retention, 5 GiB PVC |
| Grafana | mgr-3 | 1 vCPU, 1 GB RAM | persistent `grafana.db` on NFS |
| node-exporter | global (all 5) | 0.1 vCPU, 64 MB | host metrics |
| cadvisor | global (all 5) | 0.2 vCPU, 128 MB | container metrics |
| promtail | global (all 5) | 0.1 vCPU, 64 MB | forwards to Loki |

Deploy via single compose file (`observability-stack.yml`) using `docker stack deploy`.

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6.  Swarm service placement rules
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```yaml
# managers only run control-plane containers
docker node update --availability drain d12-m{1,2,3}

constraints:
  - node.labels.role==worker   # user workloads
  - node.labels.role==monitor  # monitoring (mgr-*)
```

Label nodes:
```bash
docker node update --label-add role=worker d12-w{1,2}
docker node update --label-add role=monitor d12-m{1,2,3}
```

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7.  Post-bake-in “rightsizing” schedule
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Monitor **Grafana → “VM Memory Utilization”** for 2 weeks.

Typical safe cuts  
| VM      | New RAM | Justification |
|---------|---------|---------------|
| d12-gw  | 512 MB  | Static routes + dnsmasq only |
| d12-m{1,2,3} | 2 GB each | 1 GB OS + 1 GB Prometheus/Loki/Grafana |
| d12-w{1,2} | 2 GB each | 1 GB OS + 1 GB workload burst |
TOTAL after resize ≈ **10.5 GB** (leaves ~8 GB for DSM & SSD-cache buffers).

Use **virtio-balloon** so DSM can reclaim unused RAM dynamically.

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8.  Security & hardening checklist
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✓ TLS on Docker socket (`dockerd --tlsverify`)  
✓ SSH key-only, fail2ban on d12-gw  
✓ `sysctl` hardening: `net.ipv4.ip_forward=1` only on d12-gw, disabled elsewhere  
✓ Secrets via Docker secrets, NOT env-vars  
✓ Weekly `dsym` offline snapshots of VM disks

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9.  Day-2 growth path
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• Add USB-NIC → LACP bond → 2 GbE for DSM + VMs.  
• When CPU becomes bottleneck, migrate workers to bare-metal NUC; keep managers on NAS.  
• Move NFS to dedicated SSD shelf via USB-C enclosure if I/O saturates.

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One-command bootstrap (after VMs exist)
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```bash
# on d12-m1
git clone https://github.com/you/swarm-lab
cd swarm-lab
./scripts/init.sh   # labels nodes, deploys observability stack
```
After 5 minutes you’ll have metrics, logs, and a resource dashboard.

---

Below is a concise, end-to-end walkthrough that takes you from “I have a fresh Linux VM” to “I have a 3-node Docker Swarm running a small demo service.”  
Everything is 100 % CLI-based and works on Debian 12, Ubuntu 22/24, or Alpine with Docker ≥ 27.3 installed .

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1.  Prerequisites
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•  3 Linux hosts (1 manager + 2 workers) on the same L2/L3 network  
•  Docker Engine installed and started on every host  
•  TCP ports **2377**, **7946** (both TCP & UDP), and **4789** (UDP) open between hosts 

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2.  Install Docker (example for Debian/Ubuntu)
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```bash
sudo apt update && sudo apt install -y docker.io
sudo systemctl enable --now docker
sudo usermod -aG docker $USER   # log out & back in
```

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3.  Initialize the swarm (on the manager)
--------------------------------------------------
```bash
# Find the interface that other nodes can reach, e.g. 192.168.1.10
docker swarm init --advertise-addr 192.168.1.10
```
You’ll receive a `docker swarm join` command with a token.  
Example output:
```
Swarm initialized ...  
To add a worker, run:
  docker swarm join --token SWMTKN-1-xxxxx 192.168.1.10:2377
```
Save that token .

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4.  Join the worker nodes
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SSH to each worker and paste the join command:
```bash
docker swarm join --token <token> 192.168.1.10:2377
```
Back on the manager, verify:
```bash
docker node ls
```
You should see 3 nodes in “Ready/Active” state .

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5.  Deploy your first service
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Create a replicated nginx service (3 instances) and expose it on port 80:
```bash
docker service create \
  --name web \
  --replicas 3 \
  --publish 80:80 \
  nginx:latest
```
Check its status:
```bash
docker service ls
docker service ps web
```
Swarm automatically spreads the 3 nginx containers across the three nodes .

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6.  Scale or update with one command
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Scale up:
```bash
docker service scale web=5
```
Rolling update to a new image:
```bash
docker service update --image nginx:1.25-alpine web
```

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7.  Optional: deploy with a stack file
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Save as `demo-stack.yml`:
```yaml
version: "3.9"
services:
  web:
    image: nginx:alpine
    ports:
      - "80:80"
    deploy:
      replicas: 3
```
Deploy:
```bash
docker stack deploy -c demo-stack.yml demo
```
You now have the same nginx cluster expressed declaratively .

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8.  Day-2 commands you’ll use often
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•  List nodes: `docker node ls`  
•  Inspect a service: `docker service inspect --pretty web`  
•  Drain a node (maintenance): `docker node update --availability drain <node>`  
•  Remove a service: `docker service rm web`  
•  Leave the swarm (worker): `docker swarm leave`  
•  Tear down the whole swarm (manager): `docker swarm leave --force`

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Key concepts in 30 seconds
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Node       = Docker host joined to the swarm  
Manager    = schedules tasks & keeps the swarm state  
Worker     = runs the containers (tasks)  
Service    = desired state for a set of containers (image, replicas, ports, etc.)  
Task       = a single container instance scheduled by the swarm  
Stack      = group of services defined in a Compose file 

That’s all you need to get productive. Once you’re comfortable, add secrets, configs, health-checks, and multiple managers for HA.