docs: Update docs for the correctness (#13)

services/docs/content/en/root/concepts/tiered-storage.md

@@ -8,37 +8,67 @@ The framework enforces a standardized storage taxonomy. This ensures that every
 
 ## The storage taxonomy {#taxonomy}
 
-Data is divided into three logical [Tiers](/root/glossary#tier) based on its lifecycle, volume, and importance.
+Data is divided into three logical [Tiers](/root/glossary#tier) based on its performance requirements, lifecycle, and importance.
 
-### Tier 1: Configuration & critical state
+### Tier 1: Configuration & Cache
 
 **Path:** `${ROOT_NODE_DIR}/@tier1`
 
-- **Contents**: Config files, small SQLite databases, SSL certificates.
-- **Characteristics**: Extremely high value, low volume.
-- **Backup strategy**: Frequent (hourly/daily), long retention.
+- **Contents**: Config files, small SQLite databases, SSL certificates, application cache.
+- **Characteristics**: Extremely high value, low to medium volume, requires low latency.
+- **Hardware**: High-speed SSD storage, typically shared with the Operating System.
 
-### Tier 2: Application data
+### Tier 2: Application Data
 
 **Path:** `${ROOT_NODE_DIR}/@tier2`
 
 - **Contents**: User uploads, large databases, persistent application state.
-- **Characteristics**: High value, medium to high volume.
-- **Backup strategy**: Daily, prioritized for integrity.
+- **Characteristics**: High value, medium to high volume, requires high reliability.
+- **Hardware**: Reliable HDD clusters, RAID arrays, or dedicated storage compute.
 
-### Tier 3: Transients & bulk media
+### Tier 3: Transients & Bulk Media
 
 **Path:** `${ROOT_NODE_DIR}/@tier3`
 
-- **Contents**: Logs, caches, temporary files, bulk media downloads.
-- **Characteristics**: Low value, high volume.
-- **Backup strategy**: Minimal or none. These directories are typically excluded from sync operations.
+- **Contents**: Logs, temporary files, bulk media (movies, music), and other non-critical large data.
+- **Characteristics**: Low value, very high volume.
+- **Hardware**: High-capacity HDDs optimized for continuous sequential write/read (e.g., WD Purple). Note that Tier 3 is **too slow for cache**.
+
+## Backup strategy {#backup}
+
+The framework does not enforce a specific backup strategy or provide a default implementation, allowing for complete node-specific flexibility. Operators are expected to implement their own backup logic.
+
+### Example: Restic backup
+
+A common best practice is to use **Restic** for backups. For example, you might create an operator compose file to run Restic:
+
+```yaml filename="operator-compose.yml"
+services:
+  restic:
+    image: restic/restic:0.18.1
+    container_name: operator-restic
+    volumes:
+      - ./@tier2/{project}/{service}/{data}:/data/@tier2/{project}/{service}/{data}:ro
+      - ./@tier1/{project}/{service}/{data}:/data/@tier1/{project}/{service}/{data}:ro
+      - ./@tier1/.operator/restic/cache:/var/restic-cache
+    environment:
+      RESTIC_REPOSITORY: "s3:example.com/bucket"
+      RESTIC_PASSWORD: ${RESTIC_PASSWORD}
+      # ... AWS/S3 credentials
+    network_mode: "host"
+    entrypoint: [ "restic" ]
+    user: "0:0"
+```
+
+### Implementing the strategy
+
+Operators can implement this strategy by defining a local action in `.operator/local/scripts/actions/sync/tiers`. The `root` dispatcher can then execute this script to trigger the backup, allowing for granular control over how data is persisted.
 
 ## Justification {#justification}
 
-1. **Backup efficiency**: By separating logs (Tier 3) from configs (Tier 1), we avoid wasting storage space on redundant backups of transient data.
-2. **Hardware optimization**: Tier 1 and 2 can be placed on fast SSD storage, while Tier 3 can be mounted on slower, high-capacity HDDs.
-3. **Instant understanding**: Any operator can enter a [Node](/root/glossary#node) directory and immediately know where the critical data resides.
+1. **Hardware optimization**: By mapping tiers to specific hardware (SSDs for Tier 1, RAID for Tier 2, Purples for Tier 3), we optimize for both performance and cost.
+2. **Selective backup**: Separating transient logs and bulk media (Tier 3) from critical configs (Tier 1) prevents wasting expensive backup storage.
+3. **Architectural clarity**: Any operator can inspect a [Node](/root/glossary#node) directory and immediately understand the storage requirements and priority of its contents.
 
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