docs: add truenas-migration plan

docs/plans/truenas-migration.md

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+# TrueNAS Migration Planning
+
+## Current State
+
+### Hardware
+- CPU: AMD Ryzen 5 5600G with Radeon Graphics
+- RAM: 32GB
+- Network: 10GbE (mlxen0)
+- Software: TrueNAS-13.0-U6.1 (Core)
+
+### Storage Status
+
+**hdd-pool**: 29.1TB total, **28.4TB used, 658GB free (97% capacity)** ⚠️
+- mirror-0: 2x Seagate ST16000NE000 16TB HDD (16TB usable)
+- mirror-1: 2x WD WD80EFBX 8TB HDD (8TB usable)
+- mirror-2: 2x Seagate ST8000VN004 8TB HDD (8TB usable)
+
+## Goal
+
+Expand storage capacity for the main hdd-pool. Since we need to add disks anyway, also evaluating whether to upgrade or replace the entire system.
+
+## Decisions
+
+### Migration Approach: Option 3 - Migrate to NixOS
+
+**Decision**: Replace TrueNAS with NixOS bare metal installation
+
+**Rationale**:
+- Aligns with existing infrastructure (16+ NixOS hosts already managed in this repo)
+- Declarative configuration fits homelab philosophy
+- Automatic monitoring/logging integration (Prometheus + Promtail)
+- Auto-upgrades via same mechanism as other hosts
+- SOPS secrets management integration
+- TrueNAS-specific features (WebGUI, jails) not heavily utilized
+
+**Service migration**:
+- radarr/sonarr: Native NixOS services (`services.radarr`, `services.sonarr`)
+- restic-rest: `services.restic.server`
+- nzbget: NixOS service or OCI container
+- NFS exports: `services.nfs.server`
+
+### Filesystem: BTRFS RAID1
+
+**Decision**: Migrate from ZFS to BTRFS with RAID1
+
+**Rationale**:
+- **In-kernel**: No out-of-tree module issues like ZFS
+- **Flexible expansion**: Add individual disks, not required to buy pairs
+- **Mixed disk sizes**: Better handling than ZFS multi-vdev approach
+- **RAID level conversion**: Can convert between RAID levels in place
+- Built-in checksumming, snapshots, compression (zstd)
+- NixOS has good BTRFS support
+
+**BTRFS RAID1 notes**:
+- "RAID1" means 2 copies of all data
+- Distributes across all available devices
+- With 6+ disks, provides redundancy + capacity scaling
+- RAID5/6 avoided (known issues), RAID1/10 are stable
+
+### Hardware: Keep Existing + Add Disks
+
+**Decision**: Retain current hardware, expand disk capacity
+
+**Hardware to keep**:
+- AMD Ryzen 5 5600G (sufficient for NAS workload)
+- 32GB RAM (adequate)
+- 10GbE network interface
+- Chassis
+
+**Storage architecture**:
+
+**Bulk storage** (BTRFS RAID1 on HDDs):
+- Current: 6x HDDs (2x16TB + 2x8TB + 2x8TB)
+- Add: 2x new HDDs (size TBD)
+- Use: Media, downloads, backups, non-critical data
+- Risk tolerance: High (data mostly replaceable)
+
+**Critical data** (small volume):
+- Use 2x 240GB SSDs in mirror (BTRFS or ZFS)
+- Or use 2TB NVMe for critical data
+- Risk tolerance: Low (data important but small)
+
+### Disk Purchase Decision
+
+**Options under consideration**:
+
+**Option A: 2x 16TB drives**
+- Matches largest current drives
+- Enables potential future RAID5 if desired (6x 16TB array)
+- More conservative capacity increase
+
+**Option B: 2x 20-24TB drives**
+- Larger capacity headroom
+- Better $/TB ratio typically
+- Future-proofs better
+
+**Initial purchase**: 2 drives (chassis has space for 2 more without modifications)
+
+## Migration Strategy
+
+### High-Level Plan
+
+1. **Preparation**:
+   - Purchase 2x new HDDs (16TB or 20-24TB)
+   - Create NixOS configuration for new storage host
+   - Set up bare metal NixOS installation
+
+2. **Initial BTRFS pool**:
+   - Install 2 new disks
+   - Create BTRFS filesystem in RAID1
+   - Mount and test NFS exports
+
+3. **Data migration**:
+   - Copy data from TrueNAS ZFS pool to new BTRFS pool over 10GbE
+   - Verify data integrity
+
+4. **Expand pool**:
+   - As old ZFS pool is emptied, wipe drives and add to BTRFS pool
+   - Pool grows incrementally: 2 → 4 → 6 → 8 disks
+   - BTRFS rebalances data across new devices
+
+5. **Service migration**:
+   - Set up radarr/sonarr/nzbget/restic as NixOS services
+   - Update NFS client mounts on consuming hosts
+
+6. **Cutover**:
+   - Point consumers to new NAS host
+   - Decommission TrueNAS
+   - Repurpose hardware or keep as spare
+
+### Migration Advantages
+
+- **Low risk**: New pool created independently, old data remains intact during migration
+- **Incremental**: Can add old disks one at a time as space allows
+- **Flexible**: BTRFS handles mixed disk sizes gracefully
+- **Reversible**: Keep TrueNAS running until fully validated
+
+## Next Steps
+
+1. Decide on disk size (16TB vs 20-24TB)
+2. Purchase disks
+3. Design NixOS host configuration (`hosts/nas1/`)
+4. Plan detailed migration timeline
+5. Document NFS export mapping (current → new)
+
+## Open Questions
+
+- [ ] Final decision on disk size?
+- [ ] Hostname for new NAS host? (nas1? storage1?)
+- [ ] IP address allocation (keep 10.69.12.50 or new IP?)
+- [ ] Timeline/maintenance window for migration?