truenas-migration: note subnet issue for 10GbE traffic

NAS and Proxmox are on the same 10GbE switch but different subnets,
forcing traffic through the router. Need to fix during migration.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/plans/truenas-migration.md

@@ -39,23 +39,17 @@ Expand storage capacity for the main hdd-pool. Since we need to add disks anyway
 - nzbget: NixOS service or OCI container
 - NFS exports: `services.nfs.server`
 
-### Filesystem: BTRFS RAID1
+### Filesystem: Keep ZFS
 
-**Decision**: Migrate from ZFS to BTRFS with RAID1
+**Decision**: Keep existing ZFS pool, import on NixOS
 
 **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
+- **No data migration needed**: Existing ZFS pool can be imported directly on NixOS
+- **Proven reliability**: Pool has been running reliably on TrueNAS
+- **NixOS ZFS support**: Well-supported, declarative configuration via `boot.zfs` and `services.zfs`
+- **BTRFS RAID5/6 unreliable**: Research showed BTRFS RAID5/6 write hole is still unresolved
+- **BTRFS RAID1 wasteful**: With mixed disk sizes, RAID1 wastes significant capacity vs ZFS mirrors
+- Checksumming, snapshots, compression (lz4/zstd) all available
 
 ### Hardware: Keep Existing + Add Disks
 
@@ -69,83 +63,94 @@ Expand storage capacity for the main hdd-pool. Since we need to add disks anyway
 
 **Storage architecture**:
 
-**Bulk storage** (BTRFS RAID1 on HDDs):
-- Current: 6x HDDs (2x16TB + 2x8TB + 2x8TB)
-- Add: 2x new HDDs (size TBD)
+**hdd-pool** (ZFS mirrors):
+- Current: 3 mirror vdevs (2x16TB + 2x8TB + 2x8TB) = 32TB usable
+- Add: mirror-3 with 2x 24TB = +24TB usable
+- Total after expansion: ~56TB usable
 - 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)
+**Decision**: 2x 24TB drives (ordered, arriving 2026-02-21)
 
 ## 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
+1. **Expand ZFS pool** (on TrueNAS):
+   - Install 2x 24TB drives (may need new drive trays - order from abroad if needed)
+   - If chassis space is limited, temporarily replace the two oldest 8TB drives (da0/ada4)
+   - Add as mirror-3 vdev to hdd-pool
+   - Verify pool health and resilver completes
+   - Check SMART data on old 8TB drives (all healthy as of 2026-02-20, no reallocated sectors)
+   - Burn-in: at minimum short + long SMART test before adding to pool
 
-2. **Initial BTRFS pool**:
-   - Install 2 new disks
-   - Create BTRFS filesystem in RAID1
-   - Mount and test NFS exports
+2. **Prepare NixOS configuration**:
+   - Create host configuration (`hosts/nas1/` or similar)
+   - Configure ZFS pool import (`boot.zfs.extraPools`)
+   - Set up services: radarr, sonarr, nzbget, restic-rest, NFS
+   - Configure monitoring (node-exporter, promtail, smartctl-exporter)
 
-3. **Data migration**:
-   - Copy data from TrueNAS ZFS pool to new BTRFS pool over 10GbE
-   - Verify data integrity
+3. **Install NixOS**:
+   - `zfs export hdd-pool` on TrueNAS before shutdown (clean export)
+   - Wipe TrueNAS boot-pool SSDs, set up as mdadm RAID1 for NixOS root
+   - Install NixOS on mdadm mirror (keeps boot path ZFS-independent)
+   - Import hdd-pool via `boot.zfs.extraPools`
+   - Verify all datasets mount correctly
 
-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
+4. **Service migration**:
+   - Configure NixOS services to use ZFS dataset paths
+   - Update NFS exports
+   - Test from consuming hosts
 
-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
+5. **Cutover**:
+   - Update DNS/client mounts if IP changes
+   - Verify monitoring integration
    - Decommission TrueNAS
-   - Repurpose hardware or keep as spare
+
+### Post-Expansion: Vdev Rebalancing
+
+ZFS has no built-in rebalance command. After adding the new 24TB vdev, ZFS will
+write new data preferentially to it (most free space), leaving old vdevs packed
+at ~97%. This is suboptimal but not urgent once overall pool usage drops to ~50%.
+
+To gradually rebalance, rewrite files in place so ZFS redistributes blocks across
+all vdevs proportional to free space:
+
+```bash
+# Rewrite files individually (spreads blocks across all vdevs)
+find /pool/dataset -type f -exec sh -c '
+  for f; do cp "$f" "$f.rebal" && mv "$f.rebal" "$f"; done
+' _ {} +
+```
+
+Avoid `zfs send/recv` for large datasets (e.g. 20TB) as this would concentrate
+data on the emptiest vdev rather than spreading it evenly.
+
+**Recommendation**: Do this after NixOS migration is stable. Not urgent - the pool
+will function fine with uneven distribution, just slightly suboptimal for performance.
 
 ### 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
+- **No data migration**: ZFS pool imported directly, no copying terabytes of data
+- **Low risk**: Pool expansion done on stable TrueNAS before OS swap
+- **Reversible**: Can boot back to TrueNAS if NixOS has issues (ZFS pool is OS-independent)
+- **Quick cutover**: Once NixOS config is ready, the OS swap is fast
 
 ## 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)
+1. ~~Decide on disk size~~ - 2x 24TB ordered
+2. Install drives and add mirror vdev to ZFS pool
+3. Check SMART data on 8TB drives - decide whether to keep or retire
+4. Design NixOS host configuration (`hosts/nas1/`)
+5. Document NFS export mapping (current -> new)
+6. Plan NixOS installation and cutover
 
 ## 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?
+- [ ] IP address/subnet: NAS and Proxmox are both on 10GbE to the same switch but different subnets, forcing traffic through the router (bottleneck). Move to same subnet during migration.
+- [x] Boot drive: Reuse TrueNAS boot-pool SSDs as mdadm RAID1 for NixOS root (no ZFS on boot path)
+- [ ] Retire old 8TB drives? (SMART looks healthy, keep unless chassis space is needed)
+- [ ] Drive trays: do new 24TB drives fit, or order trays from abroad?
+- [ ] Timeline/maintenance window for NixOS swap?