# NAS Storage Notes

## Current Issue: XFS Metadata Overhead

The XFS filesystem on `/var/lib/multimedia` uses ~100GB more than the actual file data due to metadata overhead.

### Root Cause

The filesystem was created with advanced features enabled:

```
rmapbt=1    # Reverse mapping btree - tracks block ownership
reflink=1   # Copy-on-write support
```

These features add metadata that scales with **filesystem size**, not file count. On a 5TB filesystem with 700GB of data, this results in ~100GB (~2%) overhead.

### Diagnostic Commands

```bash
# Compare file data vs filesystem usage
du -sh /var/lib/multimedia           # Actual file data
df -h /var/lib/multimedia            # Filesystem reports

# Check XFS features
xfs_info /var/lib/multimedia

# Verify block allocation
xfs_db -r -c "freesp -s" /dev/mapper/vg--data-lv--multimedia
```

## Recommendation: LVM + ext4

For media storage (write-once, read-many), ext4 with minimal reserved space offers the lowest overhead:

```bash
# Create filesystem with 0% reserved blocks
mkfs.ext4 -m 0 /dev/vg/lv

# Or adjust existing ext4
tune2fs -m 0 /dev/vg/lv
```

### Why ext4 over XFS for this use case

| Consideration | ext4 | XFS (current) |
|---------------|------|---------------|
| Reserved space | 0% with `-m 0` | N/A |
| Metadata overhead | ~0.5% | ~2% (with rmapbt) |
| Shrink support | Yes | No |
| Performance for 4K stream | Identical | Identical |

A single 4K remux stream requires ~12 MB/s. Any filesystem handles this trivially.

## Migration Path

1. Backup data from XFS volumes
2. Recreate LVs with ext4 (`mkfs.ext4 -m 0`)
3. Restore data
4. Update `/etc/fstab` or NixOS `fileSystems` config