nixos/hosts/nas/STORAGE.md

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

# 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:

# 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