Calculate usable NAS capacity after RAID redundancy and filesystem overhead.
NAS usable capacity accounts for two layers of overhead: RAID redundancy (parity or mirroring) and filesystem metadata. The RAID level determines how many disks' worth of capacity is consumed by redundancy, and the filesystem (ext4, XFS, Btrfs, ZFS) reserves additional space for metadata, journaling, and internal structures — typically 3-7% depending on configuration.
Usable NAS capacity
usable = raid_capacity × (1 - fs_overhead%)
NAS capacity is reduced by RAID redundancy, filesystem overhead, and often additional reserves for snapshots or system partitions — vendors show usable capacity after all these deductions.
Yes. XFS and ext4 typically use 1-5% for metadata, while copy-on-write filesystems like ZFS and Btrfs may use 5-10% due to additional metadata for checksums, snapshots, and self-healing capabilities.