Calculate usable network-attached storage capacity across drives and RAID configuration.
NAS usable capacity starts from raw capacity (disk count × per-disk size), applies the RAID level's efficiency factor to account for parity or mirroring, then subtracts filesystem overhead for inode tables, journaling, and reserved blocks that most filesystems (ext4, XFS, ZFS) set aside. The result is a realistic estimate of the space actually available for files, typically 5-15% below the RAID-adjusted figure.
Usable capacity
usable = disks × disk_size × raid_efficiency% × (1 − filesystem_overhead%)
RAID 5 efficiency is (n-1)/n where n is the disk count in the group — 87.5% for 8 disks. RAID 6 is (n-2)/n — 75% for 8 disks. RAID 10 is always 50% regardless of disk count since every disk is mirrored.
NAS exposes a filesystem directly (NFS/SMB shares) rather than raw blocks, so it always carries filesystem structure overhead — inode tables, journal, superblocks — on top of any RAID overhead, whereas some SAN/block deployments let the host filesystem's overhead be counted separately.
No — this calculates raw usable capacity before any data reduction. Deduplication and compression happen at the data layer on top of usable capacity and their effective ratio depends heavily on data type, so they're modeled separately rather than as a fixed overhead percentage.