RAID 5 with four 4 TB drives: capacity, parity penalty, and rebuild-risk reality

How this is calculated

A small partial-stripe write usually requires RAID 5 to read the old data and parity, calculate the new parity, and write the changed data and parity. A full-stripe write can calculate parity from the new data without reading the old stripe. The partial-stripe read-modify-write penalty can slow random writes compared with RAID 0 or RAID 10. The bigger concern in 2026 is rebuild time. A 4 TB drive takes 8-24 hours to rebuild depending on controller speed and array activity. During that window, a second drive failure can destroy the array. With 4 TB and larger drives, some storage administrators prefer RAID 6 for arrays of 4 or more drives.

Verdict

This config provides 12 TB usable from 16 TB raw with single distributed parity. Reads can use the three data drives, but write costs depend on stripe size. A partial-stripe write typically reads old data and parity before calculating and writing the new parity. A full-stripe write can calculate parity from all-new data without that read-modify-write step. The risk that remains during a degraded rebuild is a second drive failure, which RAID 5 cannot tolerate. Replacing a 4 TB drive may take 8-24 hours depending on the system and workload. RAID 6 trades another drive's capacity for tolerance of a second failure.