Using the calculator
List the drives, either as a count, `4 x 8TB`, or one by one, `8TB, 8TB, 12TB`, and pick the level. You get the usable space, how many drives the array can lose, the share of raw space you keep, and how much goes to parity or mirror copies. Mixed sizes like `2x4TB, 2x8TB` work too.
Usable space is given in TB, as drives are sold, and in TiB, the 1,024-based unit Windows and many NAS screens show with the label TB. That's why a 24 TB array appears as about 21.8 "TB" once it's set up.
The levels
RAID 0 stripes data across every drive. You get all the space and the speed, and lose everything if any one drive fails. RAID 1 mirrors: every drive holds the same data, so the array is the size of one drive and survives all but one failing.
RAID 5 stripes with one drive's worth of parity spread across all of them, so it needs at least three drives, gives up one drive's capacity, and survives one failure. RAID 6 uses two drives' worth of parity, needs four, and survives any two. RAID 10 builds mirrored pairs and stripes across them. It keeps half the space and survives one failure for certain, more if they're in different pairs.
ZFS's RAIDZ1, RAIDZ2 and RAIDZ3 match RAID 5, RAID 6 and a triple-parity version in capacity. ZFS keeps some space in reserve, so a real pool shows a little less than the arithmetic here.
Mixed drive sizes
Classic RAID uses every drive only up to the size of the smallest. Three drives of 4, 4 and 8 TB in RAID 5 give 8 TB usable, as if all three were 4 TB, and the extra 4 TB on the big drive sits unused. The calculator shows that waste so you can decide whether to match the drives or use a system that handles mixed sizes.
Inexpensive, then independent
David Patterson, Garth Gibson and Randy Katz at the University of California, Berkeley coined the term in 1987, and presented "A Case for Redundant Arrays of Inexpensive Disks (RAID)" at the SIGMOD conference in June 1988. The point was that several cheap disks working together could beat one expensive one. Once cheap disks were the only kind, the RAID Advisory Board redefined the I as independent.
Questions
How much space does RAID 5 give with 4 x 8 TB?
24 TB, about 21.8 TiB. One drive's worth goes to parity, and the array survives one drive failing.
RAID 5 or RAID 6?
RAID 6 gives up another drive of space to survive a second failure. With large drives, which take a long time to rebuild, that second drive of protection is often worth it.
Is RAID a backup?
No. RAID keeps an array running when a drive fails. A deleted file, ransomware or a failed controller affects every copy at once, so keep a separate backup.
Why does my 8 TB drive show as 7.28 TB?
Drives are sold in decimal terabytes. Your operating system divides by 1,024 three times and still calls the result TB, so 8 TB shows as about 7.28. Nothing is missing.
Sources
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