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RAID 5 with three 8 TB drives: the smallest RAID 5 and its rebuild exposure

By TechCompare · Updated · How we check

Three 8 TB drives in RAID 5 give 16 TB usable and survive one failure. Its rebuild reads 16 TB, so keep a backup somewhere else.

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RAID Calculator

Usable Capacity
16 TB
Total Capacity
24 TB

Storage Efficiency

67%
Fault Tolerance:

Survives 1 drive failure

This exact 3 × 8000 GB RAID 5 array, pre-loaded. Resize the drives, change the bay count, or compare another level.
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RAID 5 with three 8 TB drives gives you 16 TB of usable space from 24 TB raw. One third of the raw space goes to parity (33%). RAID 5 can't go steeper than that, because it needs at least three drives. The array survives one failure and has no spare margin after that.

How this is calculated

Three drives is the bottom of the RAID 5 range, and every drive you add cuts the parity share. A fourth 8 TB drive brings it down to 25% and lifts usable space to 24 TB.

The rebuild is where three drives get tight. A failed drive is rebuilt by reading the two that are left, 16 TB in all. On a quiet array, one 8 TB drive takes about 14 hours at best. A busy array takes longer.

Those reads carry a risk too. At a spec rate of 1 in 10^14 bits, the chance that a rebuild hits at least one unrecoverable read error is at most about 72.2%. At 1 in 10^15 bits, it's at most about 12.01%. A spec rate is a ceiling, so both figures are upper bounds. Check your drive's datasheet for its rate.

Pick it over a mirror when you need more than one drive's worth of space. Two 8 TB drives in RAID 1 give 8 TB, and this array doubles that to 16 TB. RAID 6 needs four drives, so a three bay box can't step up to it without another bay. If the data matters more than the space, a mirror plus a separate backup can be the better call, since this array has no margin for a second failure.

Three 8 TB drives and two 16 TB drives hold the same 16 TB, so the choice comes down to layout and rebuild behavior.

Verdict

Pick this for a three bay NAS or a small server that already has a backup. Skip it when a second failure during a rebuild would cost you data you can't replace, because the array reads 16 TB during a rebuild and keeps no spare parity.

More RAID scenarios

RAID 0 with 2×2 TB
RAID 0 with two 2 TB drives stripes data across both disks, giving you 4 TB of usable capacity and roughly double the sequential read and write speeds of a single drive.
View details
RAID 1 with 2×4 TB
RAID 1 with two 4 TB drives mirrors all data identically across both disks, giving you 4 TB of usable capacity from 8 TB of raw storage.
View details
RAID 5 with 4×4 TB
RAID 5 with four 4 TB drives uses single distributed parity, giving you 12 TB of usable capacity (3 drives' worth) from 16 TB raw.
View details

Related guides

RAID 5 with 4x4 TB
The same RAID 5 layout with four 4 TB drives and a lower parity share.
View guide
RAID 6 with 4x8 TB
The double parity step up, on the same 8 TB drives.
View guide
RAID 1 with 2x4 TB
The mirror option when you only need one drive's worth of space.
View guide

Frequently asked questions

How much usable space does RAID 5 with 3 drives of 8 TB give?
16 TB, from 24 TB raw. One drive's worth goes to parity, so you keep 2 drives of data. Counted in binary units, that's 14.6 TiB.
How many drives can fail in RAID 5 with three drives?
One. RAID 5 keeps one drive's worth of parity, so it survives a single failure. A second failure before the rebuild finishes loses the array, and the rebuild reads 16 TB to finish.
How long does a RAID 5 rebuild take with 8 TB drives?
On a quiet array, about 14 hours at best for one 8 TB drive, at the calculator's modeled hard drive speed. A busy array takes longer. At a spec rate of 1 in 10^14 bits, the chance of a read error during that rebuild is at most about 72.2%.

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