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RAID levels explained

RAID joins several drives into one volume. Each level trades usable space, speed and how many drive failures the volume survives. This page lists the trade for each level as one source gives it.

Checked
2026-10-16
Source
backblaze.com
Sentences checked
38

Read on 2026-10-16. Level names and drive minimums vary by product. Your NAS maker's own pages decide what your device offers.

What RAID is

Backblaze, in its post “NAS RAID Levels Explained: RAID Types and Benefits” (April 11, 2025), says that RAID stands for redundant array of independent disks (or “inexpensive disks” depending on who you ask). It adds that both NAS devices and RAID are disk arrays, that is, a set of several hard disk drives (HDDs) and/or solid state drives (SSDs) that store large amounts of data, orchestrating the drives to work as one unit. It also says that RAID fault tolerance refers to the ability of a RAID configuration to continue functioning even in the event of a hard disk failure.

Three techniques

The same post says that striping distributes your data over multiple drives, but striping doesn't provide any redundancy whatsoever. It adds that in a striped array, which has no redundancy, if a single drive fails in the storage array, all data on the device can be lost. It also says that mirroring makes a copy of your data, because data is written simultaneously to two disks, thereby providing redundancy by having two copies of the data. The same post says that the disadvantage of mirroring is that it requires twice as many disks for the same total storage capacity. It adds that parity is all about error detection and correction, and parity is commonly used in RAID 5 and RAID 6, where the latter uses double parity, meaning that two sets of parity data are stored for additional protection. It also says that with parity, as in RAID 5 and RAID 6, if a drive fails, you can install a new drive and the NAS device will restore your files based on the previously created ECC.

Levels side by side

The summary table in the same post lists the storage method and fault tolerance of each level.

LevelStorage methodFault tolerance
JBODJust a bunch of disksNone
RAID 0Block-level stripingNone
RAID 1MirroringOne drive
RAID 5Block-level striping with distributed parityOne drive
RAID 6Block-level striping with dual distributed parityTwo drives
RAID 1+0 (RAID 10)Block-level striping with mirroringOne drive per mirrored set

Usable space and minimum drives

Backblaze, in the same post, says that 67% of a three-drive RAID 5 array's disk space is available for data, compared with just 50% of the RAID 1 array's disk space. It adds that increasing the number of drives in the RAID 5 array to four increases the available space to 75%. It also says that with a four-drive RAID 6 array, 50% of the storage is available for data, and a five-drive RAID 6 array allows use of 60% of the storage. The same post says that a RAID 5 array requires a minimum of three disks. It adds that RAID 1+0 requires a minimum of four disks, and only 50% of the total disk space is usable. It also says that it's not uncommon for NAS vendors to offer proprietary RAID configurations offering features such as the ability to combine different sizes of disks into a single array.

What each level gives up

Backblaze, in the same post, says that RAID 0 doesn't provide fault tolerance or redundancy, and the failure of one drive can cause the entire storage array to fail, resulting in total loss of data. It adds that with an annual failure rate of 1% per drive, the two-drive RAID 0 array is twice as likely to fail as a single disk. It also says that after a RAID 1 disk fails, the array can be repaired by installing a replacement disk and copying all the data from the remaining drive, but there is still a small chance that the remaining disk will fail before the copy is complete. The same post says that RAID 5 can only tolerate one disk failure, and if two disks fail, data will be lost. It adds that with RAID 6, due to dual parity, write transactions are slow, and it also takes longer to repair the array because of its complex structure.

Choosing a level for a NAS

Backblaze, in its NAS buying guide, says that RAID 6 can survive the loss of two drives, and delivers performance nearly equal to RAID 5 with better protection. It adds that when a drive fails and you replace it with a fresh drive, the rebuilding process to restore that drive's data and parity information can take a long time, and though it's rare, it's possible to have another drive fail during the rebuilding process.

The RAID post adds that a common approach is to buy a NAS device that has space for a “hot spare”, so that the replacement drive is always ready for action.

The storage calculator shows how many drives each level takes for a given amount of data.

RAID is not a backup

Backblaze, in the same RAID post, says that while different RAID levels offer different levels of data redundancy, they are not enough to provide complete data protection for NAS devices. It adds that RAID does not protect against the broader range of events that could result in data loss, including natural disasters, theft, or ransomware attacks. It also says that RAID does not protect against user error either, because if you inadvertently delete an important file from your NAS device, it's gone from that array.

Where the other copies go is covered in NAS or external hard drive.

Sources

Checked on 2026-10-16 against Backblaze: NAS RAID Levels Explained, Backblaze: NAS Buying Guide. Read the source again before you rely on a number, because posts get updated and drive models change.