RAID levels explained: 0, 1, 5, 6 and 10

RAID combines several disks so they act as one. Depending on the level, it makes storage faster, lets it survive a failed disk, or both.

Level	Minimum disks	Survives	Usable space	In short
RAID 0	2	No failures	100%	Speed, no protection
RAID 1	2	All but one disk	50% with two disks	Mirror
RAID 5	3	1 disk	Total minus one disk	Striping with parity
RAID 6	4	2 disks	Total minus two disks	Double parity
RAID 10	4	1 per mirror pair	50%	Mirrors, striped

RAID combines several disks into one, either for speed, for protection against a disk failing, or both. The level number says how. For servers, RAID 1 (two mirrored disks) and RAID 10 (mirrors, striped) are the most common choices. No RAID level is a backup.

RAID 0: striping

Data is split across all disks, so reads and writes are fast. If any one disk fails, everything is lost. Only for data you can rebuild easily.

RAID 1: mirroring

Every disk holds a full copy. Simple and reliable, and reads can be faster. You get the capacity of a single disk. A common choice for a server’s system disks.

RAID 5: parity

Data is striped with parity, extra information that can rebuild any one missing disk. Efficient on space, but writes are slower, and rebuilding a large failed disk takes a long time and strains the remaining ones. During the rebuild, a second failure loses the array.

RAID 6: double parity

Like RAID 5 but survives two failed disks. The safer choice for large arrays of big disks, at the cost of slower writes.

RAID 10: mirrors, then striped

Disks are paired into mirrors, and the mirrors are striped. Fast for both reads and writes, rebuilds quickly, and survives one failure in each pair. The usual choice for busy databases.

Which level to choose

Situation	Choose
Two disks in a small server	RAID 1
A busy database, four or more disks	RAID 10
Large storage where space matters, writes are light	RAID 6
Scratch space or caches you can rebuild	RAID 0
A single cloud VPS	None: the provider’s storage already handles disk failure

RAID 5 was the default choice for years. With today’s large disks, rebuilds take many hours, and the risk of a second failure or a read error during that time is real, so RAID 6 or RAID 10 is now usually preferred for important data.

Worked example: usable space

With four 4 TB disks:

Level	Usable	Can lose
RAID 0	16 TB	Nothing
RAID 5	12 TB	Any 1 disk
RAID 6	8 TB	Any 2 disks
RAID 10	8 TB	1 disk in each pair, so up to 2 if they are in different pairs

RAID 6 and RAID 10 give the same space here; RAID 6 survives any two failures, while RAID 10 is faster and rebuilds more quickly.

Hardware or software RAID

Hardware RAID uses a controller card. Software RAID, such as Linux’s mdadm or ZFS, is done by the operating system and is now the common choice on servers. Check the state of Linux software RAID with:

cat /proc/mdstat

A healthy two-disk mirror looks like this, where [UU] means both disks are up:

md0 : active raid1 sdb1[1] sda1[0]
      976630464 blocks super 1.2 [2/2] [UU]

[U_] means one disk has failed and the array is running on the other. Replace the disk promptly; until you do, there is no protection. For more detail, run sudo mdadm --detail /dev/md0.

Watch the disks, not just the array

RAID only helps if you notice a failure. Set up alerts: mdadm can email when an array degrades (set MAILADDR in /etc/mdadm/mdadm.conf), and smartctl from the smartmontools package reads each disk’s own health data, which often warns before a disk fails:

sudo smartctl -H /dev/sda

On dedicated servers, ask your host whether they monitor disks and replace failed ones automatically.

RAID is not a backup. It protects against a disk failing. It does nothing about deleted files, ransomware, a bad update or a fire. Keep real backups as well.