There are some common misconceptions on RAID levels with their level of redundancy, etc. I thought I would write a quick guide with some information on them.
RAID0
Striped data. There is no level of redundancy, any loss of a drive will result in a loss of the array. Striped data incurs no storage overhead, you get access to all of the raw storage.
RAID1
Mirrored data. Data is mirrored across all devices so all devices contain the same data preventing data loss from the loss of a device. As long as one device is in tact, the data is safe. Most RAID1s consist of a 2-drive mirror, therefore they can withstand 1 drive loss.
RAID10
A RAID10, also known as RAID1+0, is a stripe of mirrors. The loss of any mirrored pair will result in the loss of the array.
Examples:
4-drive Typical RAID10:
RAID0+1
Less commonly used, the opposite of a RAID10, a mirror of stripes. More complex to manage and rebuild therefore RAID10 is recommended.
RAID5 / RAIDZ / RAID-1P
A parity array with 1 parity calculation. Parity arrays use calculations to allow for the failure of any drive in the array. One parity calculation allows for one drive failure before the loss of the array. RAID5 is generally no longer recommended due to larger drives and therefore longer rebuild times. The chance of an unreadable error or second drive failure during the rebuild is high which would result in data corruption or the failure of the entire array. RAID5 may still be reasonable for small drives (SAS/SSD).
Example:
RAID50
A stripe of RAID5s. Loss of any RAID5 array will result in failure. Contains the same high risk of failure during rebuilds as RAID5.
RAID6 / RAIDZ2 / RAID-2P
A parity array with 2 parity calculations. Parity arrays use calculations to allow for the failure of any drive in the array. Two parity calculations allow for any two drive failures before the loss of the array.
A RAID6 can consist of 4 or more drives and has only a storage overhead of two drives. The higher number of drives, the higher the risk of failure.
RAID60
A stripe of RAID6s. Loss of any RAID6 array will result in failure. Can withstand a loss of two drives in each RAID6.
RAID7 / RAIDZ3 / RAID-3P
A parity array with 3 parity calculations. Parity arrays use calculations to allow for the failure of any drive in the array. Three parity calculations allow for any three drive failures before the loss of the array.
A RAID7 can consist of 5 or more drives and has only a storage overhead of three drives. The higher number of drives, the higher the risk of failure.
So which is the safest RAID level?
It depends on circumstances and how they are configured. RAID1 with more than 2 drives is probably the safest but also contains a huge storage overhead. A RAID10 with 6 drives (3 mirrors) could withstand 3 drive failures but it could also fail if 2 drives in the same mirror fail.
RAID6, RAID60, and RAID7 are the only typical configurations that can withstand the loss of any two drives.
RAID60 and RAID7 are becoming the recommended configuration for very large storage systems. This is due to the fact that when a drive fails and you attempt to rebuild a large array, it can take a very long time and adds stress to the other drives in the array. Therefore you want to be able to withstand multiple drive failures within the same array.
RAID0
Striped data. There is no level of redundancy, any loss of a drive will result in a loss of the array. Striped data incurs no storage overhead, you get access to all of the raw storage.
RAID1
Mirrored data. Data is mirrored across all devices so all devices contain the same data preventing data loss from the loss of a device. As long as one device is in tact, the data is safe. Most RAID1s consist of a 2-drive mirror, therefore they can withstand 1 drive loss.
RAID10
A RAID10, also known as RAID1+0, is a stripe of mirrors. The loss of any mirrored pair will result in the loss of the array.
Examples:
4-drive Typical RAID10:
- RAID0 Stripe
RAID1: 2 Pair Mirror - RAID1: 2 Pair Mirror
- RAID0 Stripe
RAID1: 2 Pair Mirror - RAID1: 2 Pair Mirror
- RAID1: 2 Pair Mirror
- RAID0 Stripe
RAID1: 3 Drive Mirror - RAID1: 3 Drive Mirror
RAID0+1
Less commonly used, the opposite of a RAID10, a mirror of stripes. More complex to manage and rebuild therefore RAID10 is recommended.
RAID5 / RAIDZ / RAID-1P
A parity array with 1 parity calculation. Parity arrays use calculations to allow for the failure of any drive in the array. One parity calculation allows for one drive failure before the loss of the array. RAID5 is generally no longer recommended due to larger drives and therefore longer rebuild times. The chance of an unreadable error or second drive failure during the rebuild is high which would result in data corruption or the failure of the entire array. RAID5 may still be reasonable for small drives (SAS/SSD).
Example:
- Drive 1:
Block 1 - Block 2
- Parity 3
- Block 1
- Parity 2
- Block 3
- Parity 1
- Block 2
- Block 3
RAID50
A stripe of RAID5s. Loss of any RAID5 array will result in failure. Contains the same high risk of failure during rebuilds as RAID5.
RAID6 / RAIDZ2 / RAID-2P
A parity array with 2 parity calculations. Parity arrays use calculations to allow for the failure of any drive in the array. Two parity calculations allow for any two drive failures before the loss of the array.
A RAID6 can consist of 4 or more drives and has only a storage overhead of two drives. The higher number of drives, the higher the risk of failure.
RAID60
A stripe of RAID6s. Loss of any RAID6 array will result in failure. Can withstand a loss of two drives in each RAID6.
RAID7 / RAIDZ3 / RAID-3P
A parity array with 3 parity calculations. Parity arrays use calculations to allow for the failure of any drive in the array. Three parity calculations allow for any three drive failures before the loss of the array.
A RAID7 can consist of 5 or more drives and has only a storage overhead of three drives. The higher number of drives, the higher the risk of failure.
So which is the safest RAID level?
It depends on circumstances and how they are configured. RAID1 with more than 2 drives is probably the safest but also contains a huge storage overhead. A RAID10 with 6 drives (3 mirrors) could withstand 3 drive failures but it could also fail if 2 drives in the same mirror fail.
RAID6, RAID60, and RAID7 are the only typical configurations that can withstand the loss of any two drives.
RAID60 and RAID7 are becoming the recommended configuration for very large storage systems. This is due to the fact that when a drive fails and you attempt to rebuild a large array, it can take a very long time and adds stress to the other drives in the array. Therefore you want to be able to withstand multiple drive failures within the same array.
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