Sign in with
Sign up | Sign in
Your question

RAID FAQ

Tags:
  • Homebuilt
  • NAS / RAID
  • Performance
  • Systems
Last response: in Systems
Share
April 7, 2007 2:36:08 PM

Standard RAID Levels

RAID 0:
Striped Set (2 disk minimum) without parity. Provides improved performance and additional storage but no fault tolerance from disk errors or disk failure. Any disk failure destroys the array, and is twice as likely to fail.

RAID 1:
Mirrored Set (2 disks minimum, and usually exactly 2) without parity. Provides fault tolerance from disk errors and single disk failure. Increased read performance occurs when using a multi-threaded operating system that supports split seeks, very small performance reduction when writing. Array continues to operate with one failed drive.

RAID 0+1:
Striped Set + Mirrored Set (4 disk minimum; Even number of disks). Provides fault tolerance and improved performance but increases complexity. Array continues to operate with one failed drive. The key difference from RAID 1+0 is that RAID 0+1 creates a second striped set to mirror a primary striped set.

RAID 1+0:
Mirrored Set + Striped Set (4 disk minimum; Even number of disks). Provides fault tolerance and improved performance but increases complexity. Array continues to operate with one failed drive. The key difference from RAID 0+1 is that RAID 1+0 creates a striped set from a series of mirrored drives.

RAID 3 and RAID 4:
Striped Set (3 disk minimum) with Dedicated Parity.
Provides improved performance and fault tolerance similar to RAID 5, but with a dedicated parity disk rather than rotated parity stripes. The single disk is a bottle-neck for writing since every write requires updating the parity data. One minor benefit is the dedicated parity disk allows the parity drive to fail and operation will continue without parity or performance penalty.

RAID 5:
Striped Set (3 disk minimum) with Distributed Parity.
Distributed parity requires all but one drive to be present to operate. Drive failure requires replacement, but the array is not destroyed by a single drive failure. Upon drive failure, any subsequent reads can be calculated from the distributed parity such that the drive failure is masked from the end user. The array will have data loss in the event of a second drive failure and is vulnerable until the data that was on the failed drive is rebuilt onto a replacement drive.

RAID 6:
Striped Set (4 disk minimum) with Dual Distributed Parity.
Provides fault tolerance from two drive failures. Array continues to operate with up to two failed drives. This makes larger RAID groups more practical. This is because the single parity RAID levels are vulnerable to data loss until the failed drive is rebuilt. The larger the drive, the longer the rebuild will take. With dual parity, it gives the array time to rebuild onto a large drive with the ability to sustain another drive failure.

Nested RAID levels

RAID 01:
A mirror of stripes.

RAID 10:
A stripe of mirrors.

RAID 50:
A stripe across distributed parity RAID systems

RAID 51:
A mirror striped set with distributed parity (some manufacturers label this as RAID 53)

More about : raid faq

April 7, 2007 7:48:49 PM

Nice info on data reliability. It would be nice to add info on storage efficiency (i.e. how much of the total individual drive space is available for use) and performance, including effects on access times and throughput for reads and writes, as the higher RAID levels get a bit complex.
April 7, 2007 8:00:49 PM

RAID 0 = # drives x capacaty = usable storage
No redundancy/fault tolerance, need a backup solution or only for disposable data

RAID 1 = Even # drives x capacity /2 = usable storage
Very safe, at the expense of any increased performance, and a high drive cost.

RAID 0+1 = Even # drives (4 min) x capacity /2 = usable storage
Very safe and excellent performance, but at a high drive cost


RAID 1+0 = See above...

RAID 3 and 5= 3+ drives -1 x capacity = usable storage
Safe and improved performance over RAID 1, and more affordable than RAID 0+1

RAID 6 = 4+ drives -2 x capacity = usable storage
Very safe and improved performance over RAID 1.
!