RAID SYSTEMS
RAID SYSTEMS
RAID
RAID, an acronym for Redundant Array of Independent Disks was invented to address problems of disk reliability, cost, and performance.
In RAID, data is stored across many disks, with extra disks added to the array to provide error correction (redundancy).
RAID INTRO
RAID 5 & 6
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4
3
Long version for greater clarity https://www.youtube.com/watch?v=nxkXNZlJDJc
RAID
RAID Level 0, also known as drive spanning, provides improved performance, but no redundancy.
Data is written in blocks across the entire array.
The disadvantage of RAID 0 is in its low reliability.
4
RAID
RAID Level 1, also known as disk mirroring, provides 100% redundancy, and good performance.
Two matched sets of disks contain the same data.
The disadvantage of RAID 1 is cost.
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RAID
A RAID Level 2 configuration consists of a set of data drives, and a set of Hamming code drives.
Hamming code drives provide error correction for the data drives.
RAID 2 performance is poor and the cost is relatively high.
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RAID
RAID Level 3 stripes bits across a set of data drives and provides a separate disk for parity.
Parity is the XOR of the data bits.
RAID 3 is not suitable for commercial applications but is good for personal systems.
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RAID
RAID Level 4 is like adding parity disks to RAID 0.
Data is written in blocks across the data disks, and a parity block is written to the redundant drive.
RAID 4 would be feasible if all record blocks were the same size.
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RAID
RAID Level 5 is RAID 4 with distributed parity.
With distributed parity, some accesses can be serviced concurrently, giving good performance and high reliability.
RAID 5 is used in many commercial systems.
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RAID
RAID Level 6 carries two levels of error protection over striped data: Reed-Soloman and parity.
It can tolerate the loss of two disks.
RAID 6 is write-intensive, but highly fault-tolerant.
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RAID
Double parity RAID (RAID DP) employs pairs of over- lapping parity blocks that provide linearly independent parity functions.
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RAID
Like RAID 6, RAID DP can tolerate the loss of two disks.
The use of simple parity functions provides RAID DP with better performance than RAID 6.
Of course, because two parity functions are involved, RAID DP’s performance is somewhat degraded from that of RAID 5.
RAID DP is also known as EVENODD, diagonal parity RAID, RAID 5DP, advanced data guarding RAID (RAID ADG) and—erroneously—RAID 6.
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RAID
Large systems consisting of many drive arrays may employ various RAID levels, depending on the criticality of the data on the drives.
A disk array that provides program workspace (say for file sorting) does not require high fault tolerance.
Critical, high-throughput files can benefit from combining RAID 0 with RAID 1, called RAID 10.
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How to reconstruct data using partiy information
Recovering the data when we lose a drive.
How does parity help us do that?
RAID 5 Example:
Start this at 2:08 or sit through a bit of RAID intro..
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RAID 50
RAID 50 combines striping and distributed parity. For good fault tolerance and high capacity.
Note: Higher RAID levels do not necessarily mean “better” RAID levels. It all depends upon the needs of the applications that use the disks.
ON THE LIGHTER SIDE
Using 8 flash drives to make a RAID setup