RAID - Redundant Array of Independent Disks

What is RAID Disk, Types of RAID, How to Work
The term RAID was born out of a paper written at the University of California Berkeley. At the time disk technology was going in two different directions. There were large 14" disk drives with high capacities and large physical size(washing machine). There was also an emerging standard of lower capacity and physically smaller(5.25" Full Height) disk drives.
 
Six different RAID levelsThe levels defined were; RAID 0RAID1RAID2RAID3RAID4 and RAID5.
 
[Note : -These RAID levels are in fact what is used today. RAID 2 was not technically feasible, RAID 0+1 was born later and a multitude of marketing RAID levels (RAID 7, RAID 5 +1 and so on) have also hit the streets.]
 
Understanding: - This paper will explain the base function and data layout for the different levels of RAID. Each level or type of RAID data layout has specific performance and redundancy characteristics. These characteristics allow you to custom configure RAID sets to your performance criteria. Understanding the basics of RAID data layout is the first step to purchasing a RAID subsystem that meets today's and tomorrow's storage requirements. A second paper, titled "Buying RAID" goes into more specific hardware configurations and features. We invite you to download "Buying RAID" after reading this piece.
 
RAID 0 - Data Striping
RAID 0 allows a number of disk drives to be combined and presented as one large disk. RAID 0 does not provide any data redundancy - if one drive fails, all data is lost.
Access Time                      Very Good
Transfer Rate                    Good
Redundancy                       None
Cost Per Megabyte            None
 
Penalty : -                         
Applications Large disk requirements, High performance databases
 
 
RAID 1 - Disk Mirroring/Disk Duplexing
RAID 1 mirrors (shadows) one disk drive to another. All data is stored twice on two or more identical disk drives. When one disk drive fails, all data is immediately available on the other without any impact on the data integrity - performance in degraded mode is also degraded. Performance is gained by splitting of functions. If multiple read requests are pending, the RAID controller will allows reads from different disk drives. If one disk is busy writing the other disk drive can supply read data, at a later time. The RAID controller will update the read drive with data from the already written disk drive. If each disk drive is connected with a separate SCSI channel, this is called "Disk Duplexing" (additional security and performance). RAID 1 represents a simple and highly efficient solution for data security and system availability. Use RAID 1 when large volumes of data are not required.
Access Time                      Very Good
Transfer Rate                    Good
Redundancy                       Yes
Cost Per Megabyte            100% or more
 
Penalty Applications : -
Small disk capacities that require redundancy
 
 
RAID 0 + 1 - Combination of RAID 1 and RAID 0
The idea behind RAID 0+1 is simply based on the combination of RAID 0 (Performance) and RAID 1 (Data Security). RAID 0+1 disk sets offer good performance and data security. Similar as in RAID 0, optimum performance is achieved in highly sequential load situations. The major draw back is a 100% "Cost Per Megabyte Penalty".

Access Time                      Very Good

Transfer Rate                    Good
Redundancy                       Yes
Cost Per Megabyte            100%
 
Penalty : -
Applications Multiuser environments, database servers, file serving, web site hosting
 
 
RAID 3 Data Bit Striping With a Dedicated Parity Drive
The data is striped at a byte/bit level across the disk drives. Additionally, the controller calculates parity information which is stored on a separate disk drive (aP, bP, ...). Even when one disk drive fails, all data is fully available. The missing data can be recalculated from the data still available and the parity information. This data calculation can also be used to restore data to a replaced defective disk. Because the data must be presented at the same time, the disk drive spindles must be synchronized for RAID 3 to be effective. This represents a practical implementation problem for RAID 3. Many RAID controller manufacturers are moving to a RAID 4 solution or using the term RAID 3 merely as a recognized marketing term for high data transfer capability.
Access Time                      GoodTransfer Rate         Very Good
Redundancy                       Yes
Cost Per Megabyte :-         Varies.  5 drive set = 20%, 6 drive set = 17%, 10 drive set = 10%
 
Penalty Applications :-
Imaging, geological, seismological, video
 
 
RAID 4 - Data Striping With a Dedicated Parity Drive
RAID 4 works just like RAID 0. The data is striped across disk drives. Additionally, the controller calculates parity information which is stored on a separate disk drive (P1, P2, ...). Even when one disk drive fails, all data is fully available. The missing data can be recalculated from the data still available and the parity information. This data calculation can also be used to restore data to replaced defective disk. RAID 4 offers excellent transfer rates when used with large contiguous blocks of data. When used with with many small data blocks, the parity disk drive becomes a throughput bottle-neck because of it's fixed position.A RAID 4 disk set can only lose one disk from it's RAID set. Loosing another disk drive, before a replacement is restored, will loose all data in the RAID set.
Access Time                      Good
Transfer Rate                    Very Good
Redundancy                       Yes
Cost Per Megabyte :-         Varies.  5 drive set = 20%, 6 drive set = 17%, 10 drive set = 10%
Penalty Applications :-
Imaging, geological, seismological, video
 
RAID 5 - Data Striping with Striped Parity
The data is striped across disk drives. Unlike RAID 4, the parity data in a RAID 5 set is striped across all disk drives. RAID 5 is designed to handle small data blocks. This makes RAID 5 the level of choice for multitasking, multiuser and database environments. RAID 5 offers the same level of security as RAID 4: when one disk drive fails, all data is fully available, the missing data is recalculated from the data still available and the parity information. This data calculation can also be used to restore data to replaced defective disks. RAID 5 is particularily suited for systems with medium to large capacity requirements, with their "Cost Per Megabyte Penalty" is relatively low. A RAID 5 disk set can only lose one disk from it's RAID set. Loosing another disk drive, before a replacement is restored, will loose all data in the RAID set.
Access Time                      Ver Good
Transfer Rate                    Good
Redundancy                       Yes
Cost Per Megabyte            Varies.  5 drive set = 20%, 6 drive set = 17%, 10 drive set = 10%
Penalty Applications :-      Multiuser environments, database servers, file serving, web site hosting

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