nagendra srivastava
DESCRIPTION
Raid TechnologyTRANSCRIPT
By Nagendra Srivastava
RAID Technology
CONTENT
IntroductionTechniques/methods
Mirroring Striping Parity
Advantages and Disadvantages Uses Conclusion
History
> It first defined by David A. Patterson, Garth A. Gibson, and Randy Katz at the University of California, Berkeley in 1987 to describe a redundant array of inexpensive disks.
>What is RAID - It is a technology that allowed computer users to achieve high levels of storage reliability from low-cost and RAID allows you to store the same data redundantly (in multiple paces) in a balanced any to improve overall performance.
What is RAID
Benefits of RAID>Data loss can be very dangerous for an organization
>RAID technology prevents data loss due to disk failure
>RAID technology can be implemented in hardware or software
>Servers make use of RAID technology
Hardware vs. Software RAID >Software RAID
• Software RAID: run on the server’s CPU• Directly dependent on server CPU performance and load• Occupies host system memory and CPU operation, degrading server performance
>Hardware RAID• Hardware RAID: run on the RAID controller’s CPU• Does not occupy any host system memory. Is not operating system dependent• Host CPU can execute applications while the array adapter's processor simultaneously executes array functions: true hardware multi-tasking
•Redundant Arrays of Independent Drives
•Benefits– Improved data availability– Improved I/O performance– Increased scalability
•Levels supported by Array controllers– RAID 0 – Data striping– RAID 1 – Drive mirroring– RAID 4 – Data guarding– RAID 5 – Distributed data guarding– RAID 6/ADG - Advanced data guarding– RAID 0+ 1 – Mirroring and striping
RAID
Single Drive and Drive Arrays
Single Drive Drive Arrays
Fixed Disk Drive Array
SingleDrive 1 2 3 4
234
1
1 2 3 4
Drive Array Features
•Data striping across multiple drives
•Multiple channels
•Request processing
Drive Array Organization
Techniques/Methods
Mirroring
Parity
10101010 XOR 11111111 = 01010101
11111111 XOR 01010101 = 10101010
10101010 XOR 01010101 = 11111111
Techniques/Methods (cont’d)
Striping
File divided into chunks (or segments) and then written (striped) across multiple drives
RAID Level 0 — Data Striping
RAID 0
Uses striping I/O performance gain No Data redundancy
Not fault tolerantNot considered “true” RAID
Striping Factor
64KB Host Data
Fixed Striping Factor Based on Drive Sizes
16KB
16KB
16KB
16KB
Data Written to Two or More Separate Mirrored Drives
RAID Level 1 — Drive Mirroring
RAID 1
Uses mirroringAlso known as duplexing
Fault tolerantHigh Disk overhead
Mirroring typically handled system software
Simplest RAID design
Disk 0 is mirrored to disk 2 and disk 1 is mirrored to disk 3.Then disk 0 is striped with disk 1 and disk 2 is striped to disk 3.
RAID Level 0+1
RAID 0+1
RAID 0+1
RAID 1+0 requires an array with four or more physical disks. The disks are mirrored in pairs and data blocks are striped across the mirrored pairs
Advantages Highest read and write performance No loss of data as long as no failed disks are mirrored to any
other failed disk
Disadvantages Expensive and Low disk capacity
Data striped across multiple drives and then its parity sum is calculated and written to a dedicated parity drive
RAID Level 4 — Data Guarding
RAID 4
Disadvantages:
Very slow write rates Even small writes fill up
parity write queue
Inefficient data recovery
Even more Complex Controller Design than RAID 3
Advantages:
Very high read ratesMultiple files read at
once
Uses:
Web Servers, and other high read, low write situations
Data is striped across multiple drives and then its parity sum calculated and striped across multiple drives. Example of 64KB striped across five drives using 4KB chunks.
RAID Level 5 — Distributed Data Guarding
RAID 5
RAID 5 uses a parity data formula to create fault tolerance. In RAID 5 each block of data stripe contains parity data
that is calculated for the other data blocks in that strip. The blocks of parity data are distributed over the physical
disks that make up the logical drive with each physical disk containing only one block of parity data
It is referred to as data guarding.Advantages
High read performance No loss of data if one physical disk fails More usable disk capacity
Disadvantages Relatively low write performance Data loss occurs if a second disk fails before data from the first
failed disk is rebuilt
RAID ADG (Advanced Data Guarding)
>RAID ADG is similar to RAID 5 except this RAID level writes 2 sets of parity stripped across all drives.
>Protects against failure of ANY 2 drives in the array
RAID ADG
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RAID ADG (Advanced Data Guarding)
>P = f1(A, B, C) = RAID 5 Parity
>Q = f2(A, B, C) = new ADG Parity
RAID ADG
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RAID ADG (Advanced Data Guarding)>If 2 parity drives are selected, the system can sustain failure of ANY 2 drives.
RAID ADG
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RAID ADG (Advanced Data Guarding) or RAID 6
RAID advanced data guarding sometimes referred to as RAID 6, is similar to RAID 5 in that parity data is generated and stored to protect against data loss caused by physical disk failure
Advantages High read performance High data availability More usable disk capacity
RAID 10
Combining RAID 0 and RAID 1 is often referred to as RAID 10 which offers higher performance than RAID 1 but at much higher cost
Uses multiple (mirrored) RAID 1 in a single arrayData striped across all mirrored setsVery high fault toleranceHigh performance rate
RAID 10
Characterized by:
Each drive duplicated
High implementation cost
Comparing RAID Levels
RAID 0 RAID 1 RAID 5 RAID 10
Read High 2X High High
Write High 1X Medium High
Fault tolerance
No Yes Yes Yes
Disk utilization
High Low High Low
Key problems
Data lost when any disk fails
Use double the disk space
Lower throughput with disk failure
Very expensive, not scalable
Key advantages
High I/O performance
Very high I/O performance
A good overall balance
High reliability with good performance
On-Line Spare
•Replacement for failed drive
•Requires hardware fault tolerance
•Background rebuild process
•Four On-Line Spares maximum (Smart Array controller)
MirroredPair
BeforeFailure
On-LineSpare
MirroredPair
AfterReplacement
On-LineSpare
MirroredPair
DuringFailure
Conclusion
So what have we learned here?
Well we have learned that RAID is not just a bug spray. RAID is a good solution for companies or individuals carving more transfer performance, redundancy and storage capacity in their data storage
systems.
Thanks