copyright © curt hill, 2003-2010 raid what every server wants!

Copyright © Curt Hill, 2003-2010

RAID

What every server wants!


History I• In the late part of the 1970s and there

were two types of disks:– Hard– Floppy

• The hard drives were only used on mainframes or minis– Quite expensive

• The floppys were mostly used on personal computers

• In about 1984 hard drives started appearing on IBM personal computers


History II

• Now there were two types of hard drives:– Professional

• Large 100-500 M• Expensive

– Amateur• Small: 5-50 M• Inexpensive


History III

• At this point economics takes charge

• The large disks were a small market, typically 10s of thousands

• The small disks were mass market, typically 10s of millions

• What happens to the prices?


History IV• Someone gets a bright idea:

– Should I buy one 500 M professional disk for $50,000 or 10 50M amateur disks at $100 each?

• RAID is born: Redundant Array Inexpensive Disks– Later Redundant Array Independent Disks

• There might even be a performance improvement if I can access the array independently

• However, we need some type of controller to treat the 10 disks as if it were one


Issues• Redundancy

– If the small is less reliable or if the data is so important we need multiple copies for safety

• Speed– Increase speed by writing part of the

data to one drive and another part to another at the same time

• Versions– There have been many, termed Levels– Currently level 0 through level 6


Redundancy

• Multiple copies allows one disk to crash without losing data

• Two forms:– Mirroring

• AKA Shadowing, Duplexing

– Error Correction Codes


Mirroring

• Write all the data twice– Once to a disk and its mirror– This is done simultaneously, so no extra

delay

• A read only has to wait for the faster of the two

• If a disk crashes, no rebuild is needed since the other disk may just be copied

• High disk overhead, needs two disks to store one disk worth of data

• Can read different parts of the two at same time to increase speed


Error Correction Code

• Even more history and background is needed here

• See the ECC.ppt presentation• Many of the EC codes were studied

by Hamming of Bell Labs– Thus known as Hamming codes


ECC

• Instead of mirroring which requires double disk space use an ECC

• Conceptually:– The eight bit data placed on eight

separate drives and the four bit ECC on another

– Any one disk that fails may be recreated from the rest


Speed

• Speed requires parallelism– Do two things at once

• With a mirrored disk read the front half from one and the back half from the other

• Transfer time cut in half• This leads to a more general

approach: stripes


Stripes

• Cut the data into stripes• If you have N disks

– Partition into N pieces– Read or write N pieces at a time

• Best if each piece goes to a separate controller


Controllers

• Controllers may be hardware or software or both

• Some levels make a software controller a problem


RAID Levels

• Originally specified with six levels– 0-5

• Some of these are seldom used and new ones have been added


Level 0

• Striped disk array• Minimum of two disks• No redundancy or error checking

– If a drive is lost all the data is lost– Only RAID that does not protect data

• Real RAID or not?


Level 0 - Striping

A1 A2 A3 A4B1 B2 B3 B4C1 C2 C3 C4

Four stripesEach block of data is written in four piecesNo redundancy


Level 1

• Mirroring• Minimum of two disks• 100% redundancy

– A rebuilt disk is just a copy, not computed

• Simple controller• Cannot be expanded on the fly


Level 1 - Mirroring

A A E EB B F FC C G G

Four disks, each is a mirror of anotherEach block of data is written twiceNeed twice as much space


Level 2• Striped disk at bit level• ECC provides the redundancy• Minimum of seven disks for storing

4 bit word• Not commercially viable• Number of parity disks is

proportional to log of number of data disks

• Not very flexible


Level 2 – Striping with ECC

A1 A2 A3 A4B1 B2 B3 B4C1 C2 C3 C4

Four stripes of data protected by three of ECCECC is computed on the fly

EAxEBx

EAzEAyEBy EBz

ECx ECy ECz


Level 3• Striped disk array with one ECC

disk• Each block is striped across disks• One disk may fail without

diminishing throughput• Minimum of three disks• High data rates• Controller should be in hardware,

not just software


Level 3 – Striping with ECC

A1 A2 A3 A4B1 B2 B3 B4C1 C2 C3 C4

Four stripes of data protected by three of ECCECC is computed on the fly

ECCAECCBECCC


Level 4• Similar to level 3 except

– Blocks are not subdivided– Different block are written to different

disks

• Minimum of three disks• Controller is complex

– Should be hardware

• Not easy to rebuild in case of failure


Level 5• Striped disk array with distributed

ECC blocks• Each disk stores both data and

ECC– A disk stores ECC data for other disks

• Minimum of three disks• Any single disk failure will result in

no loss of data• Most complex controller design


Level 5 – Striping with Distributed ECC

A1A2A3A4

B1B2B3

B5

C1C2

C5C4

Four stripes of dataEach ECC protects other fourNo disks are only data or only ECC

ECC5ECC4

ECC3ECC2

ECC1D1

D5D4D3

E2E3E4E5


Level 6• Striped disk array

– Similar to five except two independent ECC schemes

– ECC blocks distributed among data disks

• Minimum of four disks• May have multiple disk failures

without loss of data


Level 6 – Striping with Two Distributed ECCs

A1A2A3

B1B2

B3

C1

C3C2

Four stripes of dataTwo types of ECC each of which protects a different groupNo disks are only data or only ECC

ECC4ECC3

ECC2

ECC1

D3D2D1

ECCbECCc

ECCd ECCa


Others

• Several combinations of these exists• 0 + 1

– Mirroring a striped disk• 10

– Striping a mirrored disk• 53

– Level 0 and 3– Level 0 whose stripes are level 3 arrays

What is coming?

• As disk arrays get larger the likelihood of a double failure becomes significant

• Although no product yet, it seems likely that a triple parity scheme is inevitable


copyright © curt hill, 2003-2010 raid what every server wants!

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