RAID

John Williams & Kevin Forkl

Brief History of RAID● Developed by a team from the University of California,

Berkely● Lead by David A. Patterson● Project began in 1987● Team had been working on RISC processors

○ Thought "Processors are going to start getting fast, improving faster than they have in the past. So what are we going to do about I/O?"

● Saw smaller disks as a building block● Wrote "The Case for Redundant Arrays of Inexpensive

Disks"○ Advocated replacing larger disks by lots of smaller ones

● Originally RAID was geared towards performance○ PC community saw it as dependability oriented

The Term RAID

● When first coined the term RAID stood for Redundant Array (of) Inexpensive Disks

● System was so expensive that the term had to be changed to be able to market it

● Was changed to also mean Redundant Array (of) Independent Disks

Striping vs Mirroring

Striping ● Process of writing date across all drives in the array● Data is written in round-robin fashion● Data is also read in the same way

Mirroring● Stores an exact replica of all data on a separate disk or

disks● Using mirroring causes the system to take a performance hit

when performing writes○ The more mirrored disks the worse the performance

● Read operations improve since they can be done in parallel

Software RAID vs Hardware RAIDSoftware RAID (Best for RAID 0, 1)

● More Flexible● Cheaper● Uses CPU● Unprotected at boot

Hardware RAID (Best for RAID 5,6)● System independent

○ Safe from viruses○ Calculations are done on the RAID card

● Disk hot-plug● Protection from power loss● Movable between operating systems

Types of RAID to be covered

RAID 0RAID 1 RAID 2RAID 3RAID 4RAID 5RAID 6RAID 1+0 RAID 0+1

RAID 0

● Known as stripe set or striped volume● Splits data evenly across two or more disks● No parity information ● Not one of the orginal RAID levels● No data redundancy● Normally used to increase performance● Can be created using disks of differing sizes

○ Is limited to the size of the smallest disk though○ If a 500 GB disk and a 750 GB disk were used:

■ Size = 2* min(500 GB, 750 GB) = 2* 500 GB = 1000 GB

Operating System Concepts (8th ed) by Silberschatz, Galvin and Gagne

RAID 0 (Performance)

Picture Source: http://tweakers.net/reviews/515/1/raid-0-hype-or-blessing-pagina-1.html

RAID 1

● Known as mirroring or shadowing● Uses twice as many disks as a non-redundant disk array● When data is written to one disk the same data must be

also written to a redundant disk● Data is read from the disk with the shorter queuing, seek

and rotational delays● If one disk fails then the other is used● Used frequently in database applications

○ Availability and transaction time of higher importance○ Storage efficiency is of less importance

Operating System Concepts (8th ed) by Silberschatz, Galvin and Gagne

RAID 2

● Stripes data at the bit level ● Uses Hamming code for error correction● Disks are synchronized by controller to spin at the same

angular orientation● Extremely high data transfer rates possible● The read/write level error correction code used later

became standard firmware feature on hard drives● No longer had an advantage over other RAID levels● No longer is used

Operating System Concepts (8th ed) by Silberschatz, Galvin and Gagne

RAID 3

● Byte-level striping with a dedicated parity disk● Required all disks operate in lockstep

○ All spindles are synchronized○ Added design design considerations○ No significant advantage over other RAID levels

Operating System Concepts (8th ed) by Silberschatz, Galvin and Gagne

RAID 4

● Block-level striping with a dedicated parity disk● When data is written to an array disk an algorithm generates

recovery information○ Recovery information is written to parity drive

● If a single disk fails the algorithm is reversed and missing data is automatically generated based on remaining data and the parity information

Operating System Concepts (8th ed) by Silberschatz, Galvin and Gagne

RAID 5

● Block-level striping with parity data distributed across all disks

● Low cost of redundancy● Advantage over RAID 4 since it does not have to write parity

information to a single drive during each write operation● Allows all disks to be used when servicing read operations

○ Parity disk systems do not use the parity disk on reads● Best performance for small reads and large writes of a

redundancy disk array● Small writes are less efficient than mirroring

○ Needs to perform read-modify-write operations to update parity

Operating System Concepts (8th ed) by Silberschatz, Galvin and Gagne

RAID 5 (Performance)

● Random Read Performance - Excellent: Better with large stripe sizes. Parity information is unused during normal reads.

● Random Write Performance - Fair: Computing the parity bit slows down writes; better then RAID 3 and 4 dur to lack of dedicated parity drive.

● Sequential Read Performance - Good: Better with small stripe sizes.

● Sequential Write Performance - Fair: Better then RAID 1+0.

RAID 6

● Block-level striping with two parity blocks distributed across all disks

○ Simply adds another parity block to RAID 5● Uses Reed-Solomon codes to protect against up to two disk

failures using a minimum of two redundant disk arrays

Operating System Concepts (8th ed) by Silberschatz, Galvin and Gagne

RAID 6 (cont)

● Random Read Performance: Excellent; Better for larger stripe sizes.

● Random Write Performance: Poor; Dual parity overhead and complexity.

● Sequential Read Performance: Good; Better for smaller stripe sizes.

● Sequential Write Performance: Fair; Slightly better then random write.

RAID 1+0 and Raid 0+1● Uses both striping and mirroring

Operating System Concepts (8th ed) by Silberschatz, Galvin and Gagne

Common RAID Disk Data Format

● Specification defines a standard data structure describing how data is formatted across the disks in a RAID group

● Allows a basic level of operation between different suppliers of RAID technology

● Benefits storage users by enabling data-in-place migration among systems from different vendors

● SNIA (Storage Networking Industry Association) currently uses the Common Raid Disk Data Format (DDF) Specification v2.0

Questions?

Works Cited

● Common RAID Disk Data format○ http://www.snia.org/tech_activities/standards/curr_standards/ddf

● RAID level information○ http://books.google.com/books?

id=RM4tahggCVcC&pg=PA6&dq=raid+2+implementation&hl=en#v=onepage&q=raid%202%20implementation&f=false

○ http://www.ecs.umass.edu/ece/koren/architecture/Raid/basicRAID.html○ http://www.chicago-data-recovery.com/raid-levels.php ○ http://tweakers.net/reviews/515/1/raid-0-hype-or-blessing-pagina-1.html ○ http://sqlblog.com/blogs/linchi_shea/archive/2007/02/07/is-raid-5-really-that-bad.aspx ○ http://www.pcguide.com/ref/hdd/perf/raid/levels/singleLevel5-c.html ○ http://www.pcguide.com/ref/hdd/perf/raid/levels/singleLevel6-c.html

● RAID Background information○ http://www.computerworld.com/s/article/87093/The_Story_So_Far