RAID A RRAYS Redundant Array of Inexpensive Discs

W HAT IS RAID A RRAYS ? RAID is an acronym for Redundant Array of Independent Drives (or Disks), also known as Redundant Array of Inexpensive Drives (or Disks) The various types of RAID are data storage schemes that divide and/or replicate data among multiple hard drives

W HY U SE RAID? Improved Reliability Improved Performance Fault Tolerance Improved Availability Higher Data Security

K EY T ERMS Mirroring - the copying of data to more than one disk Striping - the splitting of data across more than one disk Parity - a redundancy check that ensures that the data is protected without having to have a full set of duplicate drives. Duplexing - an extension of mirroring that is based on the same principle as that technique expect it goes one step further in that it also duplicates the hardware that controls the two hard drives (or sets of hard drives). RAID Arrays

RAID - R EDUNDANT A RRAY OF I NDEPENDENT D ISKS RAID Arrays RAID Controller RAID Array Host

RAID C OMPONENTS RAID Arrays RAID Controller Logical Array Physical Array RAID Array Host

D ATA O RGANIZATION : S TRIPS AND S TRIPES RAID Arrays Stripe 1 Stripe 2 Stripe 3 Strips

RAID L EVELS 0 Striped array with no fault tolerance 1 Disk mirroring 3 Parallel access array with dedicated parity disk 4 Striped array with independent disks and a dedicated parity disk 5 Striped array with independent disks and distributed parity 6 Striped array with independent disks and dual distributed parity Combinations of levels (I.e., 1 + 0, 0 + 1, etc.) RAID Arrays

RAID 0 A striped set of at least two disks without parity The data is broken down into blocks and each block is written to a separate disk drive Best performance is achieved when data is striped across multiple controllers with only one drive per controller

RAID 0 – S TRIPED A RRAY WITH NO F AULT T OLERANCE RAID Arrays - 10 RAID Controller Block 4 Block 3 Block 2 Block 1 Block 0 Host

A DVANTAGES OF RAID 0 I/O performance is greatly improved by spreading the I/O load across many channels and drives No parity calculation overhead is involved Very simple design Easy to implement

D ISADVANTAGES OF RAID 0 Not a "True" RAID because it is NOT fault-tolerant The failure of just one drive will result in all data in an array being lost Should never be used in mission critical environments

RAID 1 – D ISK M IRRORING RAID Arrays - 13 RAID Controller Block 1 Block 0 Host

RAID 1 A DVANTAGES High data availability and high I/O rate (small block size). Improves read performance - twice the read transaction rate of single disks, same write transaction rate as single disks 100% redundancy of data means no rebuild is necessary in case of a disk failure, just a copy to the replacement disk Simplest RAID storage subsystem design – easy to maintain

RAID 1 D ISADVANTAGES Expensive due to the extra capacity required to duplicate data. Overhead cost equals 100%, while usable storage capacity is 50%. May not support hot swap of failed disk when implemented with software. Use hardware implementation.

RAID 0+1 – S TRIPING AND M IRRORING RAID Arrays RAID Controller Block 3 Block 2 Block 1 Block 0 Host

RAID 1+0 – M IRRORING AND S TRIPING RAID Arrays RAID Controller Block 3 Block 2 Block 1 Block 0 Host

RAID 0+1 VS. RAID 1+0 Benefits are identical under normal operations Rebuild operations are very different RAID 1+0 uses a mirrored pair – only 1 disk is rebuilt if a disk fails RAID 0+1 if a single drive fails, the entire stripe is faulted RAID is 0+1 is a poorer solution and is less common RAID Arrays

RAID R EDUNDANCY : P ARITY - 19 RAID Arrays Parity Disk RAID Controller Host

P ARITY C ALCULATION RAID Arrays Parity Data = 14 The middle drive fails: ? + 2 = 14 ? = 14 – 5 – 3 – 2 ? = 4 RAID Array

RAID 3 – P ARALLEL T RANSFER WITH D EDICATED P ARITY D ISK RAID Arrays RAID Controller Block 1 Block 2 Block 3 P Block 0 Block 3 Block 2 Block 1 Block 0 Parity Generated Host

RAID 4 – S TRIPING WITH D EDICATED P ARITY D ISK RAID Arrays RAID Controller P Block 0 Block 4 Block 1 Block 5 Block 2 Block 6 Block 3 Block 7 P P Parity Generated Block 0 P Host

RAID 5 – I NDEPENDENT D ISKS WITH D ISTRIBUTED P ARITY RAID Arrays Block 0 P Block 7 RAID Controller P Block 0Block 4Block 0 Block 1 Block 5 Block 2 Block 6 Block 3 Parity Generated Block 0 P Block 4 P Block 4 P Block 4 Parity Generated Host

RAID 6 – D UAL P ARITY RAID Two disk failures in a RAID set leads to data unavailability and data loss in single-parity schemes, such as RAID-3, 4, and 5 Increasing number of drives in an array and increasing drive capacity leads to a higher probability of two disks failing in a RAID set RAID-6 protects against two disk failures by maintaining two parities Horizontal parity which is the same as RAID-5 parity Diagonal parity is calculated by taking diagonal sets of data blocks from the RAID set members Even-Odd, and Reed-Solomon are two commonly used algorithms for calculating parity in RAID-6 RAID Arrays

RAID I MPLEMENTATIONS Hardware (usually a specialized disk controller card) Controls all drives attached to it Performs all RAID-related functions, including volume management Array(s) appear to the host operating system as a regular disk drive Dedicated cache to improve performance Generally provides some type of administrative software Software Generally runs as part of the operating system Volume management performed by the server Provides more flexibility for hardware, which can reduce the cost Performance is dependent on CPU load Has limited functionality RAID Arrays

H OT S PARES RAID Arrays RAID Controller

H OT S WAP RAID Arrays RAID Controller

C HECK Y OUR K NOWLEDGE What is a RAID array? What benefits do RAID arrays provide? What methods can be used to provide higher data availability in a RAID array? What is the primary difference between RAID 3 and RAID 5? What is a hot spare? RAID Arrays