Disk Redundancy (RAID)

Transcription

1 A Primer fr Business Dvana s Primers fr Business series are a set f shrt papers r guides intended fr business decisin makers, wh feel they are being bmbarded with terms and want t understand a cmplex tpic. This guide will break dwn the technlgy and explain the imprtant pints withut jargn. The intentin is t give yu the knwledge needed t make business decisins, enabling yu can use the right tl fr the jb. We understand that nt everyne can be an expert with the intricacies f mdern cmputing and IT technlgies, unless it is yur passin f curse! This guide will explain the cncept f a Redundant Array f Independent Disks (RAID). It cvers the different levels, their advantages and disadvantages, alng with a brief discussin n where t use each level. dvana.cm

2 Intrductin It is imprtant t understand that the techniques presented in this reprt are intended t prtect the data n the system frm the failure f disk drives. They d nt prvide any backup f the data, nr d they facilitate archiving f the data. Fr bth f thse requirements, ther techniques need t be emplyed, which are beynd the scpe f this reprt. Please see ther reprts in the series fr infrmatin n backups and archiving. Please nte that all f these techniques d nt prtect against accidentally deleting data r prtect against a virus affecting the data. The different Redundant Array f Independent Disks (RAID) levels attempt t slve a particular prblem, namely the inherent lack f reliability in hard disk drives. RAID cmbines tw r mre physical hard disks int a single lgical unit. A set f disk drives cnfigured t wrk tgether as a grup is referred t as a RAID Array r mre simply Array. There are tw basic methds f achieving this; the first is sftware RAID, while the secnd is hardware RAID. There are advantages and disadvantages t bth. Sftware implementatins are typically implemented in the perating system and usually present the RAID vlume as a single drive t applicatins running within the perating system. Hardware implementatins are typically designed t present themselves t the attached system as a single hard drive, s that the perating system wuld be unaware that the drive was nt a single disk drive. There are three key basic cncepts in RAID: 1. Mirrring is the writing f the same data t mre than ne disk 2. Striping is the splitting f the data acrss mre than ne disk 3. Errr uses redundant infrmatin called "parity" which is stred n the disk drives t prvide a mechanism t recver data shuld a drive failure ccur. This is knwn as fault tlerance In this reprt we will cnsider the standard levels and cnsider the different uses fr the different ptins. The purpse f these techniques is t increase the reliability f data strage and t make the data mre available. This prtects the stred data frm hardware failure, while ften increasing the perfrmance f the system. As additinal hard disks are added t a system, the prbability that ne f the disk drives will fail increases, this is the reasn fr redundancy. This redundancy hwever, cmes at a cst, in that the redundant data must be stred smewhere. It is typically stred n additinal hard disk drives. The RAID techniques presented here are thse mst cmmnly used, there are ther variants which have either been superseded r are n lnger cmmnly utilized fr ne reasn r anther. Each f the different techniques has an impact n the perfrmance and the reliability f the system, with their wn quirks, advantages and disadvantages as we shall see in the fllwing sectins. Striping r RAID 0 Descriptin Striping in its mst basic frm is used t imprve the speed f data access, bth reading and writing. It wrks by writing the data equally t all the disks at nce. In a Striped Set there can be tw r mre disks all 2

3 wrking tgether. Since this methd spreads the data acrss all disk drives, in the event f a disk drive failure, the whle striped set will be lst. The mre disks are in the striped set, the higher the risk f data lss. Cnfiguratin Abslute Minimum Tw data disks Practical Tw data disks Three data disks Practical Maximum Five data disks Six data disks Advantages Faster than a single drive Utilizes the full capacity f all drives Best price t perfrmance rati Disadvantages Increased risk f data lss All disks must be the same size t eliminate wasted space Ideal Uses Wrkstatin Vide editing scratch disks Phtshp scratch disks Temprary users space Server Temprary file strage Backup snapshts prir t archiving General Temprary strage fr files r intermediate results Striping is used where the perfrmance is f benefit, but the significant disadvantage f 100% data lss in the event f a single disk drive failure is nt an issue. This limits its use t niche applicatins, as mst data strage situatins require the data t be available in the event f a failure. Mirrring r RAID 1 3

4 Descriptin The mirrring f data between tw r mre disk drives means that shuld a single drive fail, all the data is available n the ther drive. The mirrring prcess writes the duplicated data t all the drives simultaneusly. Writing f data t the disk drives is clse t the same speed as fr a single disk drive. Hwever, all the drives must write the data befre it is written and this can take a little lnger than fr a single disk drive. Data is read frm all the disk drives at nce, significantly increasing the read perfrmance, since files can be read in parallel. Cnfiguratin Abslute Minimum Tw data disks Practical Tw data disks Practical Maximum Tw data disks Nte: It is pssible t use mre than tw disks in thery, but in practice this is rarely implemented in hardware, which is the perfrmance ptin. Advantages Multiple exact duplicates f the data Very fast and easy recvery f data Ptentially faster than a single disk drive Read perfrmance is increased Easy t upgrade, bth fr disks r the system Disadvantages Uses twice the number f disk drives All disks must be the same size t eliminate wasted space A small write penalty might be incurred Mst expensive methd fr the capacity Ideal Uses Wrkstatin System drive Server System drive Database servers General Any situatin in which the data must nt be lst, and read perfrmance is required 4

5 Mirrring is the mst basic f the redundant methds; it prvides gd perfrmance and enables easy access t the data in the event f a failure. It is mst ideally suited fr use where perfrmance is imprtant, but data cannt be lst. Redundant Striping r RAID 5 Descriptin Redundant striping f data is ne f the mst cmmn frms f prtecting data against lss. In this system, an additinal disk drive is used t add capacity which is then used fr string parity r redundant data. The data is striped ver all the disks, which gives the advantages f simple striping and the parity is rtated between all disks, this imprves perfrmance. All but ne f the disk drives are required fr the data t be recvered, s a single failure can be tlerated. It is imperative that in the event f a single disk failure, the defective disk drive be replaced as sn a pssible and the array allwed t rebuild itself. If a secnd disk drive fails, all the data n the array is lst. During a rebuild f the additinal redundant data, the perfrmance f the system will be degraded. Cnfiguratin Abslute Minimum Three disks, tw fr data and ne fr parity Practical Any number f data disks and a single parity drive Practical Maximum Fifteen data disks and ne parity Advantages Gd cmprmise between capacity and perfrmance Gd cmprmise n cst Read perfrmance is increased Disadvantages Small writes are slwer than a single disk All disks must be the same size t eliminate wasted space A rebuild can be very slw fr large arrays Ideal Uses Wrkstatin Nt cmmnly needed r used in wrkstatins Server General User shared data General files 5

6 Any situatin a large capacity is required Read perfrmance is mre imprtant than write. Redundant Striping is a gd general purpse slutin fr reasnable perfrmance and capacity which has a degree f prtectin against hardware failures. It is pssible t have a Ht Spare disk assciated with the array; in this case, a disk drive is designated t be used t rebuild the redundant infrmatin autmatically in the event f a disk drive failure. With the advent f very large disks, it wuld be generally better t have Multi-Redundant Striping (RAID 6) instead f Redundant Striping (RAID 5) with Ht Spare. Multi-Redundant Striping r RAID 6 Multi-Redundant Striping is very similar t Redundant Striping (Raid 5), but differs in the use f tw r mre disks fr redundancy. This means that fr each additinal parity disk drive, it is pssible t lse that number f drives withut lsing all the data. This is particularly imprtant when the number f disks in an array passes 15, since the extra prtectin f the additinal parity drive r drives is needed. Cnfiguratin Abslute Minimum Fur disks, tw fr data and tw fr parity Practical Any number f data disks and tw r mre parity Practical Maximum Any number f data disks and tw r mre parity Advantages As fr Redundant Striping (RAID 5) Ability t increase array size withut practical limit Prtectin against a secnd disk drive failure Disadvantages As fr Redundant Striping (RAID 5) The use f a secnd parity drive affects write perfrmance Additinal cst as cmpared t Redundant Striping (RAID 5) Ideal Uses Wrkstatin Nt cmmnly needed r used in wrkstatins Server 6

7 User shared data General files General Any situatin where a large capacity is required Read perfrmance is mre imprtant than write Multi-Redundant Striping (RAID 6) is suited t situatins where mre disks are needed fr capacity than can be prvided by Redundant Striping (RAID 5) r where the additinal prtectin is needed. Advanced It is pssible t cmbine the varius techniques presented in this reprt t create a whle hst f ther cmbinatins. Fr example, it is pssible t have tw Redundant Stripes (RAID 5) and Mirrr (RAID 1) them tgether s that all the data written t ne is present n the ther. In this case, it wuld be pssible t lse all f ne array and the data wuld still be present and safe. Hwever, it wuld mean dubling the ttal equipment requirements. Utilising this arrangement, it is pssible t have the tw Redundant Stripe arrays at different lcatins and update in parallel bth cpies f the data, this can prvide prtectin against the lss f data at a single site. It is als pssible t Stripe (RAID 0) data ver Redundant Stripes (RAID 5) t gain the benefits f writing the data t different strage units, but at the same time eliminating the vulnerability inherent in Striping (RAID 0). This is f curse at the cst f dubling the hardware requirements. Everything applicable t Redundant Striping is als applicable t Multi-Redundant Striping. Cnclusin There are varius mechanisms t ensure that data can be prtected against the lss f a single disk drive, with a little design thught, it is pssible t make the system highly available. By using redundancy, it is pssible t sacrifice a little capacity t ensure that data will nt be lst in the event f a hardware failure. As the need fr strage grws and the reliability f the strage devices remains almst cnstant, the use f the techniques utlined here are f particular interest t businesses wh are ever mre reliant n their data being Just There when they need it. The use f specialized hardware and strage devices can prvide the perfrmance in terms f speed and capacity a business needs. Scalability and upgradability shuld als be cnsidered in prvisining such a system. Spending a little mre time and mney frm the utset will save large amunts f bth in the future, when the systems need upgrading as yur business need grw. 7

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