Assignment 5 CH

8/14/2019 Assignment 5 CH

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1.Explain the RAID terminology. (What isRAID)From Wikipedia, the free encyclopedia

RAID is an acronym 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,[1] a technology that allowed computer users to achieve high levels of storage reliability

from low-cost and less reliable PC-class disk-drive components, via the technique of arranging

the devices into arrays for redundancy.

Marketers representing industry RAID manufacturers later reinvented the term to describe a

redundant array ofindependentdisks as a means of dissociating a "low cost" expectation from

RAID technology.[2]

"RAID" is now used as an umbrella term forcomputer data storage schemes that can divide and

replicate data among multiple hard disk drives. The different schemes/architectures are named by

the word RAID followed by a number, as in RAID 0, RAID 1, etc. RAID's various designs

involve two key design goals: increase data reliability and/or increase input/output performance.

When multiple physical disks are set up to use RAID technology, they are said to be in aRAID

array. This array distributes data across multiple disks, but the array is seen by the computer userand operating system as one single disk. RAID can be set up to serve several different purposes.

Purpose and basics

Redundancy is achieved by either writing the same data to multiple drives (known as mirroring),

or collecting data (known asparity data) across the array, calculated such that the failure of one

(or possibly more, depending on the type of RAID) disks in the array will not result in loss of

data. A failed disk may be replaced by a new one, and the lost data reconstructed from the

remaining data and the parity data.

How Parity Is Calculated, And How Failed Drives Are

Rebuilt
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Parity data in a RAID environment is calculated using the Boolean XORfunction. For example,

here is a simple RAID 4 three-disk setup consisting of two drives that hold 8 bits of data each

and a third drive that will be used to hold parity data.

Drive 1: 01101101

Drive 2: 11010100

To calculate parity data for the two drives, an XOR is performed on their data.

i.e. 01101101 XOR11010100 = 10111001

The resulting parity data, 10111001, is then stored on Drive 3, the dedicated parity drive.

Should any of the three drives fail, the contents of the failed drive can be reconstructed on a

replacement (or "hot spare") drive by subjecting the data from the remaining drives to the same

XOR operation. If Drive 2 were to fail, its data could be rebuilt using the XOR results of the

contents of the two remaining drives, Drive 3 and Drive 1:

Drive 3: 10111001

Drive 1: 01101101

i.e. 10111001 XOR01101101 = 11010100

The result of that XOR calculation yields Drive 2's contents. 11010100 is then stored on Drive 2,

fully repairing the array. This same XOR concept applies similarly to larger arrays, using any

number of disks. In the case of a RAID 3 array of 12 drives, 11 drives participate in the XOR

calculation shown above and yield a value that is then stored on the dedicated parity drive.

Organization

Organizing disks into a redundant array decreases the usable storage capacity. For instance, a 2-

disk RAID 1 array loses half of the total capacity that would have otherwise been available using

both disks independently, and a RAID 5 array with several disks loses the capacity of one disk.Other types of RAID arrays are arranged, for example, so that they are faster to write to and read

from than a single disk.

There are various combinations of these approaches giving different trade-offs of protection

against data loss, capacity, and speed. RAID levels 0, 1, and 5 are the most commonly found,

and cover most requirements.
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RAID 0 (striped disks) distributes data across multiple disks in a way that gives improved

speed at any given instant. If one disk fails, however, all of the data on the array will be

lost, as there is neither parity nor mirroring. In this regard, RAID 0 is somewhat of a

misnomer, in that RAID 0 is non-redundant. A RAID 0 array requires a minimum of two

drives. A RAID 0 configuration can be applied to a single drive provided that the RAID

controller is hardware and not software (i.e. OS-based arrays) and allows for such

configuration. This allows a single drive to be added to a controller already containing

another RAID configuration when the user does not wish to add the additional drive to

the existing array. In this case, the controller would be set up as RAID only (as opposed

to SCSI only (no RAID)), which requires that each individual drive be a part of some sort

of RAID array.

RAID 1 mirrors the contents of the disks, making a form of 1:1 ratio realtime backup.The contents of each disk in the array are identical to that of every other disk in the array.

A RAID 1 array requires a minimum of two drives. RAID 1 mirrors, though during the

writing process copy the data identically to both drives, would not be suitable as a

permanent backup solution, as RAID technology by design allows for certain failures to

take place.

RAID 3 or 4 (striped disks with dedicated parity) combines three or more disks in a way

that protects data against loss of any one disk.Fault tolerance is achieved by adding an

extra disk to the array and dedicating it to storing parity information. The storage

capacity of the array is reduced by one disk. A RAID 3 or 4 array requires a minimum of

three drives: two to hold striped data, and a third drive to hold parity data.

RAID 5 (striped disks with distributed parity) combines three or more disks in a way that

protects data against the loss of any one disk. It is similar to RAID 3 but the parity is not

stored on one dedicated drive, instead parity information is interspersed across the drive

array. The storage capacity of the array is a function of the number of drives minus the

space needed to store parity. The maximum number of drives that can fail in any RAID 5

configuration without losing data is only one. Losing two drives in a RAID 5 array is

referred to as a "double fault" and results in data loss.

RAID 6 (striped disks with dual parity) combines four or more disks in a way that

protects data against loss of any two disks.


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RAID 1+0 (or 10) is a mirrored data set (RAID 1) which is then striped (RAID 0), hence

the "1+0" name. A RAID 10 array requires a minimum of two drives, but is more

commonly implemented with 4 drives to take advantage of speed benefits.

RAID 0+1 (or 01) is a striped data set (RAID 0) which is then mirrored (RAID 1). A

RAID 0+1 array requires a minimum of four drives: two to hold the striped data, plus

another two to mirror the first pair.

RAID can involve significant computation when reading and writing information. With

traditional "real" RAID hardware, a separate controller does this computation. In other cases the

operating system or simpler and less expensive controllers require the host computer's processor

to do the computing, which reduces the computer's performance on processor-intensive tasks

(see Operating system based ("software RAID") and Firmware/driver-based RAID below).

SimplerRAID controllers may provide only levels 0 and 1, which require less processing.

RAID systems with redundancy continue working without interruption when one (or possibly

more, depending on the type of RAID) disks of the array fail, although they are then vulnerable

to further failures. When the bad disk is replaced by a new one the array is rebuilt while the

system continues to operate normally. Some systems have to be powered down when removing

or adding a drive; others support hot swapping, allowing drives to be replaced without powering

down. RAID with hot-swapping is often used in high availability systems, where it is important

that the system remains running as much of the time as possible.

RAID is not a good alternative to backing up data. Data may become damaged or destroyed

without harm to the drive(s) on which they are stored. For example, some of the data may be

overwritten by a system malfunction; a file may be damaged or deleted by user error or malice

and not noticed for days or weeks; and, of course, the entire array is at risk of physical damage.

Note that a RAID controller itself can become the single point of failure within a system.

Principles

RAID combines two or more physical hard disks into a single logical unit by using either special

hardware or software. Hardware solutions often are designed to present themselves to the

attached system as a single hard drive, so that the operating system would be unaware of the

technical workings. For example, you might configure a 1TB RAID 5 array using three 500GB

hard drives in hardware RAID, the operating system would simply be presented with a "single"
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1TB volume. Software solutions are typically implemented in the operating system and would

present the RAID drive as a single volume to applications running upon the operating system.

There are three key concepts in RAID: mirroring, the copying of data to more than one disk;

striping, the splitting of data across more than one disk; and error correction, where redundant

data is stored to allow problems to be detected and possibly fixed (known as fault tolerance).

Different RAID levels use one or more of these techniques, depending on the system

requirements. RAID's main aim can be either to improve reliability and availability of data,

ensuring that important data is available more often than not (e.g. a database of customer orders),

or merely to improve the access speed to files (e.g. for a system that delivers video on demand

TV programs to many viewers).

The configuration affects reliability and performance in different ways. The problem with using

more disks is that it is more likely that one will fail, but by using error checking the total system

can be made more reliable by being able to survive and repair the failure. Basic mirroring can

speed up reading data as a system can read different data from both the disks, but it may be slow

for writing if the configuration requires that both disks must confirm that the data is correctly

written. Striping is often used for performance, where it allows sequences of data to be read from

multiple disks at the same time. Error checking typically will slow the system down as data

needs to be read from several places and compared. The design of RAID systems is therefore a

compromise and understanding the requirements of a system is important. Modern disk arrays

typically provide the facility to select the appropriate RAID configuration.

Standard levels

Main article: Standard RAID levels

A number of standard schemes have evolved which are referred to as levels. There were five

RAID levels originally conceived, but many more variations have evolved, notably several

nested levels and many non-standard levels (mostlyproprietary).

Following is a brief summary of the most commonly used RAID levels. [3] Space efficiency is

given as amount of storage space available in an array of n disks, in multiples of the capacity of a

single drive. For example if an array holds n=5 drives of 250GB and efficiency is n-1 then

available space is 4 times 250GB or roughly 1TB.
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Level DescriptionMinimum# of disks

SpaceEfficiency

Image

RAID0

"Striped set without parity" or"Striping". Provides improved performance and additionalstorage but no redundancy or faulttolerance. Because there is noredundancy, this level is notactually a Redundant Array ofInexpensive Disks, i.e. not trueRAID. However, because of thesimilarities to RAID (especiallythe need for a controller todistribute data across multiple

disks), simple stripe sets arenormally referred to as RAID 0.Any disk failure destroys thearray, which has greater consequences with more disks inthe array (at a minimum,catastrophic data loss is twice assevere compared to single driveswithout RAID). A single diskfailure destroys the entire arraybecause when data is written to a

RAID 0 drive, the data is brokeninto fragments. The number offragments is dictated by thenumber of disks in the array. Thefragments are written to theirrespective disks simultaneously onthe same sector. This allowssmaller sections of the entirechunk of data to be read off thedrive in parallel, increasing bandwidth. RAID 0 does notimplement error checking so any

error is unrecoverable. More disksin the array means higherbandwidth, but greater risk of dataloss.

2 n
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RAID1

'Mirrored set without parity' or'Mirroring'. Provides faulttolerance from disk errors andfailure of all but one of the drives.

Increased read performance occurswhen using a multi-threadedoperating system that supportssplit seeks, as well as a very small performance reduction whenwriting. Array continues tooperate so long as at least onedrive is functioning. Using RAID1 with a separate controller foreach disk is sometimes calledduplexing.

2

1 (size ofthesmallestdisk)

RAID2

Hamming code parity. Disks aresynchronized and striped in verysmall stripes, often in single bytes/words. Hamming codeserror correction is calculatedacross corresponding bits on disks,and is stored on multiple paritydisks.

3

RAID3

Striped set with dedicated parityor bit interleaved parity or bytelevel parity.

This mechanism provides faulttolerance similar to RAID 5.However, because the strip acrossthe disks is a lot smaller than afilesystem block, reads and writesto the array perform like a singledrive with a high linear write performance. For this to work properly, the drives must havesynchronised rotation. If one drivefails, the performance doesn'tchange.

3 n-1
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RAID4

Block level parity. Identical toRAID 3, but does block-levelstriping instead of byte-levelstriping. In this setup, files can bedistributed between multiple disks.

Each disk operates independentlywhich allows I/O requests to beperformed in parallel, though datatransfer speeds can suffer due tothe type of parity. The errordetection is achieved throughdedicated parity and is stored in aseparate, single disk unit.

3 n-1

RAID5

Striped set with distributedparity or interleave parity.Distributed parity requires all

drives but one to be present tooperate; drive failure requiresreplacement, but the array is notdestroyed by a single drive failure.Upon drive failure, anysubsequent reads can be calculatedfrom the distributed parity suchthat the drive failure is maskedfrom the end user. The array willhave data loss in the event of asecond drive failure and isvulnerable until the data that was

on the failed drive is rebuilt onto areplacement drive. A single drivefailure in the set will result inreduced performance of the entireset until the failed drive has beenreplaced and rebuilt.

3 n-1

RAID6

Striped set with dual distributedparity. Provides fault tolerancefrom two drive failures; arraycontinues to operate with up totwo failed drives. This makes

larger RAID groups more practical, especially for highavailability systems. This becomesincreasingly important becauselarge-capacity drives lengthen thetime needed to recover from thefailure of a single drive. Singleparity RAID levels are vulnerable

4 n-2
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to data loss until the failed drive isrebuilt: the larger the drive, thelonger the rebuild will take. Dual parity gives time to rebuild thearray without the data being at risk

if a (single) additional drive failsbefore the rebuild is complete.

Nested (hybrid) RAID

Main article:Nested RAID levels

In what was originally termed hybrid RAID,[4]many storage controllers allow RAID levels to

be nested. The elements of aRAID may be either individual disks or RAIDs themselves. Nesting

more than two deep is unusual.

As there is no basic RAID level numbered larger than 9, nested RAIDs are usually

unambiguously described by concatenating the numbers indicating the RAID levels, sometimes

with a "+" in between. For example, RAID 10 (or RAID 1+0) consists of several level 1 arrays of

physical drives, each of which is one of the "drives" of a level 0 array striped over the level 1

arrays. It is not called RAID 01, to avoid confusion with RAID 1, or indeed, RAID 01. When the

top array is a RAID 0 (such as in RAID 10 and RAID 50) most vendors omit the "+", though

RAID 5+0 is clearer.

RAID 0+1: striped sets in a mirrored set (minimum four disks; even number of disks)provides fault tolerance and improved performance but increases complexity. The key

difference from RAID 1+0 is that RAID 0+1 creates a second striped set to mirror a

primary striped set. The array continues to operate with one or more drives failed in the

same mirror set, but if drives fail on both sides of the mirror the data on the RAID system

is lost.

RAID 1+0: mirrored sets in a striped set (minimum two disks but more commonly four

disks to take advantage of speed benefits; even number of disks) provides fault tolerance

and improved performance but increases complexity.

The key difference from RAID 0+1 is that RAID 1+0 creates a striped set from a series of

mirrored drives. In a failed disk situation, RAID 1+0 performs better because all the

remaining disks continue to be used. The array can sustain multiple drive losses so long

as no mirror loses all its drives.
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RAID 5+1: mirror striped set with distributed parity (some manufacturers label this as

RAID 53).

Non-standard levels

Main article:Non-standard RAID levels

Many configurations other than the basic numbered RAID levels are possible, and many

companies, organizations, and groups have created their own non-standard configurations, in

many cases designed to meet the specialised needs of a small niche group. Most of these non-

standard RAID levels are proprietary.

Storage Computer Corporation used to call a cached version of RAID 3 and 4, RAID 7.

Storage Computer Corporation is now defunct.

EMC Corporation used to offerRAID Sas an alternative to RAID 5 on their Symmetrix

systems. Their latest generations of Symmetrix, the DMX and the V-Max series, do not

support RAID S (instead they support RAID-1, RAID-5 and RAID-6.)

The ZFS filesystem, available in Solaris, OpenSolaris, FreeBSD and Mac OS X, offers

RAID-Z, which solves RAID 5's write hole problem.

Hewlett-Packard 'sAdvanced Data Guarding(ADG) is a form of RAID 6.

NetApp's Data ONTAP uses RAID-DP (also referred to as "double", "dual", or

"diagonal" parity), is a form of RAID 6, but unlike many RAID 6 implementations, does

not use distributed parity as in RAID 5. Instead, two unique parity disks with separate

parity calculations are used. This is a modification of RAID 4 with an extra parity disk.

Accusys Triple Parity (RAID TP) implements three independent parities by extending

RAID 6 algorithms on its FC-SATA and SCSI-SATA RAID controllers to tolerate three-

disk failure.

Linux MD RAID10 (RAID10) implements a general RAID driver that defaults to a

standard RAID 1+0 with four drives, but can have any number of drives. MD RAID10

can run striped and mirrored with only two drives with the f2 layout (mirroring with

striped reads, normal Linux software RAID 1 does not stripe reads, but can read in

parallel).[5]
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Infrant (now part ofNetgear) X-RAID offers dynamic expansion of a RAID5 volume

without having to back up or restore the existing content. Just add larger drives one at a

time, let it resync, then add the next drive until all drives are installed. The resulting

volume capacity is increased without user downtime. (It should be noted that this is also

possible in Linux, when utilizing Mdadm utility. It has also been possible in the EMC

Clariion and HP MSA arrays for several years.) The new X-RAID2 found on x86

ReadyNas, that is ReadyNas with Intel CPUs, offers dynamic expansion of a RAID-5 or

RAID-6 volume (note X-RAID2 Dual Redundancy not available on all X86 ReadyNas)

without having to back up or restore the existing content etc. A major advantage overX-

RAID, is that usingX-RAID2 you do not need to replace all the disks to get extra space,

you only need to replace two disks using single redundancy or four disks using dual

redundancy to get more redundant space.

BeyondRAID, created by Data Robotics and used in the Drobo series of products,

implements both mirroring and striping simultaneously or individually dependent on disk

and data context. It offers expandability without reconfiguration, the ability to mix and

match drive sizes and the ability to reorder disks. It supports NTFS, HFS+, FAT32, and

EXT3 file systems[6]. It also uses thin provisioning to allow for single volumes up to

16 TB depending on the host operating system support.

Hewlett-Packard 's EVA series arrays implement vRAID - vRAID-0, vRAID-1, vRAID-5,

and vRAID-6. The EVA allows drives to be placed in groups (called Disk Groups) that

form a pool of data blocks on top of which the RAID level is implemented. Any Disk

Group may have "virtual disks" or LUNs of any vRAID type, including mixing vRAID

types in the same Disk Group - a unique feature. vRAID levels are more closely aligned

to Nested RAID levels - vRAID-1 is actually a RAID 1+0 (or RAID10), vRAID-5 is

actually a RAID 5+0 (or RAID50), etc. Also, drives may be added on-the-fly to an

existing Disk Group, and the existing virtual disks data is redistributed evenly over all the

drives, thereby allowing dynamic performance and capacity growth.

IBM (Among others) has implemented a RAID 1E (Level 1 Enhanced). With an even

number of disks it is similar to a RAID 10 array, but, unlike a RAID 10 array, it can also

be implemented with an odd number of drives. In either case, the total available disk

space is n/2. It requires a minimum of three drives.
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Non-RAID drive architectures

Main article:Non-RAID drive architectures

Non-RAID drive architectures also exist, and are often referred to, similarly to RAID, by

standard acronyms, several tongue-in-cheek. A single drive is referred to as a SLED (Single

Large Expensive Drive), by contrast with RAID, while an array of drives without any additional

control (accessed simply as independent drives) is referred to as a JBOD (Just a Bunch Of

Disks). Simple concatenation is referred to a SPAN, or sometimes as JBOD, though this latter is

proscribed in careful use, due to the alternative meaning just cited.

Implementations

It has been suggested that Vinum volume managerbe merged into this article orsection. (Discuss)

(Specifically, the section comparing hardware / software raid)

The distribution of data across multiple drives can be managed either by dedicated hardware or

by software. When done in software the software may be part of the operating system or it may

be part of the firmware and drivers supplied with the card.

Operating system based ("software RAID")

Software implementations are now provided by many operating systems. A software layer sits

above the (generallyblock-based) diskdevice drivers and provides an abstraction layer between

the logical drives (RAIDs) andphysical drives. Most common levels are RAID 0 (striping across

multiple drives for increased space and performance) and RAID 1 (mirroring two drives),

followed by RAID 1+0, RAID 0+1, and RAID 5 (data striping with parity) are supported.

Apple's Mac OS X Serversupports RAID 0, RAID 1, RAID 5 and RAID 1+0.[7]

FreeBSD supports RAID 0, RAID 1, RAID 3, and RAID 5 and all layerings of the above

via GEOM modules[8][9]

and ccd.[10]

, as well as supporting RAID 0, RAID 1, RAID-Z, and

RAID-Z2 (similar to RAID-5 and RAID-6 respectively), plus nested combinations of

those via ZFS.

Linux supports RAID 0, RAID 1, RAID 4, RAID 5, RAID 6 and all layerings of the

above.[11][12]
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Microsoft 's server operating systems support 3 RAID levels; RAID 0, RAID 1, and RAID

5. Some of the Microsoft desktop operating systems support RAID such as Windows XP

Professional which supports RAID level 0 in addition to spanning multiple disks but only

if using dynamic disks and volumes. Windows XP supports RAID 0, 1, and 5 with a

simple file patch [13]. RAID functionality in Windows is slower than hardware RAID, but

allows a RAID array to be moved to another machine with no compatibility issues.

NetBSD supports RAID 0, RAID 1, RAID 4 and RAID 5 (and any nested combination of

those like 1+0) via its software implementation, named RAIDframe.

OpenBSD aims to support RAID 0, RAID 1, RAID 4 and RAID 5 via its software

implementation softraid.

OpenSolaris and Solaris 10 supports RAID 0, RAID 1, RAID 5 (or the similar "RAID Z"

found only on ZFS), and RAID 6 (and any nested combination of those like 1+0) via ZFS

and now has the ability to boot from a ZFS volume on both x86 and UltraSPARC.

Through SVM, Solaris 10 and earlier versions support RAID 1 for the boot filesystem,

and adds RAID 0 and RAID 5 support (and various nested combinations) for data drives.

Software RAID has advantages and disadvantages compared to hardware RAID. The software

must run on a host server attached to storage, and server's processor must dedicate processing

time to run the RAID software. This is negligible for RAID 0 and RAID 1, but may become

significant when using parity-based arrays and either accessing several arrays at the same time or

running many disks. Furthermore all the busses between the processor and the disk controller

must carry the extra data required by RAID which may cause congestion.

Another concern with operating system-based RAID is the boot process. It can be difficult or

impossible to set up the boot process such that it can fail over to another drive if the usual boot

drive fails. Such systems can require manual intervention to make the machine bootable again

after a failure. There are exceptions to this, such as the LILO bootloader for Linux, loader for

FreeBSD[14] , and some configurations of the GRUB bootloader natively understand RAID-1

and can load a kernel. If the BIOS recognizes a broken first disk and refers bootstrapping to the

next disk, such a system will come up without intervention, but the BIOS might or might not do

that as intended. A hardware RAID controller typically has explicit programming to decide that a

disk is broken and fall through to the next disk.
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Hardware RAID controllers can also carry battery-powered cache memory. For data safety in

modern systems the user of software RAID might need to turn the write-back cache on the disk

off (but some drives have their own battery/capacitors on the write-back cache, a UPS, and/or

implement atomicity in various ways, etc). Turning off the write cache has a performance

penalty that can, depending on workload and how well supported command queuing in the disk

system is, be significant. The battery backed cache on a RAID controller is one solution to have

a safe write-back cache.

Finally operating system-based RAID usually uses formats specific to the operating system in

question so it cannot generally be used for partitions that are shared between operating systems

as part of a multi-boot setup. However, this allows RAID disks to be moved from one computer

to a computer with an operating system or file system of the same type, which can be more

difficult when using hardware RAID (e.g. #1: When one computer uses a hardware RAIDcontroller from one manufacturer and another computer uses a controller from a different

manufacturer, drives typically cannot be interchanged. e.g. #2: If the hardware controller 'dies'

before the disks do, data may become unrecoverable unless a hardware controller of the same

type is obtained, unlike with firmware-based or software-based RAID).

Most operating system-based implementations allow RAIDs to be created frompartitions rather

than entire physical drives. For instance, an administrator could divide an odd number of disks

into two partitions per disk, mirror partitions across disks and stripe a volume across the mirrored

partitions to emulate IBM's RAID 1E configuration. Using partitions in this way also allows

mixing reliability levels on the same set of disks. For example, one could have a very robust

RAID 1 partition for important files, and a less robust RAID 5 or RAID 0 partition for less

important data. (Some BIOS-based controllers offer similar features, e.g. Intel Matrix RAID.)

Using two partitions on the same drive in the same RAID is, however, dangerous. (e.g. #1:

Having all partitions of a RAID-1 on the same drive will, obviously, make all the data

inaccessible if the single drive fails. e.g. #2: In a RAID 5 array composed of four drives 250 +

250 + 250 + 500 GB, with the 500-GB drive split into two 250 GB partitions, a failure of thisdrive will remove two partitions from the array, causing all of the data held on it to be lost).

Hardware-based

Hardware RAID controllers use different, proprietary disk layouts, so it is not usually possible to

span controllers from different manufacturers. They do not require processor resources, the BIOS
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can boot from them, and tighter integration with the device driver may offer better error

handling.

A hardware implementation of RAID requires at least a special-purpose RAID controller. On a

desktop system this may be a PCI expansion card, PCI-e expansion card or built into the

motherboard. Controllers supporting most types of drive may be used IDE/ATA, SATA, SCSI,

SSA, Fibre Channel, sometimes even a combination. The controller and disks may be in a stand-

alone disk enclosure, rather than inside a computer. The enclosure may be directly attached to a

computer, or connected via SAN. The controller hardware handles the management of the drives,

and performs any parity calculations required by the chosen RAID level.

Most hardware implementations provide a read/write cache, which, depending on the I/O

workload, will improve performance. In most systems the write cache is non-volatile (i.e.

battery-protected), so pending writes are not lost on a power failure.

Hardware implementations provide guaranteed performance, add no overhead to the local CPU

complex and can support many operating systems, as the controller simply presents a logical disk

to the operating system.

Hardware implementations also typically support hot swapping, allowing failed drives to be

replaced while the system is running.

Firmware/driver-based RAID

Operating system-based RAID doesn't always protect the boot process and is generally

impractical on desktop versions of Windows (as described above). Hardware RAID controllers

are expensive and proprietary. To fill this gap, cheap "RAID controllers" were introduced that do

not contain a RAID controller chip, but simply a standard disk controller chip with special

firmware and drivers. During early stage bootup the RAID is implemented by the firmware;

when a protected-mode operating system kernel such as Linux or a modern version ofMicrosoft

Windows is loaded the drivers take over.

These controllers are described by their manufacturers as RAID controllers, and it is rarely made

clear to purchasers that the burden of RAID processing is borne by the host computer's central

processing unit, not the RAID controller itself, thus introducing the aforementioned CPU

overhead which hardware controllers don't suffer from. Firmware controllers often can only use

certain types of hard drives in their RAID arrays (e.g. SATA for Intel Matrix RAID, as there is
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neither SCSI nor PATA support in modern Intel ICH southbridges; however, motherboard

makers implement RAID controllers outside of the southbridge on some motherboards). Before

their introduction, a "RAID controller" implied that the controller did the processing, and the

new type has become known in technically knowledgeable circles as "fake RAID" even though

the RAID itself is implemented correctly. Adaptec calls them "HostRAID".

Network-attached storage

Main article:Network-attached storage

While not directly associated with RAID, Network-attached storage (NAS) is an enclosure

containing disk drives and the equipment necessary to make them available over a computer

network, usually Ethernet. The enclosure is basically a dedicated computer in its own right,

designed to operate over the network without screen or keyboard. It contains one or more disk

drives; multiple drives may be configured as a RAID.

Hot spares

Both hardware and software RAIDs with redundancy may support the use of hot sparedrives, a

drive physically installed in the array which is inactive until an active drive fails, when the

system automatically replaces the failed drive with the spare, rebuilding the array with the spare

drive included. This reduces the mean time to recovery (MTTR), though it doesn't eliminate it

completely. Subsequent additional failure(s) in the same RAID redundancy group before the

array is fully rebuilt can result in loss of the data; rebuilding can take several hours, especially on

busy systems.

Rapid replacement of failed drives is important as the drives of an array will all have had the

same amount of use, and may tend to fail at about the same time rather than randomly. RAID 6

without a spare uses the same number of drives as RAID 5 with a hot spare and protects data

against simultaneous failure of up to two drives, but requires a more advanced RAID controller.

Further, a hot spare can be shared by multiple RAID sets.

Reliability terms

Failure rate

Failure rate is only meaningful if failure is defined. If a failure is defined as the loss of a

single drive (logistical failure rate), the failure rate will be the sum of individual drives'

failure rates. In this case the failure rate of the RAID will be larger than the failure rate of
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its constituent drives. On the other hand, if failure is defined as loss of data (system

failure rate), then the failure rate of the RAID will be less than that of the constituent

drives. How much less depends on the type of RAID.

Mean time to data loss (MTTDL)

In this context, the average time before a loss of data in a given array. [15]. Mean time to

data loss of a given RAID may be higher or lower than that of its constituent hard drives,

depending upon what type of RAID is employed. The referenced report assumes times to

data loss are exponentially distributed. This means 63.2% of all data loss will occur

between time 0 and the MTTDL.

Mean time to recovery (MTTR)

In arrays that include redundancy for reliability, this is the time following a failure to

restore an array to its normal failure-tolerant mode of operation. This includes time to

replace a failed disk mechanism as well as time to re-build the array (i.e. to replicate data

for redundancy).

Unrecoverable bit error rate (UBE)

This is the rate at which a disk drive will be unable to recover data after application of

cyclic redundancy check (CRC) codes and multiple retries.

Write cache reliability

Some RAID systems use RAM write cache to increase performance. A power failure can

result in data loss unless this sort ofdisk bufferis supplemented with a battery to ensure

that the buffer has enough time to write from RAMback to disk.

Atomic write failure

Also known by various terms such as torn writes, torn pages, incomplete writes,

interrupted writes, non-transactional, etc.

Problems with RAID

Correlated failures

The theory behind the error correction in RAID assumes that failures of drives are independent.

Given these assumptions it is possible to calculate how often they can fail and to arrange the

array to make data loss arbitrarily improbable.

In practice, the drives are often the same ages, with similar wear. Since many drive failures are

due to mechanical issues which are more likely on older drives, this violates those assumptions
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and failures are in fact statistically correlated. In practice then, the chances of a second failure

before the first has been recovered is not nearly as unlikely as might be supposed, and data loss

can in practice occur at significant rates.[16]

A common misconception is that "server-grade" drives fail less frequently than consumer-grade

drives. Two different, independent studies, by Carnegie Mellon University and Google, have

shown that failure rates are largely independent of the supposed "grade" of the drive.[17]

Atomicity

This is a little understood and rarely mentioned failure mode for redundant storage systems that

do not utilize transactional features. Database researcherJim Gray wrote "Update in Place is a

Poison Apple"[18] during the early days of relational database commercialization. However, this

warning largely went unheeded and fell by the wayside upon the advent of RAID, which many

software engineers mistook as solving all data storage integrity and reliability problems. Many

software programs update a storage object "in-place"; that is, they write a new version of the

object on to the same disk addresses as the old version of the object. While the software may also

log some delta information elsewhere, it expects the storage to present "atomic write semantics,"

meaning that the write of the data either occurred in its entirety or did not occur at all.

However, very few storage systems provide support for atomic writes, and even fewer specify

their rate of failure in providing this semantic. Note that during the act of writing an object, a

RAID storage device will usually be writing all redundant copies of the object in parallel,

although overlapped or staggered writes are more common when a single RAID processor is

responsible for multiple drives. Hence an error that occurs during the process of writing may

leave the redundant copies in different states, and furthermore may leave the copies in neither the

old nor the new state. The little known failure mode is that delta logging relies on the original

data being either in the old or the new state so as to enable backing out the logical change, yet

few storage systems provide an atomic write semantic on a RAID disk.

While the battery-backed write cache may partially solve the problem, it is applicable only to apower failure scenario.

Since transactional support is not universally present in hardware RAID, many operating systems

include transactional support to protect against data loss during an interrupted write. Novell

Netware, starting with version 3.x, included a transaction tracking system. Microsoft introduced

transaction tracking via the journaling feature in NTFS. Ext4 has journaling with checksums;
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ext3 has journaling without checksums but an "append-only" option, or ext3COW (Copy on

Write). If the journal itself in a filesystem is corrupted though, this can be problematic. The

journaling in NetApp WAFL file system gives atomicity by never updating the data in place, as

does ZFS. An alternative method to journaling is soft updates, which are used in some BSD-

derived system's implementation ofUFS.

This can present as a sector read failure. Some RAID implementations protect against this failure

mode by remapping thebad sector, using the redundant data to retrieve a good copy of the data,

and rewriting that good data to the newly mapped replacement sector. The UBE (Unrecoverable

Bit Error) rate is typically specified at 1 bit in 1015 for enterprise class disk drives (SCSI, FC,

SAS) , and 1 bit in 1014 for desktop class disk drives (IDE/ATA/PATA, SATA). Increasing disk

capacities and large RAID 5 redundancy groups have led to an increasing inability to

successfully rebuild a RAID group after a disk failure because an unrecoverable sector is foundon the remaining drives. Double protection schemes such as RAID 6 are attempting to address

this issue, but suffer from a very high write penalty.

Write cache reliability

The disk system can acknowledge the write operation as soon as the data is in the cache, not

waiting for the data to be physically written. This typically occurs in old, non-journaled systems

such as FAT32, or if the Linux/Unix "writeback" option is chosen without any protections like

the "soft updates" option (to promote I/O speed whilst trading-away data reliability). A poweroutage or system hang such as a BSOD can mean a significant loss of any data queued in such

cache.

Often a battery is protecting the write cache, mostly solving the problem. If a write fails because

of power failure, the controller may complete the pending writes as soon as restarted. This

solution still has potential failure cases: the battery may have worn out, the power may be off for

too long, the disks could be moved to another controller, the controller itself could fail. Some

disk systems provide the capability of testing the battery periodically, however this leaves the

system without a fully charged battery for several hours.

An additional concern about write cache reliability exists, specifically regarding devices

equipped with a write-back cachea caching system which reports the data as written as soon as

it is written to cache, as opposed to the non-volatile medium. [19] The safer cache technique is
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write-through, which reports transactions as written when they are written to the non-volatile

medium.

Equipment compatibility

The disk formats on different RAID controllers are not necessarily compatible, so that it may not

be possible to read a RAID on different hardware. Consequently a non-disk hardware failure

may require using identical hardware, or a data backup, to recover the data. Software RAID

however, such as implemented in the Linux kernel, alleviates this concern, as the setup is not

hardware dependent, but runs on ordinary disk controllers. Additionally, Software RAID1 disks

(and some hardware RAID1 disks, for example Silicon Image 5744) can be read like normal

disks, so no RAID system is required to retrieve the data. Data recovery firms typically have a

very hard time recovering data from RAID drives, with the exception of RAID1 drives with

conventional data structure.

Data recovery in the event of a failed array

With larger disk capacities the odds of a disk failure during rebuild are not negligible. In that

event the difficulty of extracting data from a failed array must be considered. Only RAID 1

stores all data on each disk. Although it may depend on the controller, some RAID 1 disks can be

read as a single conventional disk. This means a dropped RAID 1 disk, although damaged, can

often be reasonably easily recovered using a software recovery program or CHKDSK. If the

damage is more severe, data can often be recovered by professional drive specialists. RAID5 and

other striped or distributed arrays present much more formidable obstacles to data recovery in the

event the array goes down.

Drive error recovery algorithms

Many modern drives have internal error recovery algorithms that can take upwards of a minute

to recover and re-map data that the drive fails to easily read. Many RAID controllers will drop a

non-responsive drive in 8 seconds or so. This can cause the array to drop a good drive because it

has not been given enough time to complete its internal error recovery procedure, leaving the rest

of the array vulnerable. So-called enterprise class drives limit the error recovery time and prevent

this problem, but desktop drives can be quite risky for this reason. A fix is known for Western

Digital drives. A utility called WDTLER.exe can limit the error recovery time of a Western

Digital desktop drive so that it will not be dropped from the array for this reason. The utility


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enables TLER (time limited error recovery) which limits the error recovery time to 7 seconds.

Western Digital enterprise class drives are shipped from the factory with TLER enabled to

prevent being dropped from RAID arrays. Similar technologies are used by Seagate, Samsung,

and Hitachi (reference http://en.wikipedia.org/wiki/TLER).

Increasing recovery time

Drive capacity has grown at a much faster rate than transfer speed, and error rates have only

fallen a little in comparison. The consequences of this are more frequent failures and longer re-

build times.[20]

Other Problems and Viruses

While RAID may protect against drive failure, the data is still exposed to operator, software,

hardware and virus destruction. Most well-designed systems include separate backup systemsthat hold copies of the data, but don't allow much interaction with it. Most copy the data and

remove it from the computer for safe storage.

History

Norman Ken Ouchi at IBM was awarded a 1978 U.S. patent 4,092,732[21] titled "System for

recovering data stored in failed memory unit." The claims for this patent describe what would

later be termed RAID 5 with full stripe writes. This 1978 patent also mentions that disk

mirroring or duplexing (what would later be termed RAID 1) and protection with dedicated

parity (that would later be termed RAID 4) were prior art at that time.

The term RAID was first defined by David A. Patterson, Garth A. Gibson and Randy Katz at the

University of California, Berkeley in 1987. They studied the possibility of using two or more

drives to appear as a single device to the host system and published a paper: "A Case for

Redundant Arrays of Inexpensive Disks (RAID)"in June 1988 at the SIGMOD conference.[1]

This specification suggested a number of prototypeRAID levels, or combinations of drives. Each

had theoretical advantages and disadvantages. Over the years, different implementations of the

RAID concept have appeared. Most differ substantially from the original idealized RAID levels,

but the numbered names have remained. This can be confusing, since one implementation of

RAID 5, for example, can differ substantially from another. RAID 3 and RAID 4 are often

confused and even used interchangeably.
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2.Explain the meaning of these terminology related to RAID.

i. RAID array

All RAID arrays provide a range of redundancy features that include all RAID levels RAID 0, 1 (0+1), 3, 5, 6,

10, 30, 50, NRAID and JBOD. Redundant hot swap fans, power supplies, hard disks. Some of the RAID

systems also feature redundant dual controller cards with hot swap, 72 hour cache memory backup.

Multi host attach support is standard and depending on the model and configuration up to 12 fibre channel

hosts can be connected to a single RAID array. Alternatively you can connect the systems to a storage

server and use iSCSI to serve up disk space.

RAID management is done through an intuitive GUI and shows detailed information relating to the status,

configuration and reporting features.

Our RAID arrays combine cost effective pricing and high capacity against SCSI or Fibre Channel disk basedRAID arrays.

Our extensive range of disk storage systems, comprise of various host attachments including fibre channelvia direct point-to-point or through a fibre channel switch, direct attached SAS / SCSI using a SAS / SCSIcontroller card, iSCSI network attached through software or hardware. All models use the latest disk drivesincluding 15,000 rpm SCSI and fibre channel disks as well as Serial ATA 7,200 rpm and 10,000 rpm drives.

Our RAID disk storage systems support 99.9% of all operating systems currently in existence including allflavours of Windows, Linux, Novell and UNIX. We can also supply the SATA RAID systems with differenthost attach cabling for connecting to legacy SCSI controllers.

SATA RAID are increasingly replacing the older generation IDE RAID arrays and offer greater performance,better cooling and easier cabling.

SATA disk comparitive performance table:Spindle Sustained transfer External transfer


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speed rate rateSATA 7,200 32-58MB/s 150MB/s

SATA-II 7,200 72MB/s 300MB/sSATA 10,000 41-74MB/s 150MB/s

Ultra 160SCSI

10,000 29-57MB/s 160MB/s

Ultra 320SCSI

10,000 38-65MB/s 320MB/s

Ultra 320SCSI

15,000 49-75MB/s 320MB/s

FC 2Gbit 15,000 49-75MB/s 200MB/s

i. Striping Stripping is a feature of a RAID configuration that offers huge performance gain. Data in a striped array is

written or read across all the array drives simultaneously. When the computer writes data into the disk, the

data is divided across several pieces and inserted into each individual disk at the same time. Similarly, when

the computer request to read a file, the multiple pieces of data from each disk drive are extracted together to

be processed by the CPU, which effectively increases read/write time. Striping involves partitioning each

drive's storage space into units which are known as the stripe block size.

There're two important variables in a striped array, namely the stripe width and stripe size. Both factors

greatly determine the performance of a striped array.

Stripe Width:

The stripe width refers to the number of drives in the array.

Stripe Size:

The stripe size represents the size of a single chunk of unit data to be written into each disk. The stripe size is

configurable and can range from 512 bytes to several megabytes. The default IDE configuration is 64K. It is

commonly a multiple of 8k.

To demonstrate this, assume we need to write a 5MB file into a disk array. If we have an array of 5 drives and

writing the data at 100K per unit, we'll need 10 write cycles to complete writing the entire file. Note that each

cycle only writes 500K of the file. Now if we have an array of 10 drives, we can effectively reduce the write

time into half since each write cycle writes 1MB of the file. This will also mean that only 5 write cycles are

needed to complete writing the file. So generally, we can see that the more drives that are implemented, the

faster its performance. It is also apparent that for larger data size, increasing the strip size will help.

However, there're certain factors that must be considered when selecting the stripe width and stripe size.

Firstly, if the stripe size is too small, writing a big file would cause the file to be broken down into manysegments across the drives hence utilizing more disk resources. A stripe size too big may exceed the size of

the data to be stored and result in space wastage. That is to say if you configure 100K as your stripe size,

you'll waste 30K of space if you are to write a 70K sized data.

Generally, the more drives that are implemented, the better its the performance. However if any one disk

breaks down, all data will be lost. This reliability is often measured by mean time between failure (MTBF),

which is a inverse proportion to the stripe width. That is to imply that a set of 3 disks is 1/3 as reliable as a


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single disk. Hence, increasing the number of disk drives in the array for data stripping will also increase the

risk of a disk failure.

The drawback of using striping is that similar hard disks must be used together in the array to prevent latency.

As the data parts are read from the various drives to be pieced together to form the original data, the slowest

drive will determine when the concatenation completes.

i. MIRRORING

Mirroring is a simple concept of simultaneously duplicating data or having more than 2 or more copies of data

written on separate disks so as to provide redundancy. If one drive would to fail, the system can still operate

because it has another copy of the data. The drawback is we need to incur more disk space as there is a

100% "disk wastage". In addition, mirroring requires two identical sized disk to function for optimised

performance.

3.Explain why is RAID configuration is important to a server.


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4.Explain these common types of RAID

a. RAID 0 : Striped array without fault tolerance

RAID 0 is not really a "true" RAID system, as there is no fault tolerence involved here. What it does is to

stripe each data block and write the striped data into different drives.

As such RAID 0 literally has no redundancy at all compared to other RAID arrays. So why use RAID 0 at all? For that

we need to know how RAID 0 is set up.

1) Data is first sent from server to the RAID controller.

2) Then the RAID controller stripes the data.

3) Finally, the striped data is sent to the array.

RAID 0 requires at least 2 drives. Files are striped and sent to the drives in the array. This process gives the

performance enhancement of any single level RAID system as there is no redundancy.

RAID 0 incurs the lowest cost, but has zero fault tolerance. As such, if you lose data, RAID 0 will take the longest

time to recover from. Additional backup is needed, unless data is not important.However, read and write performance is great. It has its uses such as high end gaming, web streaming and editing

for videos and music and other uses which require fast read write speeds but do not have critical data.

ii. RAID 1 : Mirrored Disk Array

RAID 1: Mirroring and Duplexing


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For Highest performance, the controller must be able to perform two concurrentseparate Reads per mirrored pair or two duplicate Writes per mirrored pair.

RAID Level 1 requires a minimum of 2 drives to implement

Characteristics/Advantages

One Write or two Reads possible per mirroredpair

Twice the Read transaction rate of singledisks, same Write transaction rate as singledisks

100% redundancy of data means no rebuild isnecessary in case of a disk failure, just a copyto the replacement disk

Transfer rate per block is equal to that of asingle disk

Under certain circumstances, RAID 1 cansustain multiple simultaneous drive failures

Simplest RAID storage subsystem design

Disadvantages

Highest disk overhead of all RAID types(100%) - inefficient

Typically the RAID function is done by systemsoftware, loading the CPU/Server and possiblydegrading throughput at high activity levels.Hardware implementation is stronglyrecommended

May not support hot swap of failed disk whenimplemented in "software"

Recommended Applications

Accounting

Payroll

Financial

Any application requiring very highavailability

i. RAID 0+1 : Striped array with mirroring


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RAID 0+1 is implemented as a mirrored array whosesegments are RAID 0 arrays

RAID 0+1 has the same fault tolerance as RAID level5

RAID 0+1 has the same overhead for fault-toleranceas mirroring alone

High I/O rates are achieved thanks to multiple stripesegments

Excellent solution for sites that need highperformance but are not concerned with achievingmaximum reliability

RAID 0+1 is NOT to be confused with RAID 10. Asingle drive failure will cause the whole array tobecome, in essence, a RAID Level 0 array

Very expensive / High overhead

All drives must move in parallel to proper tracklowering sustained performance

Very limited scalability at a very high inherent cost

Imaging applications General fileserver

JetStor SATA 416S

JetStor SATA 412S

JetStor SATA 516F
http://www.acnc.com/02_01_jetstor_sata_416s.htmlhttp://www.acnc.com/02_01_jetstor_sata_412s.htmlhttp://www.acnc.com/02_01_jetstor_sata_516f.htmlhttp://www.acnc.com/02_01_jetstor_sata_416s.htmlhttp://www.acnc.com/02_01_jetstor_sata_412s.htmlhttp://www.acnc.com/02_01_jetstor_sata_516f.html


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5.Compare between Hardware RAID and Software RAID

Hardware RAID - A hardware RAID uses the configuration software, which is provided by the RAID

manufacturer, to group one or more physical hard drives into what appears to the operating system as a single

hard drive. This is commonly referred to as a virtual disk or logical unit. When you create the virtual hard

drive, you also select a RAID level. For this type of configuration, a server can be configured to use RAID-0

(also known as striped), RAID-1 (also known as mirrored), RAID-5 (also known as striped with parity), and in

some servers, RAID-0+1 (also known as a mirrored stripe). Hardware RAIDs have better overall server

performance.

Software RAID - A software RAID uses the configuration software provided with the operating system to create

a RAID-0, RAID-1, or RAID-5 volume on dynamic hard drives. Software RAIDs have lower performance than

hardware RAIDs. One reason software RAIDs are used is because they are inexpensive and simple to

configure. There are also no hardware requirements other than requiring multiple hard drives.

Documents

Assignment 5 CH