IPV4 to IPV6: Redundant Array of Independent Disks

<div dir="ltr" style="text-align: left;" trbidi="on"><span style="color: blue;"><span style="font-size: large;">Redundant Array of Independent Disks, History of RAID :</span></span><br /><br /> <br /><div style="text-align: justify;"><b>RAID</b> (<b>redundant array of independent disks</b>, originally <b>redundant array of inexpensive disks</b> is a storage technology that combines multiple <a href="http://en.wikipedia.org/wiki/Disk_drive" title="Disk drive"><span style="color: windowtext; mso-fareast-font-family: "Times New Roman"; mso-fareast-theme-font: major-fareast;">disk drive</span></a> components into a logical unit. Data is distributed across the drives in one of several ways called "RAID levels", depending on the level of <a href="http://en.wikipedia.org/wiki/Redundancy_%28engineering%29" title="Redundancy (engineering)"><span style="color: windowtext; mso-fareast-font-family: "Times New Roman"; mso-fareast-theme-font: major-fareast;">redundancy</span></a> and performance required.</div><div style="text-align: justify;">The term "RAID" was first defined by <a href="http://en.wikipedia.org/wiki/David_Patterson_%28scientist%29" title="David Patterson (scientist)"><span style="color: windowtext; mso-fareast-font-family: "Times New Roman"; mso-fareast-theme-font: major-fareast;">David Patterson</span></a>, <a href="http://en.wikipedia.org/wiki/Garth_A._Gibson" title="Garth A. Gibson"><span style="color: windowtext; mso-fareast-font-family: "Times New Roman"; mso-fareast-theme-font: major-fareast;">Garth A. Gibson</span></a>, and <a href="http://en.wikipedia.org/wiki/Randy_Katz" title="Randy Katz"><span style="color: windowtext; mso-fareast-font-family: "Times New Roman"; mso-fareast-theme-font: major-fareast;">Randy Katz</span></a> at the <a href="http://en.wikipedia.org/wiki/University_of_California,_Berkeley" title="University of California, Berkeley"><span style="color: windowtext; mso-fareast-font-family: "Times New Roman"; mso-fareast-theme-font: major-fareast;">University of California, Berkeley</span></a> in 1987.<a href="http://en.wikipedia.org/wiki/RAID#cite_note-patterson-3"><sup><span style="color: windowtext; mso-fareast-font-family: "Times New Roman"; mso-fareast-theme-font: major-fareast;">[3]</span></sup></a> Marketers representing industry RAID manufacturers later attempted to reinvent the term to describe a <i>redundant array of independent disks</i> as a means of disassociating a low-cost expectation from RAID technology.<a href="http://en.wikipedia.org/wiki/RAID#cite_note-4"><sup><span style="color: windowtext; mso-fareast-font-family: "Times New Roman"; mso-fareast-theme-font: major-fareast;">[4]</span></sup></a><a href="http://en.wikipedia.org/wiki/RAID#cite_note-Patterson_1994-5"><sup><span style="color: windowtext; mso-fareast-font-family: "Times New Roman"; mso-fareast-theme-font: major-fareast;">[5]</span></sup></a></div><div class="MsoNoSpacing"><b style="mso-bidi-font-weight: normal;"><span style="font-size: 14.0pt; mso-font-kerning: 18.0pt;">Data striping:</span></b></div><div class="MsoNoSpacing" style="text-align: justify;"><span style="font-family: "Times New Roman","serif"; font-size: 12.0pt;">In </span><a href="http://en.wikipedia.org/wiki/Computer_data_storage" title="Computer data storage"><span style="color: windowtext; font-family: "Times New Roman","serif"; font-size: 12.0pt;">computer data storage</span></a><span style="font-family: "Times New Roman","serif"; font-size: 12.0pt;">, <b>data striping</b> is the technique of segmenting logically sequential data, such as a file, in a way that accesses of sequential segments are made to different physical storage devices. Striping is useful when a processing device requests access to data more quickly than a storage device can provide access. By performing segment accesses on multiple devices, multiple segments can be accessed concurrently. This provides more data access throughput, which avoids causing the processor to idly wait for data accesses. Striping is used across </span><a href="http://en.wikipedia.org/wiki/Disk_drive" title="Disk drive"><span style="color: windowtext; font-family: "Times New Roman","serif"; font-size: 12.0pt;">disk drives</span></a><span style="font-family: "Times New Roman","serif"; font-size: 12.0pt;">in </span><a href="http://en.wikipedia.org/wiki/Redundant_array_of_independent_disks" title="Redundant array of independent disks"><span style="color: windowtext; font-family: "Times New Roman","serif"; font-size: 12.0pt;">RAID</span></a><span style="font-family: "Times New Roman","serif"; font-size: 12.0pt;"> storage, </span><a href="http://en.wikipedia.org/wiki/Network_interface_controller" title="Network interface controller"><span style="color: windowtext; font-family: "Times New Roman","serif"; font-size: 12.0pt;">network interfaces</span></a><span style="font-family: "Times New Roman","serif"; font-size: 12.0pt;"> in </span><a href="http://en.wikipedia.org/wiki/Grid-oriented_Storage" title="Grid-oriented Storage"><span style="color: windowtext; font-family: "Times New Roman","serif"; font-size: 12.0pt;">Grid-oriented Storage</span></a><span style="font-family: "Times New Roman","serif"; font-size: 12.0pt;">, and </span><a href="http://en.wikipedia.org/wiki/Random-access_memory" title="Random-access memory"><span style="color: windowtext; font-family: "Times New Roman","serif"; font-size: 12.0pt;">RAM</span></a><span style="font-family: "Times New Roman","serif"; font-size: 12.0pt;">in some systems.<span style="mso-font-kerning: 18.0pt;"></span></span></div><h2><span class="mw-headline"><span style="color: windowtext;">RAID parity</span></span><span style="color: windowtext;"></span></h2><div class="MsoNormal">Further information: <a href="http://en.wikipedia.org/wiki/Parity_bit" title="Parity bit"><span style="color: windowtext;">Parity bit</span></a></div><div style="text-align: justify;">Many RAID levels employ an error protection scheme called "parity", a widely used method in information technology to provide fault tolerance in a given set of data. Most use the simple <a href="http://en.wikipedia.org/wiki/Exclusive_or" title="Exclusive or"><span style="color: windowtext; mso-fareast-font-family: "Times New Roman"; mso-fareast-theme-font: major-fareast;">XOR</span></a> parity described in this section, but RAID 6 uses two separate parities based respectively on addition and multiplication in a particular Galois Field<sup><a href="http://en.wikipedia.org/wiki/RAID#cite_note-13"><span style="color: windowtext; mso-fareast-font-family: "Times New Roman"; mso-fareast-theme-font: major-fareast;">[12]</span></a></sup>or <a href="http://en.wikipedia.org/wiki/Reed-Solomon_error_correction" title="Reed-Solomon error correction"><span style="color: windowtext; mso-fareast-font-family: "Times New Roman"; mso-fareast-theme-font: major-fareast;">Reed-Solomon error correction</span></a>.</div><div style="text-align: justify;">The XOR operator is central to how parity data are created and used within an array. It is used both for the protection of data, as well as for the recovery of missing data.</div><div style="text-align: justify;">As an example, consider a simple RAID made up of 6 drives (4 for data, 1 for parity, and 1 for use as a <a href="http://en.wikipedia.org/wiki/Hot_spare" title="Hot spare"><span style="color: windowtext; mso-fareast-font-family: "Times New Roman"; mso-fareast-theme-font: major-fareast;">hot spare</span></a>), where each drive has only a single byte worth of storage (a '<code><span style="font-size: 10.0pt;">-</span></code>' represents a bit, the value of which doesn't matter at this point in the discussion):</div><pre>Drive #1: -------- (Data)</pre><pre>Drive #2: -------- (Data)</pre><pre>Drive #3: -------- (Data)</pre><pre>Drive #4: -------- (Data)</pre><pre>Drive #5: -------- (Hot Spare)</pre><pre>Drive #6: -------- (Parity)</pre>Suppose the following data are written to the drives:<br />  <pre>Drive #1: 00101010 (Data)</pre><pre>Drive #2: 10001110 (Data)</pre><pre>Drive #3: 11110111 (Data)</pre><pre>Drive #4: 10110101 (Data)</pre><pre>Drive #5: -------- (Hot Spare)</pre><pre>Drive #6: -------- (Parity)</pre><div style="text-align: justify;">Every time data are written to the data drives, a parity value must be calculated in order for the array to be able to recover in the event of a failure. To calculate the parity for this RAID, a <a href="http://en.wikipedia.org/wiki/Bitwise_XOR" title="Bitwise XOR"><span style="color: windowtext; mso-fareast-font-family: "Times New Roman"; mso-fareast-theme-font: major-fareast;">bitwise XOR</span></a> of each drive's data is calculated as follows, the result of which is the parity data:</div><div class="MsoNormal" style="margin-left: .5in;"><code><span style="font-size: 10.0pt; line-height: 115%; mso-fareast-font-family: Calibri; mso-fareast-theme-font: minor-latin;">00101010 XOR 10001110 XOR 11110111 XOR 10110101 = 11100110</span></code></div>The parity data <code><span style="font-size: 10.0pt;">11100110</span></code>are then written to the dedicated parity drive:<br />  <pre>Drive #1: 00101010 (Data)</pre><pre>Drive #2: 10001110 (Data)</pre><pre>Drive #3: 11110111 (Data)</pre><pre>Drive #4: 10110101 (Data)</pre><pre>Drive #5: -------- (Hot Spare)</pre><pre>Drive #6: 11100110 (Parity)</pre>Suppose Drive #3 fails. In order to restore the contents of Drive #3, the same XOR calculation is performed against the data of all the remaining data drives and data on the parity drive (11100110) which was stored in Drive #6:<br />  <div class="MsoNormal" style="margin-left: .5in;"><code><span style="font-size: 10.0pt; line-height: 115%; mso-fareast-font-family: Calibri; mso-fareast-theme-font: minor-latin;">00101010 XOR 10001110 XOR 11100110 XOR 10110101 = 11110111</span></code></div>The XOR operation will yield the missing data. With the complete contents of Drive #3 recovered, the data are written to the hot spare, which then acts as a member of the array and allows the group as a whole to continue operating.<br />  <pre>Drive #1: 00101010 (Data)</pre><pre>Drive #2: 10001110 (Data)</pre><pre>Drive #3: --Dead-- (Data)</pre><pre>Drive #4: 10110101 (Data)</pre><pre>Drive #5: 11110111 (Hot Spare)</pre><pre>Drive #6: 11100110 (Parity)</pre><div style="text-align: justify;">At this point the failed drive has to be replaced with a working one of the same size. Depending on the implementation, the new drive becomes a new hot spare, and the old hot spare drive continues to act as a data drive of the array, or (as illustrated below) the original hot spare's contents are automatically copied to the new drive by the array controller, allowing the original hot spare to return to its original purpose. The resulting array is identical to its pre-failure state:</div><pre>Drive #1: 00101010 (Data)</pre><pre>Drive #2: 10001110 (Data)</pre><pre>Drive #3: 11110111 (Data)</pre><pre>Drive #4: 10110101 (Data)</pre><pre>Drive #5: -------- (Hot Spare)</pre><pre>Drive #6: 11100110 (Parity)</pre><div style="text-align: justify;">This same basic XOR principle applies to parity within RAID groups regardless of capacity or number of drives. As long as there are enough drives present to allow for an XOR calculation to take place, parity can be used to recover data from any single drive failure. (A minimum of three drives must be present in order for parity to be used for fault tolerance, because the XOR operator requires two operands, and a place to store the result).</div><h2><span class="mw-headline"><span style="color: windowtext;">Mirroring versus parity RAID levels in relational databases</span></span><span style="color: windowtext;"></span></h2><div style="text-align: justify;">A common opinion (and one which serves to illustrate the dynamics of proper RAID deployment) is that <a href="http://en.wikipedia.org/wiki/RAID#Nested_.28hybrid.29_RAID"><span style="color: windowtext; mso-fareast-font-family: "Times New Roman"; mso-fareast-theme-font: major-fareast;">RAID 10</span></a> (a non-parity, mirrored RAID) is inherently better for <a href="http://en.wikipedia.org/wiki/Relational_databases" title="Relational databases"><span style="color: windowtext; mso-fareast-font-family: "Times New Roman"; mso-fareast-theme-font: major-fareast;">relational databases</span></a>than <a href="http://en.wikipedia.org/wiki/RAID#Standard_levels"><span style="color: windowtext; mso-fareast-font-family: "Times New Roman"; mso-fareast-theme-font: major-fareast;">RAID 5</span></a>, because RAID 5 requires the recalculation and redistribution of parity data on a per-write basis.<a href="http://en.wikipedia.org/wiki/RAID#cite_note-14"><sup><span style="color: windowtext; mso-fareast-font-family: "Times New Roman"; mso-fareast-theme-font: major-fareast;">[13]</span></sup></a></div><div style="text-align: justify;">There are, however, other considerations which must be taken into account other than simply those regarding performance. RAID 5 and other non-mirror-based arrays offer a lower degree of resiliency than RAID 10 by virtue of RAID 10's mirroring strategy. In a RAID 10, I/O can continue even in spite of multiple drive failures. By comparison, in a RAID 5 array, any failure involving more than one drive renders the array itself unusable by virtue of parity recalculation being impossible to perform. Thus, RAID 10 is frequently favored because it provides the lowest level of risk.<a href="http://en.wikipedia.org/wiki/RAID#cite_note-bytepile1-15"><sup><span style="color: windowtext; mso-fareast-font-family: "Times New Roman"; mso-fareast-theme-font: major-fareast;">[14]</span></sup></a></div><div class="MsoNoSpacing">Modern SAN design largely masks any performance hit while a RAID is in a degraded state, by virtue of being able to perform rebuild operations both in-band or out-of-band with respect to existing I/O traffic. Given the rare nature of drive failures in general, and the exceedingly low probability of multiple concurrent drive failures occurring within the same RAID, the choice of RAID 5 over RAID 10 often comes down to the preference of the storage administrator, particularly when weighed against other factors such as cost, throughput requirements, and physical spindle availability.</div><div class="MsoNoSpacing"><span class="mw-headline"><b style="mso-bidi-font-weight: normal;">Basic concepts used by RAID systems</b></span></div><div class="MsoNoSpacing">RAID uses a few basic ideas, which were described in the article "RAID: High-Performance, Reliable Secondary Storage" by Peter Chen and others, published in 1994.<a href="http://simple.wikipedia.org/wiki/RAID#cite_note-chen-2"><sup><span style="color: windowtext;">[2]</span></sup></a></div><div class="MsoNoSpacing"><span class="mw-headline"><b style="mso-bidi-font-weight: normal;">Caching</b></span></div><div class="MsoNoSpacing"><a href="http://simple.wikipedia.org/wiki/Cache" title="Cache"><span style="color: windowtext;">Caching</span></a> is a technology that also has its uses in RAID systems. There are different kinds of caches that are used in RAID systems:</div><ul type="disc"><li class="MsoNormal" style="line-height: normal; mso-list: l0 level1 lfo5; mso-margin-bottom-alt: auto; mso-margin-top-alt: auto; tab-stops: list .5in;"><a href="http://simple.wikipedia.org/wiki/Operating_system" title="Operating system"><span style="color: windowtext;">Operating system</span></a></li><li class="MsoNormal" style="line-height: normal; mso-list: l0 level1 lfo5; mso-margin-bottom-alt: auto; mso-margin-top-alt: auto; tab-stops: list .5in;">RAID      controller</li><li class="MsoNormal" style="line-height: normal; mso-list: l0 level1 lfo5; mso-margin-bottom-alt: auto; mso-margin-top-alt: auto; tab-stops: list .5in;">Enterprise      disk array</li></ul><div class="MsoNoSpacing">In modern systems, a write request is shown as done when the data has been written to the cache. This does not mean that the data has been written to the disk. Requests from the cache are not necessarily handled in the same order than they were written to the cache. This makes it possible that, if the system fails, sometimes some data has not been written to the disk involved. For this reason, many systems have a cache that is backed by a <a href="http://simple.wikipedia.org/wiki/Battery" title="Battery"><span style="color: windowtext;">battery</span></a>.</div><div class="MsoNoSpacing"><span class="mw-headline"><b style="mso-bidi-font-weight: normal;">Mirroring</b>: More than one copy of the data</span></div><div class="MsoNoSpacing">When talking about a <a href="http://simple.wikipedia.org/wiki/Mirror" title="Mirror"><span style="color: windowtext;">mirror</span></a>, this is a very simple idea. Instead of the data being in only one place, there are several copies of the data. These copies usually are on different hard disks (or disk partitions). If there are two copies, one of them can fail without the data being affected (as it still is on the other copy). Mirroring can also give a boost when reading data. It will always be taken from the fastest disk that responds. Writing data is slower though, because all disks need to be updated.</div><div class="MsoNoSpacing"><br /></div><div class="MsoNoSpacing"><span class="mw-headline"><b style="mso-bidi-font-weight: normal;">Striping</b>: Part of the data is on another disk</span></div><div class="MsoNoSpacing">With striping, the data is split into different parts. These parts then end up on different disks (or disk partitions). This means that writing data is faster, as it can be done in parallel. This does not mean that there will not be faults, as each block of data is only found on one disk.</div><div class="MsoNoSpacing"><span class="mw-headline">Error correction and faults</span></div><div class="MsoNoSpacing">It is possible to calculate different kinds of <a href="http://simple.wikipedia.org/wiki/Checksum" title="Checksum"><span style="color: windowtext;">checksums</span></a>. Some methods of calculating checksums allow finding a mistake. Most RAID levels that use redundancy can do this. Some methods are more difficult to do, but they allow to not only detect the error, but to fix it.</div><div class="MsoNoSpacing"><span class="mw-headline"><b style="mso-bidi-font-weight: normal;">Hot spares</b>: using more disks than needed</span></div><div class="MsoNoSpacing">Many of the ways to have RAID support something is called a <i>hot spare</i>. A hot spare is an empty disk that is not used in normal operation. When a disk fails, data can directly be copied onto the hot spare disk. That way, the failed disk needs to be replaced by a new empty drive to become the hot spare.</div><div class="MsoNoSpacing"><span class="mw-headline"><b style="mso-bidi-font-weight: normal;">Stripe size and chunk size</b>: spreading the data over several disks</span></div><div class="MsoNoSpacing">RAID works by spreading the data over several disks. Two of the terms often used in this <a href="http://simple.wikipedia.org/wiki/Context" title="Context"><span style="color: windowtext;">context</span></a> are <i>stripe size</i> and <i>chunk size</i>.</div><div class="MsoNoSpacing">The <i>chunk size</i> is the smallest data block that is written to a single disk of the array. The <i>stripe size</i> is the size of a block of data that will be spread over all disks. That way, with four disks, and a stripe size of 64 <a href="http://simple.wikipedia.org/wiki/Kilobyte" title="Kilobyte"><span style="color: windowtext;">kilobytes</span></a> (kB), 16 kB will be written to each disk. The chunk size in this example is therefore 16 kB. Making the stripe size bigger will mean a faster data transfer rate, but also a bigger maximum <a href="http://simple.wikipedia.org/wiki/Latency" title="Latency"><span style="color: windowtext;">latency</span></a>. In this case, this is the time needed to get a block of data.</div><h3><span class="mw-headline"><span style="color: windowtext;">Putting disk together: JBOD, concatenation or spanning</span></span><span style="color: windowtext;"></span></h3><div class="MsoNormal"><a href="http://simple.wikipedia.org/wiki/File:JBOD.svg"><span style="color: windowtext; mso-no-proof: yes; text-decoration: none; text-underline: none;"><span style="mso-ignore: vglayout;"><img alt="http://upload.wikimedia.org/wikipedia/commons/thumb/e/e2/JBOD.svg/200px-JBOD.svg.png" border="0" height="200" src="file:///C:\DOCUME~1\Aasraful\LOCALS~1\Temp\msohtmlclip1\01\clip_image002.gif" width="200" /></span></span></a></div><div class="MsoNormal"><a href="http://simple.wikipedia.org/wiki/File:JBOD.svg" title=""Enlarge" "><span style="color: windowtext; mso-no-proof: yes; text-decoration: none; text-underline: none;"><span style="mso-ignore: vglayout;"><img alt="http://bits.wikimedia.org/static-1.21wmf7/skins/common/images/magnify-clip.png" border="0" height="11" src="file:///C:\DOCUME~1\Aasraful\LOCALS~1\Temp\msohtmlclip1\01\clip_image003.gif" width="15" /></span></span></a></div><div class="MsoNormal">JBOD with 3 disks of different sizes</div>Many controllers (and also software) can put disks together in the following way: Take the first disk, till it ends, then they take the second, and so on. In that way, several smaller disks look like a larger one. This is not really RAID, as there is no redundancy. Also, spanning can combine disks where RAID 0 cannot do anything. Generally, this is called <i>just a bunch of disks</i>(JBOD).<br />  This is like a distant relative of RAID because the logical drive is made of different physical drives. Concatenation is sometimes used to turn several small drives into one larger useful drive. This can not be done with RAID 0. For example, JBOD could combine 3 GB, 15 GB, 5.5 GB, and 12 GB drives into a logical drive at 35.5 GB, which is often more useful than the drives alone.<br />  In the diagram to the right, data are concatenated from the end of disk 0 (block A63) to the beginning of disk 1 (block A64); end of disk 1 (block A91) to the beginning of disk 2 (block A92). If RAID 0 were used, then disk 0 and disk 2 would be truncated to 28 blocks, the size of the smallest disk in the array (disk 1) for a total size of 84 blocks.<br />  Some RAID controllers use JBOD to talk about working on drives without RAID features. Each drive shows up separately in the operating system. This JBOD is not the same as concatenation.<br />  Many <a href="http://simple.wikipedia.org/wiki/Linux" title="Linux"><span style="color: windowtext; mso-fareast-font-family: "Times New Roman"; mso-fareast-theme-font: major-fareast;">Linux</span></a> systems use the terms "linear mode" or "append mode". The <a href="http://simple.wikipedia.org/wiki/Mac_OS_X" title="Mac OS X"><span style="color: windowtext; mso-fareast-font-family: "Times New Roman"; mso-fareast-theme-font: major-fareast;">Mac OS X</span></a> 10.4 implementation — called a "Concatenated Disk Set" — does not leave the user with any usable data on the remaining drives if one drive fails in a concatenated disk set, although the disks otherwise operate as described above.<br />  Concatenation is one of the uses of the <a href="http://simple.wikipedia.org/wiki/Logical_volume_management" title="Logical volume management"><span style="color: windowtext; mso-fareast-font-family: "Times New Roman"; mso-fareast-theme-font: major-fareast;">Logical Volume Manager</span></a>in Linux. It can be used to create <a href="http://simple.wikipedia.org/w/index.php?title=Virtual&action=edit&redlink=1" title="Virtual (not yet started)"><span style="color: windowtext; mso-fareast-font-family: "Times New Roman"; mso-fareast-theme-font: major-fareast;">virtual</span></a>drives.<br />  <h3><span class="mw-headline"><span style="color: windowtext;">Drive Clone</span></span><span style="color: windowtext;"></span></h3>Most modern hard disks have a standard called <a href="http://simple.wikipedia.org/wiki/Self-Monitoring,_Analysis_and_Reporting_Technology" title="Self-Monitoring, Analysis and Reporting Technology"><span style="color: windowtext; mso-fareast-font-family: "Times New Roman"; mso-fareast-theme-font: major-fareast;">Self-Monitoring, Analysis and Reporting Technology</span></a>(S.M.A.R.T). SMART allows to monitor certain things on a hard disk drive. Certain controllers allow to replace a single hard disk even before it fails, for example because S.M.A.R.T or another disk test reports too many correctable errors. To do this, the controller will copy all the data onto a hot spare drive. After this, the disk can be replaced by another (which will simply be a hot spare)<br />  <div style="text-align: justify;"><br /></div><div class="MsoNoSpacing"><span class="mw-headline"><b style="mso-bidi-font-weight: normal;"><span style="font-size: 14.0pt;">Standard levels</span></b></span></div><ul type="disc"><li class="MsoNormal" style="line-height: normal; mso-list: l3 level1 lfo1; mso-margin-bottom-alt: auto; mso-margin-top-alt: auto; tab-stops: list .5in; text-align: justify;"><b><i><span style="font-family: "Times New Roman","serif"; font-size: 12.0pt; mso-fareast-font-family: "Times New Roman";">RAID 0</span></i></b><span style="font-family: "Times New Roman","serif"; font-size: 12.0pt; mso-fareast-font-family: "Times New Roman";"> (block-level </span><a href="http://en.wikipedia.org/wiki/Data_striping" title="Data striping"><span style="color: windowtext; font-family: "Times New Roman","serif"; font-size: 12.0pt; mso-fareast-font-family: "Times New Roman";">striping</span></a><span style="font-family: "Times New Roman","serif"; font-size: 12.0pt; mso-fareast-font-family: "Times New Roman";"> without </span><a href="http://en.wikipedia.org/wiki/Parity_bit" title="Parity bit"><span style="color: windowtext; font-family: "Times New Roman","serif"; font-size: 12.0pt; mso-fareast-font-family: "Times New Roman";">parity</span></a><span style="font-family: "Times New Roman","serif"; font-size: 12.0pt; mso-fareast-font-family: "Times New Roman";"> or </span><a href="http://en.wikipedia.org/wiki/Disk_mirroring" title="Disk mirroring"><span style="color: windowtext; font-family: "Times New Roman","serif"; font-size: 12.0pt; mso-fareast-font-family: "Times New Roman";">mirroring</span></a><span style="font-family: "Times New Roman","serif"; font-size: 12.0pt; mso-fareast-font-family: "Times New Roman";">) has no (or zero) redundancy. It provides improved      performance and additional storage but no fault tolerance. Hence simple      stripe sets are normally referred to as RAID 0. Any drive failure destroys      the array, and the likelihood of failure increases with more drives in the      array (at a minimum, potential for catastrophic data loss is double that      of isolated drives without RAID). A single drive failure destroys the      entire array because when data are written to a RAID 0 volume, the data      are broken into fragments called blocks. The number of blocks is dictated      by the <b>stripe size</b>, which is a configuration parameter of the      array. The blocks are written to their respective drives simultaneously on      the same sector. This allows smaller sections of the entire chunk of data      to be read off each drive in parallel, increasing bandwidth. RAID 0 does      not implement error checking, so any read error is uncorrectable. More      drives in the array means higher bandwidth, but greater risk of data loss.</span></li><li class="MsoNormal" style="line-height: normal; mso-list: l3 level1 lfo1; mso-margin-bottom-alt: auto; mso-margin-top-alt: auto; tab-stops: list .5in; text-align: justify;"><span style="font-family: "Times New Roman","serif"; font-size: 12.0pt; mso-fareast-font-family: "Times New Roman";">In <b><i>RAID 1</i></b>     (mirroring without parity or striping), data are written identically to      two drives, thereby producing a "mirrored set"; the read request      is serviced by either of the two drives containing the requested data,      whichever one involves least </span><a href="http://en.wikipedia.org/wiki/Seek_time" title="Seek time"><span style="color: windowtext; font-family: "Times New Roman","serif"; font-size: 12.0pt; mso-fareast-font-family: "Times New Roman";">seek time</span></a><span style="font-family: "Times New Roman","serif"; font-size: 12.0pt; mso-fareast-font-family: "Times New Roman";"> plus </span><a href="http://en.wikipedia.org/wiki/Rotational_latency" title="Rotational latency"><span style="color: windowtext; font-family: "Times New Roman","serif"; font-size: 12.0pt; mso-fareast-font-family: "Times New Roman";">rotational      latency</span></a><span style="font-family: "Times New Roman","serif"; font-size: 12.0pt; mso-fareast-font-family: "Times New Roman";">. Similarly, a write request      updates the stripes of both drives. The write performance depends on the      slower of the two writes (i.e., the one that involves larger </span><a href="http://en.wikipedia.org/wiki/Seek_time" title="Seek time"><span style="color: windowtext; font-family: "Times New Roman","serif"; font-size: 12.0pt; mso-fareast-font-family: "Times New Roman";">seek time</span></a><span style="font-family: "Times New Roman","serif"; font-size: 12.0pt; mso-fareast-font-family: "Times New Roman";"> and </span><a href="http://en.wikipedia.org/wiki/Rotational_latency" title="Rotational latency"><span style="color: windowtext; font-family: "Times New Roman","serif"; font-size: 12.0pt; mso-fareast-font-family: "Times New Roman";">rotational      latency</span></a><span style="font-family: "Times New Roman","serif"; font-size: 12.0pt; mso-fareast-font-family: "Times New Roman";">); at least two drives are      required to constitute such an array. While more constituent drives may be      employed, many implementations deal with a maximum of only two. The array      continues to operate as long as at least one drive is functioning. With      appropriate operating system support, there can be increased read      performance, and only a minimal write performance reduction; implementing      RAID 1 with a separate controller for each drive in order to perform      simultaneous reads (and writes) is sometimes called      "multiplexing" (or "duplexing" when there are only two      drives).</span></li><li class="MsoNormal" style="line-height: normal; mso-list: l3 level1 lfo1; mso-margin-bottom-alt: auto; mso-margin-top-alt: auto; tab-stops: list .5in; text-align: justify;"><span style="font-family: "Times New Roman","serif"; font-size: 12.0pt; mso-fareast-font-family: "Times New Roman";">In <b><i>RAID 10</i></b>     (mirroring and striping), data are written in stripes across primary disks      that have been mirrored to the secondary disks. A typical RAID 10      configuration consists of four drives, two for striping and two for      mirroring. A RAID 10 configuration takes the best concepts of RAID 0 and      RAID 1, and combines them to provide better performance along with the      reliability of parity without actually having parity as with RAID 5 and      RAID 6. RAID 10 is often referred to as RAID 1+0 (mirrored+striped) (see      also </span><a href="http://en.wikipedia.org/wiki/RAID#Nested_.28hybrid.29_RAID"><span style="color: windowtext; font-family: "Times New Roman","serif"; font-size: 12.0pt; mso-fareast-font-family: "Times New Roman";">Nested (hybrid) RAID</span></a><span style="font-family: "Times New Roman","serif"; font-size: 12.0pt; mso-fareast-font-family: "Times New Roman";"> below).</span></li><li class="MsoNormal" style="line-height: normal; mso-list: l3 level1 lfo1; mso-margin-bottom-alt: auto; mso-margin-top-alt: auto; tab-stops: list .5in; text-align: justify;"><span style="font-family: "Times New Roman","serif"; font-size: 12.0pt; mso-fareast-font-family: "Times New Roman";">In <b><i>RAID 2</i></b>     (bit-level striping with dedicated Hamming-code parity), all disk spindle      rotation is synchronized, and data are striped such that each sequential </span><a href="http://en.wikipedia.org/wiki/Bit" title="Bit"><span style="color: windowtext; font-family: "Times New Roman","serif"; font-size: 12.0pt; mso-fareast-font-family: "Times New Roman";">bit</span></a><span style="font-family: "Times New Roman","serif"; font-size: 12.0pt; mso-fareast-font-family: "Times New Roman";"> is      on a different drive. </span><a href="http://en.wikipedia.org/wiki/Hamming_code" title="Hamming code"><span style="color: windowtext; font-family: "Times New Roman","serif"; font-size: 12.0pt; mso-fareast-font-family: "Times New Roman";">Hamming-code</span></a><span style="font-family: "Times New Roman","serif"; font-size: 12.0pt; mso-fareast-font-family: "Times New Roman";"> parity is calculated across corresponding bits and stored      on at least one parity drive. This theoretical RAID level is not used in      practice.</span></li><li class="MsoNormal" style="line-height: normal; mso-list: l3 level1 lfo1; mso-margin-bottom-alt: auto; mso-margin-top-alt: auto; tab-stops: list .5in; text-align: justify;"><span style="font-family: "Times New Roman","serif"; font-size: 12.0pt; mso-fareast-font-family: "Times New Roman";">In <b><i>RAID 3</i></b>     (byte-level striping with dedicated parity), all disk spindle rotation is      synchronized, and data are striped so each sequential </span><a href="http://en.wikipedia.org/wiki/Byte" title="Byte"><span style="color: windowtext; font-family: "Times New Roman","serif"; font-size: 12.0pt; mso-fareast-font-family: "Times New Roman";">byte</span></a><span style="font-family: "Times New Roman","serif"; font-size: 12.0pt; mso-fareast-font-family: "Times New Roman";">     is on a different drive. Parity is calculated across corresponding bytes      and stored on a dedicated parity drive. Although implementations exist,<sup><a href="http://en.wikipedia.org/wiki/RAID#cite_note-10"><span style="color: windowtext;">[9]</span></a></sup> RAID 3 is not commonly used      in practice.</span></li><li class="MsoNormal" style="line-height: normal; mso-list: l3 level1 lfo1; mso-margin-bottom-alt: auto; mso-margin-top-alt: auto; tab-stops: list .5in; text-align: justify;"><b><i><span style="font-family: "Times New Roman","serif"; font-size: 12.0pt; mso-fareast-font-family: "Times New Roman";">RAID 4</span></i></b><span style="font-family: "Times New Roman","serif"; font-size: 12.0pt; mso-fareast-font-family: "Times New Roman";"> (block-level striping with dedicated parity) is      equivalent to RAID 5 (see below) except that all parity data are stored on      a single drive. In this arrangement files may be distributed between      multiple drives. Each drive operates independently, allowing I/O requests      to be performed in parallel. However, the use of a dedicated parity drive      could create a performance </span><a href="http://en.wikipedia.org/wiki/Bottleneck" title="Bottleneck"><span style="color: windowtext; font-family: "Times New Roman","serif"; font-size: 12.0pt; mso-fareast-font-family: "Times New Roman";">bottleneck</span></a><span style="font-family: "Times New Roman","serif"; font-size: 12.0pt; mso-fareast-font-family: "Times New Roman";">; because the parity data must be written to a single,      dedicated parity drive for each block of non-parity data, the overall      write performance may depend a great deal on the performance of this      parity drive.</span></li><li class="MsoNormal" style="line-height: normal; mso-list: l3 level1 lfo1; mso-margin-bottom-alt: auto; mso-margin-top-alt: auto; tab-stops: list .5in; text-align: justify;"><b><i><span style="font-family: "Times New Roman","serif"; font-size: 12.0pt; mso-fareast-font-family: "Times New Roman";">RAID 5</span></i></b><span style="font-family: "Times New Roman","serif"; font-size: 12.0pt; mso-fareast-font-family: "Times New Roman";"> (block-level striping with distributed parity)      distributes parity along with the data and requires all drives but one to      be present to operate; the array is not destroyed by a single drive      failure. Upon drive failure, any subsequent reads can be calculated from      the distributed parity such that the drive failure is masked from the end      user. However, a single drive failure results in reduced performance of      the entire array until the failed drive has been replaced and the      associated data rebuilt, because each block of the failed disk needs to be      reconstructed by reading all other disks i.e. the parity and other data      blocks of a RAID stripe. Additionally, there is the potentially disastrous      </span><a href="http://en.wikipedia.org/wiki/RAID_5_write_hole" title="RAID 5 write hole"><span style="color: windowtext; font-family: "Times New Roman","serif"; font-size: 12.0pt; mso-fareast-font-family: "Times New Roman";">RAID 5 write      hole</span></a><span style="font-family: "Times New Roman","serif"; font-size: 12.0pt; mso-fareast-font-family: "Times New Roman";">. RAID 5 requires at least      three disks.</span></li><li class="MsoNormal" style="line-height: normal; mso-list: l3 level1 lfo1; mso-margin-bottom-alt: auto; mso-margin-top-alt: auto; tab-stops: list .5in; text-align: justify;"><b><i><span style="font-family: "Times New Roman","serif"; font-size: 12.0pt; mso-fareast-font-family: "Times New Roman";">RAID 6</span></i></b><span style="font-family: "Times New Roman","serif"; font-size: 12.0pt; mso-fareast-font-family: "Times New Roman";"> (block-level striping with double distributed parity)      provides fault tolerance of two drive failures; the array continues to      operate with up to two failed drives. This makes larger RAID groups more      practical, especially for high-availability systems. This becomes      increasingly important as large-capacity drives lengthen the time needed      to recover from the failure of a single drive. Single-parity RAID levels      are as vulnerable to data loss as a RAID 0 array until the failed drive is      replaced and its data rebuilt; the larger the drive, the longer the      rebuild takes. Double parity gives additional time to rebuild the array      without the data being at risk if a single additional drive fails before      the rebuild is complete. Like RAID 5, a single drive failure results in      reduced performance of the entire array until the failed drive has been      replaced and the associated data rebuilt.</span></li></ul><div class="MsoNoSpacing" style="text-align: justify;"><span class="mw-headline"><b style="mso-bidi-font-weight: normal;"><span style="font-family: "Times New Roman","serif"; font-size: 12.0pt;">Nested (hybrid) RAID</span></b></span></div><div class="MsoNoSpacing" style="text-align: justify;"><span style="font-family: "Times New Roman","serif"; font-size: 12.0pt;">In what was originally termed <b>hybrid RAID</b>,<sup><a href="http://en.wikipedia.org/wiki/RAID#cite_note-Vijayan-11"><span style="color: windowtext;">[10]</span></a></sup> many storage controllers allow RAID levels to be nested. The elements of a <i>RAID</i> may be either individual drives or RAIDs themselves. However, if a RAID is itself an element of a larger RAID, it is unusual for its elements to be themselves RAIDs.</span></div><div class="MsoNoSpacing" style="text-align: justify;"><span style="font-family: "Times New Roman","serif"; font-size: 12.0pt;">As there is no basic RAID level numbered larger than 9, nested RAIDs are usually clearly described by attaching the numbers indicating the RAID levels, sometimes with a "+" in between. The order of the digits in a nested RAID designation is the order in which the nested array is built: For a RAID 1+0, drives are first combined into multiple level 1 RAIDs that are themselves treated as single drives to be combined into a single RAID 0; the reverse structure is also possible (RAID 0+1).</span></div><div style="text-align: justify;">The final RAID is known as the top array. When the top array is a RAID 0 (such as in RAID 1+0 and RAID 5+0), most vendors omit the "+" (yielding RAID 10 and RAID 50, respectively).</div><ul type="disc"><li class="MsoNormal" style="line-height: normal; mso-list: l1 level1 lfo2; mso-margin-bottom-alt: auto; mso-margin-top-alt: auto; tab-stops: list .5in; text-align: justify;"><span style="font-family: "Times New Roman","serif"; font-size: 12.0pt;">RAID      0+1: striped sets in a mirrored set (minimum four drives; even number of      drives) provides fault tolerance and improved performance but increases      complexity.</span></li></ul><div class="MsoNormal" style="margin-bottom: .0001pt; margin-bottom: 0in; margin-left: .5in; margin-right: 0in; margin-top: 0in; text-align: justify;"><span style="font-family: "Times New Roman","serif"; font-size: 12.0pt; line-height: 115%;">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.</span></div><ul type="disc"><li class="MsoNormal" style="line-height: normal; mso-list: l4 level1 lfo3; mso-margin-bottom-alt: auto; mso-margin-top-alt: auto; tab-stops: list .5in; text-align: justify;"><span style="font-family: "Times New Roman","serif"; font-size: 12.0pt;">RAID      1+0: (a.k.a. RAID 10) mirrored sets in a striped set (minimum four drives;      even number of drives) provides fault tolerance and improved performance      but increases complexity.</span></li></ul><div class="MsoNormal" style="margin-bottom: .0001pt; margin-bottom: 0in; margin-left: .5in; margin-right: 0in; margin-top: 0in; text-align: justify;"><span style="font-family: "Times New Roman","serif"; font-size: 12.0pt; line-height: 115%;">The key difference from RAID 0+1 is that RAID 1+0 creates a striped set from a series of mirrored drives. The array can sustain multiple drive losses so long as no mirror loses all its drives.</span><a href="http://en.wikipedia.org/wiki/RAID#cite_note-layton-lm-12"><sup><span style="color: windowtext; font-family: "Times New Roman","serif"; font-size: 12.0pt; line-height: 115%;">[11]</span></sup></a><span style="font-family: "Times New Roman","serif"; font-size: 12.0pt; line-height: 115%;"></span></div><ul type="disc"><li class="MsoNormal" style="line-height: normal; mso-list: l2 level1 lfo4; mso-margin-bottom-alt: auto; mso-margin-top-alt: auto; tab-stops: list .5in; text-align: justify;"><span style="font-family: "Times New Roman","serif"; font-size: 12.0pt;">RAID      5+3: mirrored striped set with distributed parity (some manufacturers      label this as RAID 53).</span></li></ul><div style="text-align: justify;">A RAID controller might support upgrading a RAID 1 array to a RAID 1+0 array on the fly, but require a lengthy off-line rebuild to upgrade from RAID 1 to RAID 0+1.<sup>[</sup><a href="http://en.wikipedia.org/wiki/Wikipedia:Citation_needed" title="Wikipedia:Citation needed"><i><sup><span style="color: windowtext; mso-fareast-font-family: "Times New Roman"; mso-fareast-theme-font: major-fareast;">citation needed</span></sup></i></a><sup>]</sup></div><div class="MsoNormal" style="line-height: normal; mso-margin-bottom-alt: auto; mso-margin-top-alt: auto; mso-outline-level: 1;"><br /></div><div class="MsoNormal" style="line-height: normal; mso-margin-bottom-alt: auto; mso-margin-top-alt: auto; mso-outline-level: 1;"><br /></div><br /><br /><br /><br /></div>

RAID Redundant Array of Independent Disks

Redundant Array of Independent Disks, History of RAID : RAID ( redundant array of independent disks , originally redundant array of inexpe...

RAID Redundant Array of Independent Disks

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