📄 asg12.htm
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<FONT SIZE=4 COLOR="#FF0000"><A NAME="I24"></A><A NAME="I25"></A><A NAME="I26"></A><A NAME="I27"></A><A NAME="I28"></A><A NAME="I29"></A><A NAME="I30"></A><A NAME="I31"></A><A NAME="I32"></A><A NAME="I33"></A><A NAME="I34"></A><A NAME="I35"></A><A NAME="I36"></A><A NAME="I37"></A><A NAME="I38"></A><A NAME="I39"></A><A NAME="I40"></A><A NAME="I41"></A><A NAME="I42"></A><A NAME="I43"></A><A NAME="I44"></A><A NAME="I45"></A><B>RAID 0: Non-Redundant Striped Array</B></FONT></CENTER></H6>
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<P>RAID 0 is a non-redundant group of striped disk drives. That means that if one drive fails, the entire array fails. Failure is an important concern of any RAID strategy. Under a RAID setup, the Mean Time Between Failures (MTBF) rating listed by a drive should be divided by the number of drives in the setup. A diagram of RAID 0 can be found in Figure 12.2.
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<P><B> <A HREF="javascript:if(confirm('http://docs.rinet.ru:8080/Apachu/12asg02.gif \n\nThis file was not retrieved by Teleport Pro, because it was redirected to an invalid location. You should report this problem to the site\'s webmaster. \n\nDo you want to open it from the server?'))window.location='http://docs.rinet.ru:8080/Apachu/12asg02.gif'" tppabs="http://docs.rinet.ru:8080/Apachu/12asg02.gif">Figure 12.2. Non-redundant striped array. Data </B><B>stripes from disks A, B, and C are interleaved to create a single </B><B>logical storage unit. Reads and writes can occur concurrently on all </B><B>drives.</A></B>
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<P>The best performance in any RAID configuration is on a RAID 0 configuration. This is because no data-integrity processing is done on read or write operations, and because disks can perform concurrently. The array can have two or more disks.
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<P>RAID 0 supports multiple concurrent read and write transactions on short requests. Longer read and write transactions can be split and handled concurrently.
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<H6 ALIGN=CENTER>
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<FONT SIZE=4 COLOR="#FF0000"><A NAME="I46"></A><A NAME="I47"></A><A NAME="I48"></A><A NAME="I49"></A><A NAME="I50"></A><A NAME="I51"></A><A NAME="I52"></A><A NAME="I53"></A><A NAME="I54"></A><A NAME="I55"></A><A NAME="I56"></A><A NAME="I57"></A><A NAME="I58"></A><A NAME="I59"></A><A NAME="I60"></A><A NAME="I61"></A><A NAME="I62"></A><A NAME="I63"></A><A NAME="I64"></A><A NAME="I65"></A><A NAME="I66"></A><A NAME="I67"></A><B>RAID 1: Mirrored Arrays</B></FONT></CENTER></H6>
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<P>RAID 1 provides disk mirroring—information is duplicated to both disks. Each adapter manages two drives. There is no striping between the two disks; information is duplicated to both disks. However, you can stripe several RAID 1 arrays together. A diagram of RAID 1 can be found in Figure 12.3.
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<P><B> <A HREF="javascript:if(confirm('http://docs.rinet.ru:8080/Apachu/12asg03.gif \n\nThis file was not retrieved by Teleport Pro, because it was redirected to an invalid location. You should report this problem to the site\'s webmaster. \n\nDo you want to open it from the server?'))window.location='http://docs.rinet.ru:8080/Apachu/12asg03.gif'" tppabs="http://docs.rinet.ru:8080/Apachu/12asg03.gif">Figure 12.3. A mirrored array. Data is written in </B><B>duplicate to drives A and B. Different read transactions can occur </B><B>concurrently on either drive.</A></B>
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<P>To maintain the mirroring, both disks write the same data. Therefore, RAID 1 offers no performance gains on write accesses. However, different read transactions can occur simultaneously, with a performance increase of 100 percent. RAID 1 delivers the best performance of any RAID in a multi-user environment. It provides faster short- and long-read transactions because the operations can resolve to either of the disks.
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<P>Write transactions are slower because both disks must write the same amount of data. Implementing RAID 1 can be quite expensive because it requires double the number of disks and double the storage capacity.
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<H6 ALIGN=CENTER>
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<FONT SIZE=4 COLOR="#FF0000"><A NAME="I68"></A><A NAME="I69"></A><A NAME="I70"></A><A NAME="I71"></A><A NAME="I72"></A><A NAME="I73"></A><A NAME="I74"></A><A NAME="I75"></A><A NAME="I76"></A><A NAME="I77"></A><A NAME="I78"></A><A NAME="I79"></A><A NAME="I80"></A><A NAME="I81"></A><A NAME="I82"></A><A NAME="I83"></A><A NAME="I84"></A><A NAME="I85"></A><A NAME="I86"></A><A NAME="I87"></A><A NAME="I88"></A><A NAME="I89"></A><B>RAID 2: Parallel Array with Error-Correcting Code (ECC) Data Protection </B><B>Disk</B></FONT></CENTER></H6>
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<P>RAID 2 uses a sector-stripe array. Data is written to a group of drives with some drives storing the ECC. Because most modern drives store ECC information at the end of a sector, this configuration offers no particular advantage over a RAID 3 configuration. A diagram of RAID 1 can be found in Figure 12.4.
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<P><B> <A HREF="javascript:if(confirm('http://docs.rinet.ru:8080/Apachu/12asg04.gif \n\nThis file was not retrieved by Teleport Pro, because it was redirected to an invalid location. You should report this problem to the site\'s webmaster. \n\nDo you want to open it from the server?'))window.location='http://docs.rinet.ru:8080/Apachu/12asg04.gif'" tppabs="http://docs.rinet.ru:8080/Apachu/12asg04.gif">Figure 12.4. Parallel array with ECC. Multiple </B><B>drives are striped for data storage. ECC is stored on one or more </B><B>drives. Read and write transactions span all drives.</A></B>
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<P>The ECC is calculated from the multiple-disks stripe and stored in a separate drive. Reads happen at normal speed. Writes are slower than normal because of the ECC that needs to be calculated and stored.
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<H6 ALIGN=CENTER>
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<FONT SIZE=4 COLOR="#FF0000"><A NAME="I90"></A><A NAME="I91"></A><A NAME="I92"></A><A NAME="I93"></A><A NAME="I94"></A><A NAME="I95"></A><A NAME="I96"></A><A NAME="I97"></A><A NAME="I98"></A><A NAME="I99"></A><A NAME="I100"></A><A NAME="I101"></A><A NAME="I102"></A><A NAME="I103"></A><A NAME="I104"></A><A NAME="I105"></A><A NAME="I106"></A><A NAME="I107"></A><A NAME="I108"></A><A NAME="I109"></A><A NAME="I110"></A><A NAME="I111"></A><B>RAID 3: Parallel Array with Parity</B></FONT></CENTER></H6>
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<P>RAID 3, like RAID 2, sectors data into stripes across a group of drives. Error detection is dependent on the ECC stored in each sector on each drive. If a failure is detected, data consistency is assured by mathematically calculating the correct data from information stored on the remaining drives. Files usually span all drives in the array, making disk transfer rates optimal. However, I/O cannot overlap because each transaction affects all drives in the array. This configuration provides the best performance for a single-user workstation. To avoid performance degradation on short-file accesses, synchronized drives, or <I>spindles</I>, are required. A diagram of RAID 3 can be found in Figure 12.5.
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<P><B> <A HREF="javascript:if(confirm('http://docs.rinet.ru:8080/Apachu/12asg05.gif \n\nThis file was not retrieved by Teleport Pro, because it was redirected to an invalid location. You should report this problem to the site\'s webmaster. \n\nDo you want to open it from the server?'))window.location='http://docs.rinet.ru:8080/Apachu/12asg05.gif'" tppabs="http://docs.rinet.ru:8080/Apachu/12asg05.gif">Figure 12.5. Parallel array with parity. Multiple </B><B>drives are striped for data storage. Parity is stored on one drive. Read </B><B>and write transactions span all drives. In the event of a hardware </B><B>failure, the data from the failed drive can be reconstructed on-the-fly </B><B>from the data stored on the other drives.</A></B>
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<H6 ALIGN=CENTER>
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<FONT SIZE=4 COLOR="#FF0000"><A NAME="I112"></A><A NAME="I113"></A><A NAME="I114"></A><A NAME="I115"></A><A NAME="I116"></A><A NAME="I117"></A><A NAME="I118"></A><A NAME="I119"></A><A NAME="I120"></A><A NAME="I121"></A><A NAME="I122"></A><A NAME="I123"></A><A NAME="I124"></A><A NAME="I125"></A><A NAME="I126"></A><A NAME="I127"></A><A NAME="I128"></A><A NAME="I129"></A><A NAME="I130"></A><A NAME="I131"></A><A NAME="I132"></A><A NAME="I133"></A><B>RAID 4: Striped Array with Parity</B></FONT></CENTER></H6>
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<P>The configuration of RAID 4 is the same as that of RAID 3, but the size of the stripes is larger, which allows read operations to be overlapped. Write operations have to update the parity drive and cannot be overlapped. This configuration offers no significant advantage over a RAID 5 configuration. RAID 4 disk arrays can have three or five disks. A diagram for RAID 4 can be found in Figure 12.6.
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<P><B> <A HREF="javascript:if(confirm('http://docs.rinet.ru:8080/Apachu/12asg06.gif \n\nThis file was not retrieved by Teleport Pro, because it was redirected to an invalid location. You should report this problem to the site\'s webmaster. \n\nDo you want to open it from the server?'))window.location='http://docs.rinet.ru:8080/Apachu/12asg06.gif'" tppabs="http://docs.rinet.ru:8080/Apachu/12asg06.gif">Figure 12.6. Striped array with parity. Stripes </B><B>in this configuration are larger, so read and write transactions can </B><B>occur concurrently. Because there is only a single-parity drive, all </B><B>write transactions need to update the parity drive.</A></B>
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<H6 ALIGN=CENTER>
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<FONT SIZE=4 COLOR="#FF0000"><A NAME="I134"></A><A NAME="I135"></A><A NAME="I136"></A><A NAME="I137"></A><A NAME="I138"></A><A NAME="I139"></A><A NAME="I140"></A><A NAME="I141"></A><A NAME="I142"></A><A NAME="I143"></A><A NAME="I144"></A><A NAME="I145"></A><A NAME="I146"></A><A NAME="I147"></A><A NAME="I148"></A><A NAME="I149"></A><A NAME="I150"></A><A NAME="I151"></A><A NAME="I152"></A><A NAME="I153"></A><A NAME="I154"></A><A NAME="I155"></A><B>RAID-5 Rotating Parity Array</B></FONT></CENTER></H6>
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<P>RAID 5 implements a rotating parity array that eliminates the bottleneck of the single-parity drive configuration used in RAID 4. RAID 5 uses large stripes to enable the overlap of multiple I/O transactions. Each drive takes a turn storing parity for a stripe. Most write operations access only a single data drive and the current parity drive. Write operations can overlap. A diagram for RAID 5 can be found in Figure 12.7.
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<P><B> <A HREF="javascript:if(confirm('http://docs.rinet.ru:8080/Apachu/12asg07.gif \n\nThis file was not retrieved by Teleport Pro, because it was redirected to an invalid location. You should report this problem to the site\'s webmaster. \n\nDo you want to open it from the server?'))window.location='http://docs.rinet.ru:8080/Apachu/12asg07.gif'" tppabs="http://docs.rinet.ru:8080/Apachu/12asg07.gif">Figure 12.7. Rotating parity array. All drives </B><B>store data and parity information. Reads and writes can occur </B><B>concurrently.</A></B>
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<P>This is the best configuration for a fault-tolerant setup in a multi-user environment that is not performance sensitive or that performs few write operations. Disk arrays can have between three and seven disks. RAID 5 is efficient because it uses parity information instead of duplicating data. However, because parity information must be calculated for all drives with each write operation, write operations are not as efficient. This setup is not recommended for applications that write data. Read operations, such as those sustained by a Web server, are better with RAID 5 than with the RAID 1 counterpart. For a Web server application, RAID 5 provides the best performance and reliability.
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<FONT SIZE=4 COLOR="#FF0000"><A NAME="I156"></A><A NAME="I157"></A><A NAME="I158"></A><A NAME="I159"></A><A NAME="I160"></A><A NAME="I161"></A><A NAME="I162"></A><A NAME="I163"></A><A NAME="I164"></A><A NAME="I165"></A><A NAME="I166"></A><A NAME="I167"></A><A NAME="I168"></A><A NAME="I169"></A><A NAME="I170"></A><A NAME="I171"></A><A NAME="I172"></A><A NAME="I173"></A><A NAME="I174"></A><A NAME="I175"></A><A NAME="I176"></A><A NAME="I177"></A><B>RAID 6</B></FONT></CENTER></H6>
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<P>RAID 6 describes a scheme involving a two-dimensional disk array that promises to tolerate any two-drive failure; however, no commercial implementation of RAID 6 exists as of this writing.
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<FONT SIZE=4 COLOR="#FF0000"><A NAME="I178"></A><A NAME="I179"></A><A NAME="I180"></A><A NAME="I181"></A><A NAME="I182"></A><A NAME="I183"></A><A NAME="I184"></A><A NAME="I185"></A><A NAME="I186"></A><A NAME="I187"></A><A NAME="I188"></A><A NAME="I189"></A><A NAME="I190"></A><A NAME="I191"></A><A NAME="I192"></A><A NAME="I193"></A><A NAME="I194"></A><A NAME="I195"></A><A NAME="I196"></A><A NAME="I197"></A><A NAME="I198"></A><A NAME="I199"></A><B>RAID 7</B></FONT></CENTER></H6>
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<P>RAID 7 is a marketing term created by Storage Computer, Inc. Information on the Internet about RAID 7 indicates that while some of RAID 7's performance claims may be worthwhile, RAID 7 is controversial because of its use of a cache. A few major vendors have started to introduce RAID 7 products. RAID 7 is basically just RAID with caching.
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<H6 ALIGN=CENTER>
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<FONT SIZE=4 COLOR="#FF0000"><A NAME="I200"></A><A NAME="I201"></A><A NAME="I202"></A><A NAME="I203"></A><A NAME="I204"></A><A NAME="I205"></A><A NAME="I206"></A><A NAME="I207"></A><A NAME="I208"></A><A NAME="I209"></A><A NAME="I210"></A><A NAME="I211"></A><A NAME="I212"></A><A NAME="I213"></A><A NAME="I214"></A><A NAME="I215"></A><B>Software Striping</B></FONT></CENTER></H6>
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<P>Some operating systems include software striping options, which allow your system to effectively implement RAID. However, there is a tradeoff in performance, as well as additional CPU overhead, when these options are used. Some of the vendors and operating systems that offer software striping options include Silicon Graphics, Linux, and Hewlett-Packard and Sun. For more information, check your UNIX documentation.
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<FONT SIZE=4 COLOR="#FF0000"><B>Network Tuning</B></FONT></CENTER></H4>
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<P>For a busy site, network tuning will be the second major performance issue you'll face. Even if your disk subsystem is tuned for optimal performance, its impact will be limited unless you are able to push data through the wire quickly enough. Here are a few issues to consider:
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<LI>The size of your connection to the Internet backbone and the speed of your WAN interface. Increasing these is the most costly enhancement, but any serious Web service requires ISDN or better. An ISDN service may be more than adequate for personal Web servers, but typically, network bandwidth of less than a DS-1 (T-1) won't be able to handle any sort of intense traffic. Large providers will need DS-3 connections.
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<LI>The speed of your local Ethernet network, 10Mbps (Megabits per second) Ethernet or 100Mbps Ethernet. This factor dictates how long network transactions will take, and is affected by local traffic on your local area network (LAN). If you have a large number of systems on your network, chances are your network is not performing well. Add traffic from the outside, and it's no wonder the network seems slow! There are hardware and organizational solutions to this problem; the organizational solutions are described in the "Network Topology Tuning" section later in this chapter.
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<P>These issues can be addressed by limiting traffic to well-defined sections of your network or by adjusting the size of your network plumbing to match your needs. The bigger the pipes and the pumps, the better the performance.
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<FONT SIZE=4 COLOR="#FF0000"><B>Network </B><B>Services</B><B> (WAN</B><B>)</B></FONT></CENTER></H5>
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<P>How much bandwidth you have depends on the type of service to which you subscribe. For Internet information providers, this is generally some sort of leased line. The faster the line, the more expensive it is. Top-of-the-line DS-3 services cost tens of thousands of dollars per month. This figure doesn't even include the initial cost of the networking hardware, which can easily be over $80,000, plus any other setup fees and the rental of the line! New technologies are going to increase bandwidth for consumers, and if the pricing is attractive, many providers may begin looking at them as possible additions to or replacements of their current leased lines. (See Table 12.1.)
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<P>Telephone companies, with their huge infrastructure, are in a position to bring the price of this technology down, making it affordable to almost anyone. Network plumbing varies greatly in price and performance. For a graphical comparison of the various options, see Figures 12.8 and 12.9. The shaded areas are proportional to the amount of data they can carry. Table 12.1 presents the various services available for a WAN interface and the monthly costs associated with them.
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<P><A NAME="I216"></A><A NAME="I217"></A><A NAME="I218"></A><A NAME="I219"></A><A NAME="I220"></A><B> <A HREF="javascript:if(confirm('http://docs.rinet.ru:8080/Apachu/12asg08.gif \n\nThis file was not retrieved by Teleport Pro, because it was redirected to an invalid location. You should report this problem to the site\'s webmaster. \n\nDo you want to open it from the server?'))window.location='http://docs.rinet.ru:8080/Apachu/12asg08.gif'" tppabs="http://docs.rinet.ru:8080/Apachu/12asg08.gif">Figure 12.8. Relative size comparison of 100Mbps </B><B>Ethernet, DS-3, and 10Mbps Ethernet.</B><B></A></B>
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<P><A NAME="I221"></A><A NAME="I222"></A><A NAME="I223"></A><A NAME="I224"></A><A NAME="I225"></A><B> <A HREF="javascript:if(confirm('http://docs.rinet.ru:8080/Apachu/12asg01.gif \n\nThis file was not retrieved by Teleport Pro, because it was redirected to an invalid location. You should report this problem to the site\'s webmaster. \n\nDo you want to open it from the server?'))window.location='http://docs.rinet.ru:8080/Apachu/12asg01.gif'" tppabs="http://docs.rinet.ru:8080/Apachu/12asg01.gif">Figure 12.9. Relative size comparison of 10Mbps </B><B>Ethernet, DS-1, ISDN, DS-0, and 28.8 modems.</B><B></A></B>
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<FONT COLOR="#000080"><A NAME="I226"></A><A NAME="I227"></A><A NAME="I228"></A><A NAME="I229"></A><A NAME="I230"></A><B>Table 12.1. Service and price comparison for dedicated services.</B></FONT></CENTER>
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<TABLE BORDERCOLOR=#000040 BORDER=1 CELLSPACING=2 WIDTH="80%" CELLPADDING=2 >
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<TD VALIGN=top BGCOLOR=#80FFFF ><FONT COLOR=#000080>
<I>Service Type</I>
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<TD VALIGN=top BGCOLOR=#80FFFF ><FONT COLOR=#000080>
<I>Bandwidth</I>
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<TD VALIGN=top BGCOLOR=#80FFFF ><FONT COLOR=#000080>
<I>Price/Month</I>
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<TD VALIGN=top BGCOLOR=#80FFFF ><FONT COLOR=#000080>
DS-3/ATM
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<TD VALIGN=top BGCOLOR=#80FFFF ><FONT COLOR=#000080>
45,000Kbps
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<TD VALIGN=top BGCOLOR=#80FFFF ><FONT COLOR=#000080>
>$25,000
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<TR>
<TD VALIGN=top BGCOLOR=#80FFFF ><FONT COLOR=#000080>
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