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Added: xml/htdocs/doc/en/articles software-raid-p1.xml |
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software-raid-p2.xml |
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two new articles from #104198 |
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Revision Changes Path |
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1.1 xml/htdocs/doc/en/articles/software-raid-p1.xml |
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file : http://www.gentoo.org/cgi-bin/viewcvs.cgi/xml/htdocs/doc/en/articles/software-raid-p1.xml?rev=1.1&content-type=text/x-cvsweb-markup&cvsroot=gentoo |
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plain: http://www.gentoo.org/cgi-bin/viewcvs.cgi/xml/htdocs/doc/en/articles/software-raid-p1.xml?rev=1.1&content-type=text/plain&cvsroot=gentoo |
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Index: software-raid-p1.xml |
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=================================================================== |
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<?xml version="1.0" encoding="UTF-8"?> |
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<!DOCTYPE guide SYSTEM "/dtd/guide.dtd"> |
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<!-- $Header: /var/cvsroot/gentoo/xml/htdocs/doc/en/articles/software-raid-p1.xml,v 1.1 2005/09/20 18:35:24 rane Exp $ --> |
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|
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<guide link="/doc/en/articles/software-raid-p1.xml"> |
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<title>Software RAID in the new Linux 2.4 kernel, Part 1</title> |
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|
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<author title="Author"> |
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<mail link="drobbins@g.o">Daniel Robbins</mail> |
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</author> |
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<!-- xmlified by: Joshua Saddler (jackdark@×××××.com) --> |
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|
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<abstract> |
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In his two-part series on the Linux 2.4 Software RAID, Daniel Robbins |
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introduces the new technology that's used to increase disk performance |
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and reliability by distributing data over multiple disks. This first |
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installment covers Software RAID setup (kernel and tools installation) |
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and shows you how to create linear and RAID-0 volumes. |
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</abstract> |
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|
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<!-- The original version of this article was first published on IBM |
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developerWorks, and is property of Westtech Information Services. This |
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document is an updated version of the original article, and contains |
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various improvements made by the Gentoo Linux Documentation team --> |
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|
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<version>1.0</version> |
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<date>2005-08-29</date> |
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|
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<chapter> |
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<title>Installation and a general introduction</title> |
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<section> |
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<title>The wonders of RAID</title> |
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<body> |
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|
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<note> |
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The original version of this article was first published on IBM |
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developerWorks, and is property of Westtech Information Services. This |
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document is an updated version of the original article, and contains |
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various improvements made by the Gentoo Linux Documentation team. |
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</note> |
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|
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<p> |
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The 2.4 kernel has a number of nifty features and additions. One of |
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these is the inclusion of a modern Software RAID implementation -- yay! |
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Software RAID allows you to dramatically increase Linux disk IO |
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performance and reliability without buying expensive hardware RAID |
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controllers or enclosures. Because it's implemented in software, Linux |
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RAID software is flexible, fast... and fun! |
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</p> |
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|
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<p> |
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The concept behind Software RAID is simple -- it allows you to combine |
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two or more block devices (usually disk partitions) into a single RAID |
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device. So let's say you have three empty partitions: |
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<path>hda3</path>, <path>hdb3</path>, and <path>hdc3</path>. Using |
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Software RAID, you can combine these partitions and address them as a |
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single RAID device, <path>/dev/md0</path>. <path>md0</path> can then |
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be formatted to contain a filesystem and used like any other |
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partition. There are also a number of different ways to configure a |
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RAID volume -- some maximize performance, others maximize |
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availability, while others provide a mixture of both. |
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</p> |
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|
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<p> |
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There are two forms of RAID: linear and RAID-0 mode. Neither one is |
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technically a form of RAID at all, since RAID stands for "redundant |
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array of inexpensive disks", and RAID-0 and linear mode don't |
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provide any kind of data redundancy. However, both modes -- especially |
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RAID-0 -- are very useful. After giving you a quick overview of these |
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two forms of "AID", I'll step you through the process of getting |
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Software RAID set up on your system. |
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</p> |
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|
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</body> |
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</section> |
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</chapter> |
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|
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<chapter> |
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<title>Introduction to linear mode</title> |
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<section> |
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<body> |
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|
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<p> |
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Linear mode is one of the simplest methods of combining two or more |
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block devices into a RAID volume -- the method of simple |
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concatenation. If you have three partitions, <path>hda3</path>, |
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<path>hdb3</path>, and <path>hdc3</path>, and each is about 2GB, they |
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will create a resultant linear volume of 6GB. The first third of the |
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linear volume will reside on <path>hda3</path>, the last third on |
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<path>hdc3</path>, and the middle third on <path>hdb3</path>. |
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</p> |
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|
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<p> |
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To configure a linear volume, you'll need at least two partitions |
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that you'd like to join together. They can be different sizes, and |
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they can even all reside on the same physical disk without |
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negatively affecting performance. |
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</p> |
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|
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</body> |
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</section> |
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<section> |
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<title>Linear applications</title> |
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<body> |
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|
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<p> |
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Linear mode is the best way to combine two or more partitions on the |
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same disk into a single volume. While doing this with any other RAID |
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technique will result in a dramatic loss of performance, linear mode |
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is saved from this problem because it doesn't write to its |
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constituent partitions in parallel (as all the other RAID modes do). |
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But for the same reason, linear mode has the liability of lacking |
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scale in performance compared to RAID-0, RAID-4, RAID-5, and to some |
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extent RAID-1. |
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</p> |
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|
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<p> |
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In general, linear mode doesn't provide any kind of performance |
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improvement over traditional non-RAID partitions. Actually, if you |
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spread your linear volume over multiple disks, your volume is more |
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likely to become unavailable due to a random hard drive failure. The |
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probability of failure of a linear volume will be equal to the sum of |
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the probabilities of failure of its constituent physical disks and |
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controllers. If one physical disk dies, the linear volume is |
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generally unrecoverable. Linear mode does not offer any additional |
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redundancy over using a single disk. |
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</p> |
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|
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<p> |
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But linear mode is a great way to avoid repartitioning a single disk. |
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For example, say your second IDE drive has two unused partitions, |
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<path>hdb1</path> and <path>hdb3</path>. And say you're unable to |
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repartition the drive due to critical data hanging out at |
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<path>hdb2</path>. You can still combine <path>hdb1</path> and |
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<path>hdb3</path> into a single, cohesive whole using linear mode. |
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</p> |
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|
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<p> |
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Linear mode is also a good way to combine partitions of different |
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sizes on different disks when you just need a single big partition |
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(and don't really need to increase performance). But for any other |
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job there are better RAID technologies you can use. |
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</p> |
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|
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</body> |
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</section> |
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</chapter> |
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|
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<chapter> |
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<title>Introduction to RAID-0 mode</title> |
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<section> |
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<body> |
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|
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<p> |
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RAID-0 is another one of those "RAID" modes that doesn't have any |
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"R" (redundancy) at all. Nevertheless, RAID-0 is immensely useful. |
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This is primarily because it offers the highest performance |
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potential of any form of RAID. |
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</p> |
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|
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<p> |
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To set up a RAID-0 volume you'll need two or more equally (or |
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almost equally) sized partitions. The RAID-0 code will evenly |
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distribute writes (and thus reads) between all constituent |
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partitions. And by parallelizing reads and writes between all |
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constituent devices, RAID-0 has the benefit of multiplying IO |
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performance. Ignoring the complexities of controller and bus |
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bandwidth, you can expect a RAID-0 volume composed of two |
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partitions on two separate identical disks to offer nearly double |
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the performance of a traditional partition. Crank your RAID-0 |
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volume up to three disks, and performance will nearly triple. |
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This is why a RAID-0 array of IDE disks can outperform the fastest |
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SCSI or FC-AL drive on the market. For truly blistering |
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performance, you can set up a bunch of SCSI or FC-AL drives in a |
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RAID-0 array. That's the beauty of RAID-0. |
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</p> |
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|
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<p> |
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To create a RAID-0 volume, you'll need two or more equally sized |
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partitions located on separate disks. The capacity of the volume |
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will be equal to the combined capacity of the constituent |
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partitions. As with linear mode, you can combine block devices |
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from various sources (such as IDE and SCSI drives) into a single |
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volume with no problems. |
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</p> |
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|
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<p> |
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If you're creating a RAID-0 volume using IDE disks, you should try |
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to use UltraDMA compliant disks and controllers for maximum |
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reliability. And you should use only one drive per IDE channel to |
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avoid sluggish performance -- a slave device, especially if it's |
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also part of the RAID-0 array, will slow things down so much as to |
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nearly eliminate any RAID-0 performance benefit. You may also need |
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to add an off-board IDE controller so that you have the extra IDE |
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channels you require. |
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</p> |
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|
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<p> |
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If you're creating a RAID-0 volume out of SCSI devices, be aware |
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|
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|
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|
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1.1 xml/htdocs/doc/en/articles/software-raid-p2.xml |
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|
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file : http://www.gentoo.org/cgi-bin/viewcvs.cgi/xml/htdocs/doc/en/articles/software-raid-p2.xml?rev=1.1&content-type=text/x-cvsweb-markup&cvsroot=gentoo |
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plain: http://www.gentoo.org/cgi-bin/viewcvs.cgi/xml/htdocs/doc/en/articles/software-raid-p2.xml?rev=1.1&content-type=text/plain&cvsroot=gentoo |
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|
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Index: software-raid-p2.xml |
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=================================================================== |
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<?xml version="1.0" encoding="UTF-8"?> |
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<!DOCTYPE guide SYSTEM "/dtd/guide.dtd"> |
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<!-- $Header: /var/cvsroot/gentoo/xml/htdocs/doc/en/articles/software-raid-p2.xml,v 1.1 2005/09/20 18:35:24 rane Exp $ --> |
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|
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<guide link="/doc/en/articles/software-raid-p2.xml"> |
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<title>Software RAID in the new Linux 2.4 kernel, Part 2</title> |
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|
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<author title="Author"> |
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<mail link="drobbins@g.o">Daniel Robbins</mail> |
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</author> |
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<!-- xmlified by: Joshua Saddler (jackdark@×××××.com) --> |
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|
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<abstract> |
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In this two-part series, Daniel Robbins introduces you to Linux 2.4 |
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Software RAID, a technology used to increase disk performance and |
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reliability by distributing data over multiple disks. In this article, |
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Daniel explains what software RAID-1, 4, and 5 can and cannot do for |
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you and how you should approach the implementation of these RAID |
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levels in a production environment. In the second half of the article, |
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Daniel walks you through the simulation of a RAID-1 failed drive |
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replacement. |
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</abstract> |
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|
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<!-- The original version of this article was first published on IBM |
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developerWorks, and is property of Westtech Information Services. This |
| 250 |
document is an updated version of the original article, and contains |
| 251 |
various improvements made by the Gentoo Linux Documentation team --> |
| 252 |
|
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<version>1.0</version> |
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<date>2005-08-30</date> |
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|
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<chapter> |
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<title>Setting up RAID-1 in a production environment</title> |
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<section> |
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<title>Real-world RAID</title> |
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<body> |
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|
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<note> |
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The original version of this article was first published on IBM |
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developerWorks, and is property of Westtech Information Services. This |
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document is an updated version of the original article, and contains |
| 266 |
various improvements made by the Gentoo Linux Documentation team. |
| 267 |
</note> |
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|
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<p> |
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In my <uri link="/doc/en/articles/software-raid-p1.xml">previous |
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article</uri>, I introduced you to Linux 2.4's software RAID |
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functionality, showing you how to set up linear, RAID-0, and RAID-1 |
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volumes. In this article, we look at what you need to know in order to |
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use RAID-1 to increase availability in a production environment. This |
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requires a lot more understanding and knowledge than just setting up |
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RAID-1 on a test server or at home -- specifically, you'll need to |
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know exactly what RAID-1 will protect you against, and how to keep |
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your RAID volume up and running in case of a disk failure. In this |
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article, we'll cover these topics, starting with an overview of what |
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RAID-1, 4, and 5 can and can't do for you, and ending with a complete |
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test simulation of a failed RAID 1 drive replacement -- something that |
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you should actually do (with this article as your guide) if at all |
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possible. After going through the simulation, you'll have all the |
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experience you need to handle a RAID-1 failure in a real-world |
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environment. |
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</p> |
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|
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</body> |
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</section> |
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<section> |
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<title>What RAID doesn't do</title> |
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<body> |
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|
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<p> |
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The fault-tolerant features of RAID are designed to protect you from |
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the negative impacts of a spontaneous complete drive failure. That's |
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a good thing. But RAID isn't a perfect fix for every kind of |
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reliability problem. Before implementing a fault-tolerant form of RAID |
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(1,4,5) in a production environment, it's extremely important that you |
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know exactly what RAID will and <b>will not</b> do for you. When we're |
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in a situation where we're depending on RAID to perform, we don't want |
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to make any false assumptions about what it does. Let's start by |
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dispelling common myths about RAID 1, 4, and 5. |
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</p> |
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|
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<p> |
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A lot of people think that if they place all their important data on a |
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RAID 1/4/5 volume, then they won't have to perform regular backups. |
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This is completely false -- here's why. RAID 1/4/5 helps to protect |
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against unplanned <e>downtime</e> caused by a random drive failure. |
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However, it offers no protection against accidental or malicious |
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<e>data corruption</e>. If you type <c>cd /; rm -rf *</c> as root on a |
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RAID volume, you'll lose a lot of very important data in a matter of |
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seconds, and the fact that you have a 10 drive RAID-5 configuration will |
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be of little significance. Also, RAID won't help you if your server is |
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physically stolen or if there's a fire in your building. And of course, |
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if you don't implement a backup strategy, you won't have an archive of |
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past data -- if someone in your office deletes a bunch of important |
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files, you won't be able to recover them. That alone should be enough |
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to convince you that, in most circumstances, you should plan and |
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implement a backup strategy <e>before</e> even thinking about tackling |
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RAID-1, 4, or 5. |
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</p> |
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|
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<p> |
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Another mistake is to implement software RAID on a system composed of |
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low-quality hardware. If you're putting together a server that's going |
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to do something important, it makes sense to purchase the |
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highest-quality hardware that's still comfortably within your budget. |
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If your system is unstable or improperly cooled, you'll run into |
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problems that RAID can't solve. On a similar note, RAID obviously can't |
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give you any additional uptime in the case of a power outage. If your |
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server is going to be doing anything relatively important, make sure |
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that it's been equipped with an uninterruptible power supply (UPS). |
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</p> |
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|
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<p> |
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Next, we move on to filesystem issues. The filesystem exists "on top" |
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of your software RAID volume. This means that using software RAID does |
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not allow you to escape filesystem issues, such as long and potentially |
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problematic <c>fsck</c>s if you happen to be using a non-journalled or |
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flaky filesystem. So, software RAID isn't going to make the ext2 |
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filesystem more reliable; that's why it's so important that the Linux |
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community has ReiserFS, as well as JFS and XFS in the works. Software |
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RAID and a reliable journalling filesystem make a great combination. |
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</p> |
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|
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</body> |
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</section> |
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<section> |
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<title>RAID - intelligent implementation</title> |
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<body> |
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|
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<p> |
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Hopefully, the previous section dispelled any RAID myths that you might |
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have had. When you implement RAID-1, 4, or 5, it's very important that |
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you view the technology as something that will enhance <e>uptime</e>. |
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When you implement one of these RAID levels, you're protecting yourself |
| 359 |
against a very specific situation -- a spontaneous complete (single or |
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multiple) drive failure. If you experience this situation, software |
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RAID will allow the system to continue running, while you make |
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arrangements to replace the failed drive with a new one. In other words, |
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if you implement RAID 1, 4, or 5, you'll be reducing your risk of |
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having a long, unplanned downtime due to a complete drive failure. |
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Instead, you can have a short planned downtime -- just enough time to |
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replace the dead drive. Obviously, this means that if having a |
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highly-available system isn't a priority for you, then you shouldn't be |
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implementing software RAID, unless you plan to use it primarily as a |
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way to boost file I/O performance. |
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</p> |
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|
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<p> |
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A smart system administrator uses software RAID for a specific purpose |
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-- to improve the reliability of an already very reliable server. If |
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you're a smart sysadmin, you've already covered the basics. You've |
| 376 |
protected your organization against catastrophe by implementing a |
| 377 |
regular backup plan. You've hooked your server up to a UPS, and have |
| 378 |
the UPS monitoring software up and running so that your server will |
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shut down safely in the case of an extended power outage. Maybe you're |
| 380 |
using a journalling filesystem such as ReiserFS to reduce <c>fsck</c> |
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time and increase filesystem reliability and performance. And hopefully, |
| 382 |
your server is well-cooled and is composed of high-quality hardware, |
| 383 |
and you've paid close attention to security issues. Now, and only now, |
| 384 |
should you consider implementing software RAID-1, 4 or 5 -- by doing so, |
| 385 |
you'll potentially give your server a few more percentage points of |
| 386 |
uptime by guarding it against a complete drive failure. Software RAID |
| 387 |
is that added layer of protection that makes an already rugged server |
| 388 |
even better. |
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</p> |
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|
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</body> |
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</section> |
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</chapter> |
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|
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<chapter> |
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<title>A RAID-1 walkthrough</title> |
| 397 |
<section> |
| 398 |
<body> |
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|
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<p> |
| 401 |
Now that you've read about what RAID can and can't do, I hope you have |
| 402 |
reasonable expectations and the right attitude. In this section, I'll |
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walk you through the process of simulating a disk failure, and then |
| 404 |
bringing your RAID volume back out of degraded mode. If you're have the |
| 405 |
ability to set up a RAID-1 volume on a test machine and follow along |
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with me, I highly recommend that you do so. This kind of simulation can |
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be fun. And having a little fun right now will help to ensure that when |
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a drive really fails, you'll be calm and collected, and know exactly |
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what to do. |
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</p> |
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|
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<impo> |
| 413 |
To perform this test, it's essential that you set up your RAID-1 volume |
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so that you can still boot your Linux system with one hard drive |
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unplugged, because this is how we're going to simulate a drive failure. |
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</impo> |
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|
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<p> |
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OK, our first step is to set up a RAID-1 volume; refer to my <uri |
| 420 |
link="/doc/en/articles/software-raid-p1.xml">previous article</uri> if |
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you need a refresher on how to do this. Once you've set up your volume, |
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you'll see something like this if you <c>cat /proc/mdstat</c>: |
| 423 |
</p> |
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|
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|
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|
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-- |
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