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Raid-What it is and how it works

 
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Explanation of RAID and installation guide

Thanks to 'Stein for the coding.

Greetings,

It's come to my attention, both from call volume in work and online forums, that most people do not realize the diversity of RAID controllers available, nor how to fully benefit from them.

This tutorial aims to explain in simple terms the four main types of RAID utilized by home users, point out the advantages and disadvantages of each setup, and give a general installation and troubleshooting overview.
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What is RAID?

RAID originally stood for "redundant array of independent disks," but you may also see the phrase "redundant array of inexpensive disks." The second was originally a marketing blurb but it has stuck.

All RAID groups (arrays) can be broken down to two types at their most basic level: striping(0) or mirroring(1).

More advanced Raid arrays (6 & 7) also make use of parity drives and distributed parity, but these are outside the scope of this tutorial.

How it works:

The hard drives are hooked up to the RAID controller, which is itself connected to the Operating System. The Operating System sees the RAID controller as a single hard drive and writes to it in this manner.The RAID controller then decides how to distribute the data between the hard drives.

Anything below the RAID controller is invisible to the Operating System unless you have specialized software to enable you to see the status of your RAID array. For this reason many people have incredible trouble installing RAID.


Different Raid controllers:

Depending on how much you're willing to pay, controller performance differs greatly. Here are some of the main features available.

Onboard cache - determines what size of files a controller can store in short term memory, thus speeding up write speeds as you don't have to write at the speed of the HD. The bigger the better.

Battery backup - Prevents losing the data stored in the RAID cache (short term memory) in the event of a power cut. If a battery isn't installed anything not yet written to the physical hard drive will be lost.

Also the types of arrays that each controller supports differ.

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The different types
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Raid 0(striping)

Raid 0 on its own is not technically a raid array as it does not contain "redundant" or "independent" disks. For this reason a Raid 0 configuration is generally just called "striping."


This diagram shows how data is distributed in a striped array for file "ABCD."

_________________
|............................|- Operating system sends
| Operating.............| "ABCD" to what it
| System................| thinks is the hard drive.
|________________|
|
|
|
|
_________________
|Raid......................|- RAID controller tells the
|Controller..............| O.S. that it has written
|________________| "ABCD" to itself.
|..............|
|..............|
|..............|

________ _______
|............||............|- To speed things up, RAID
| HD1.....|| HD2.....| distributes A & C to HD1 and
|_______||_______| B & D. See why it's called
|..A........||...B.......| "Striping."
|_______||_______|
|..C........||...D.......|
|............||............|
|_______||_______|


Advantages: Fast read/write; Utilizes full HD space.

Disadvantages: All data lost with single HD failure; Drives operate at speed of slowest.


I would only recommend a Striped array if your files really don't matter to you and your main use would be games. In this case, make sure you buy two hard drives of the same speed.

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Mirroring(raid 1)

Mirroring uses a backup hard drive to keep an exact copy of your files. The main problem is that you only utilize the space available in your smallest hard drive (if I have a 40 GB and a 150 GB hooked up to a mirrored array, my total HD size is only 40 GB.)


Again this diagram is based on the file "ABCD."


_________________
|............................|- Operating system sends
|Operating.............| "ABCD" to what it thinks is
|System................| the hard drive.
|________________|
|
|
|
|
_________________
|..Raid....................|- RAID controller tells the
|..Controller............| O.S. that it has written
|............................| "ABCD" to itself. If write
|............................| back caching is enabled, it
|............................| stores the file in memory
|________________| to speed up the process.
|..........|
|..........|
|..........|

________ _______
|............||............|- RAID writes the file twice:
|HD1.....||HD2.....| once on each hard drive. In
|............||............| this way, you maintain a
|............||............| "mirror" image of your
|............||............| hard drive. If one hard drive
|............||............| fails, RAID switches to the
|............||............| backup and the O.S.
|"ABCD".||"ABCD".| doesn't even blink.
|............||............|
|............||............|
|_______||_______|


With mirroring, the write speeds will be slower and the read speeds will not be noticeably quicker unless you enable "write back" and "read ahead" caching respectively.The disadvantage of "write back" caching is that any files stored in the cache but not yet written will be lost in a power cut, unless you're controller is equipped with a battery. Read ahead caching will read faster if you are reading contiguous data or if it guesses correctly what you will look at next (again, all depending on the price you pay).

If a hard drive does fail, you can replace the broken one and RAID will build a new image onto it, independent of your CPU and RAM, so Windows works fine (HD's are obviously busy though)

Advantages: Hard drive can run at independent speeds; backup copy is maintained and can be written to new hard drives; Much improved speed (assuming caching is enabled).

Disadvantages: Possible data loss with power cut (if no RAID battery is present); only utilizes 50% of available space (assuming matched hard drives).

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RAID 0+1 and RAID 10

By far the best option, if you can afford it, is four hard drives. Both arrays utilize the best parts of both mirroring and striping and are only slightly different in how they store data.


RAID 0 + 1 uses striping and maintains a backup of the array so you can sustain one HD failure, and possibly two, if you are lucky enough to retain one of each striped disk.

Raid 10 is very similar to 0 + 1 with the single exception that the backup striped disk is on the same sub array as the original.
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Installing a Raid array:

Once you have connected all the hardware together, you need to install the RAID drivers and BIOS. Unfortunately in most cases, this means re-installing Windows and pressing F6 with the floppy in the drive.

Once this is done, you restart the computer and during POST hold "ctrl" and "first letter of controller manufacturer (example Nvidea = "ctrl + "n")."

Generally, a RAID BIOS has available HDs on the left which you move to the right to create an array. Select which type of array you would like and then follow the instructions at the bottom of the screen to create an array.

Voila, "Robert's you're father's brother", as they say in England.
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Please Note - I have omitted quite a lot of detail, both for the sake of simplicity and the fact that there are quite a lot of things that can go wrong. If anyone is having a RAID problem P.M. me the link to your post and I'll have a look A.S.A.P.

Please do not mail problems directly to me!

 
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Nice write-up.

 
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Excuse my ignorance but I feel the question "What is it?" is not answered--or at least not completely for I fail to still know what it is. Is it a physical device that u can buy and plug into your mother board install, is it software? In my BIOS, RAID is disabled, hence my interest in reading this since I saw the term many times. Therefore I asked myslef, Why is it disabled is it because it's, whatever it is, does not exist within my PC or is it something else?
Based on the rest of your explanation, it appears to be something that one may want/need if installing 2 or more Hard Drives within a computer or network (but not needed if only one HD is connected). Is this assumption correct?

Knowledge presupposes previous knowledge and in this case I feel people reading this already have some idea therefore able to understand what was written.

My intentions for writing this are good, my purpose not being to bad mouth what was written (my apologies if it sounded that way for it was not my intention) but only to inform what was not understood, which I feel may be of importance.

 
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thanks for sharing the info...

 
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Well it seems to be a operating system to me and the flag like structure are the tracks of hard drive...

 
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RAID originally stood for "redundant array of independent disks," but you may also see the phrase "redundant array of inexpensive disks

My server from ~1996 uses the second definition.

 
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A clear and well written article.
Gerryx1

 
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But, realistically:
"Why RAID is (usually) a Terrible Idea"
http://www.pugetsystems.com/articles.php?id=29
This will make more sense when you've had a couple of RAID failures, as I have.

 
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Yea, the article from Pugetsys certainly reinforces the notion that Raid is not the 'bees' knees' of happy HD usage. Your input suggests tha you've had probs with it. I had considered it as a mean of finding my 'missing drv but, Gigabyte sprt. warned me that there might be issues especially when mixing Sata1 board and Sata2 drv. The other related issue Sata: how safe or stable is it? For how or why did adding a DVD writer (Sata2) to a Sata 1 port cause my HD to change from Basic to Dynamic and thus become 'unreadable'?

 
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I got SCSI RAID (Raid level 1 - Mirroring) on my server and it works good. Handy as all the drives are hotswap and on rails. Makes for minumum downtime. i still backup very regularly to tape and dvds of course.

 
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I had a SCSI Barracuda drive a few years ago and it was a flyer, so, if you could get a Raid SCSI setup going, as you have, i could imagine it would be something. Isn't a server a pc setup where a central pc controls/communicates with a number of others?

 
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thats a cluster

 
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So, a server is, likea work-horse, serving other pcs ie. for printing, data storage etc. Well, you'd need a reliable set up for that,

 
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yup downtime = -$$$$$

Thats why servers generally have lots of hotswappable raid-ed disks and regularly back up

 
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yup downtime = -$$$$$

Thats why servers generally have lots of hotswappable raid-ed disks and regularly back up

Hot-swapping Raid..sounds good. So, with a Raid 1 mirroiing setup, you're guaranteed 24/7/365 uptime with your server. That's a neat trick, where, because all the server data is mirrored onto multiple hot-swap disks, you can replace (hot-swap) the disk without powering down the system. I take it, that this type of setup uses HDDs, but what's it with the disks, Do they resemble CDs or DVDs?

 
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by disk i mean HDD

Hard Disk drive

 
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by disk i mean HDD

Hard Disk drive

Then, it would seem that Raid is a really a super idea for businesses and large establishments but, maybe not for the home user.

 
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Exactly. People who dont fully understand it, and who wont make full use of it, should stay away from it as for them it generally causes more trouble then its worth.

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