| Article Index |
|---|
| RAID 101 - Introduction to RAID |
| RAID 0 - Striping |
| RAID 1 - Mirroring |
| RAID 3 - Striping with Dedicated Parity |
| RAID 5 - Striping with Distributed Parity |
| Wrap-Up |
| All Pages |
Introduction to RAID 101
Welcome to this introduction to RAID technologies. Once the domain of expensive UNIX servers, mainframes and Storage Area Networks (SANs), RAID is now available in most computers either in software (Windows, Linux and BSD all support RAID in one form or another) or in hardware (such as a dedicated RAID card). A third category is "Driver-Based" RAID, where there is hardware configuration but the hard work is performed by a software driver rather than a hardware chip or the operating system.
RAID takes 2 or more hard disks and combines them using special algorithms so that to the user, they appear to be a single disk.
Why do we use RAID?
RAID generally provides one or both of the following three key benefits:
- Performance: Because RAID uses more than one hard disk at the same time, RAID systems can generally access data faster than a single hard disk;
- Redundancy: Most RAID types are designed so that if one of the disks in the RAID "set" fails, the rest of the disks are able to continue operating and the computer does not crash;
- Size: While the biggest hard disk available at the time of writing this article is a 2TB drive, combining hard disks in a RAID set can create a virtual hard disk that is 10TB, 20TB or even bigger.
Why don't we use RAID everywhere then?
Today, the primary reasons not to have RAID in almost all systems are cost and complexity. Specifically:
- You need to buy at least 2 disks, not just one;
- You need a motherboard that supports RAID, or;
- You need to know how to configure RAID in your operating system;
- RAID is more complex to troubleshoot if something goes wrong. And something will go wrong.
Note: RAID is not backup. Nothing you do with a RAID set can ever protect your data from a virus, prevent someone from deleting the only copy of a photograph, or save the world after you realise you didn't really want to delete that folder after all.
For the purposes of this article, we shall focus on the basic RAID types; the next articles to come in this series, RAID 201 - Advanced RAID I and RAID 202 - Advanced RAID II, discusses more complex configurations.
Let's start our journey through the world of RAID with the most primitive type of RAID, Striping.
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|192.101.136.xxx |2009-05-28 06:46:55 doublemint - homebuilt NAS ownerI'd also like to know what the recovery process is like, especially if the RAID controller fails. Can a failed RAID controller be replaced with one of a different brand and still recover? As a novice, the RAID BIOS is a bit confusing, some pointers would be helpful. I had a motherboard with RAID fail and assumed I could take one of my RAID 1 drives, plug it into another PC and read it however I couldn’t, why?
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|70.17.178.xxx |2009-05-28 13:53:44 zaphod - replyIm pretty sure youd need both HDs for that to work... not just one.
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|134.211.129.xxx |2009-05-29 17:13:22 David Rawling - Your Mileage Will VaryBasically, the answer is no - you need the same model of RAID controller to replace a failed one.
This applies from the virtually-free Intel RAID controllers on the motherboard to the thousand (and multiple-thousand) dollar add-in cards.
Each controller has its own way of marking the disks as being part of a RAID set - the disk "signature". One brand might write "RAID1-1" to the first disk and "RAID1-2" to the second. Others might use numbers with the brand (MYRAID-716825).
I guess it comes back to the same comment I made before. RAID is not backup. It only protects you from disk failure.
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|71.198.81.xxx |2009-06-01 02:54:12 doublemint - homebuilt NAS ownerDavid,
While unfortunate there isn't more standardization, I understand why this might be. I can't however understand why this would apply to RAID 1 (mirroring). Why do these drives need to be treated any different than non-RAID drives? Just write the same data two places rather than just one.
