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RAID Explained
Written on April 15, 2006 by William GeorgeAlways look at the date when you read a hardware article. Some of the content in this article is most likely out of date, as it was written on April 15, 2006. For newer information, see our more recent articles.
Introduction
Modern hard disks come in sizes as large as 500GB, and continue to expand. Soon they will likely reach 1TB (Terra-Byte = 1000GB) and beyond. However, sometimes even the largest or fastest hard disks are not enough for certain applications.
The acronym 'RAID' stands for Redundant Array of Independent (or Inexpensive) Disks. It is generally recommended that all disks in a RAID should be identical (or at the very least, the same size and speed). There are several variations designed to meet different needs. Some are for making larger, faster storage solutions. Others trade off size for increased reliability. Yet others try and accomplish both. Here is a rundown of the basic types of RAID available today.
The acronym 'RAID' stands for Redundant Array of Independent (or Inexpensive) Disks. It is generally recommended that all disks in a RAID should be identical (or at the very least, the same size and speed). There are several variations designed to meet different needs. Some are for making larger, faster storage solutions. Others trade off size for increased reliability. Yet others try and accomplish both. Here is a rundown of the basic types of RAID available today.
RAID 0 - a.k.a. Striping
RAID 1 - a.k.a. Mirroring
RAID 0+1 - Mirroring Two RAID 0 Stripes
This is our first RAID style that attempts to give both a performance and reliability boost. It requires four or more disks to function, and effectively creates two RAID 0 arrays, and then mirrors them. Hence, either array could fail (either one or two disks in it) and the other would continue to function. This allows you a similar performance boost to a two-disk RAID 0 array, but with a good measure of the stability of a RAID 1 array. However, just like RAID 1, only half the total amount of disk space is usable (since all data is written twice).
RAID 10 - Striping Two RAID 1 Mirrors
RAID 5 - Striping with Parity
RAID 6 - RAID 5 on Steroids
JBOD - Just a Bunch Of Disks
While not a real RAID style, JBOD is an option on many disk controllers built into motherboards. Depending on the controller it can mean one of two things:
1) Multiple, independent drives - hence the term 'just a bunch of disks', shortened to JBOD. This effectively just uses the RAID card as a typical SATA controller, though it still requires the installation of drivers in the operating system in order for the drives to be detected and used.
2) It can also be a compilation of drives across which a single logical unit is spanned. This usage is more properly called a SPAN, though, or in some controllers the term BIG is used. As it is in no way redundant, it offers no data safety. This setup is only useful if you have multiple hard disks but only want to work with a single, larger drive within your operating system. Also, unlike other true RAID forms, this allows you to use disks of different sizes at whim - and without wasting space.
1) Multiple, independent drives - hence the term 'just a bunch of disks', shortened to JBOD. This effectively just uses the RAID card as a typical SATA controller, though it still requires the installation of drivers in the operating system in order for the drives to be detected and used.
2) It can also be a compilation of drives across which a single logical unit is spanned. This usage is more properly called a SPAN, though, or in some controllers the term BIG is used. As it is in no way redundant, it offers no data safety. This setup is only useful if you have multiple hard disks but only want to work with a single, larger drive within your operating system. Also, unlike other true RAID forms, this allows you to use disks of different sizes at whim - and without wasting space.
Hot Spare Disks
In larger arrays, where data integrity is of paramount importance, a disk failure can still be a dangerous thing. For example, if a single disk in a RAID 5 fails, then the data is at additional risk until that drives is replaced. Until a new replacement disk can be installed and rebuilt into the array another disk failure would cause all data to be lost. While it is unlikely that a second failure would occur, sometimes it just isn't worth the risk. The best option when this is the case is to have an extra disk already installed and ready to take over for whichever disk dies. This is called having a Hot Spare disk. Sometimes it is denoted by a +1 after the RAID title (i.e. RAID 5 +1). With a Hot Spare system that is properly setup, if a failure occurs then the spare is immediately rebuilt in place of the failed disk. Then, when the failed disk is replaced, it becomes the new spare.
Other types of RAID
There are other forms of RAID that are not as commonly used. Those variations include RAID 2, 3, 4, and 7. More detailed info one these can be found at other websites, including these:
http://www.pcguide.com/ref/hdd/perf/raid/levels/index.htm
http://www.acnc.com/04_00.html - good illustrations!
Additionally, there are variations on RAID 5 and 6 that are worth noting. RAID 50 and 60 are basically striped pairs of RAID 5 or 6 arrays, giving yet again increased performance at the cost of a small amount of statistical reliability. However, they require 6 or 8 disks (respectively) at a minimum and require advanced and often expensive controller cards, making them very rarely used options.
http://www.pcguide.com/ref/hdd/perf/raid/levels/index.htm
http://www.acnc.com/04_00.html - good illustrations!
Additionally, there are variations on RAID 5 and 6 that are worth noting. RAID 50 and 60 are basically striped pairs of RAID 5 or 6 arrays, giving yet again increased performance at the cost of a small amount of statistical reliability. However, they require 6 or 8 disks (respectively) at a minimum and require advanced and often expensive controller cards, making them very rarely used options.
Informational Chart
| RAID Type | Minimum # of Disks | Space lost to redundancy | Read/Write Performance | Data Safety |
| RAID 0 | 2 | None | Excellent | Poor |
| RAID 1 | 2 | 50% | Average | Good |
| RAID 0+1 | 4 | 50% | Good | Good |
| RAID 10 | 4 | 50% | Good | Good |
| RAID 5 | 3 | 1 Disk | Good | Good |
| RAID 6 | 4 | 2 Disks | Good | Excellent |
Summary
As you can see, there are a lot of options when it comes to RAID. But whether you need speed, data protection, or a combination of the two there are solutions available. If you have any questions or are interested in getting a system with a RAID setup, we are happy to give you one-on-one help, just drop us an email!
I'm real glad you guys posted this information; my boss recently mentioned setting up a RAID configuration with two HDD's, but I didn't know much about it at all until I'd read your post. I'm currently ordering parts to build a new computer, and plan to set up two HDD's set in a RAID 0 configuration. Can you give me any information on how to make this work? Here's the intel on the HDD's I'm ordering:
Hard Drive:
Seagate Barracuda 7200.10 NCQ 500GB
http://www.tigerdirect.com/...
Processor Interface: Socket 775
Chipset: Intel 975X Express Northbridge.
Memory Expansion: 4 DIMM sockets; 240-pin DDR2 with a capacity of up to 8GB. 533/667MHz mem.
PCI Express: 3 Slots
PCI: 2 Slots
IDE: Built in IDE facilities that support Ultra DMA 33 and Ultra ATA 66/100.
Audio: Integrated Intel High Def. Audio Sybsystem, supports 8-channel audio.
Serial ATA Raid Support: Serial ATA II, up to 3.0Gb/s with RAID Support.
How data is rebuilt if one disk fails is the most important part in parity.
Parity is used in both 5 and 6 level of raid.
The below link might be helpful in understanding parity.
http://www.slashroot.in/rai...
if you are using SSD drive do you need RIAD
Please advise
You can use RAID with SSDs, but the question of whether it is "needed" or not is the same as with hard drives. If you are in a critical server environment, and you need to avoid downtime, then a redundant array of some sort is a good idea. If you need more speed than a single SSD can offer, then an array that boosts speed (like RAID 0, or 0+1 / 10 / 5 if you also need redundancy) is fine.
However, for the vast majority of users, a single SSD will provide plenty of speed and is also a very reliable storage solution (when using quality brands like Intel or Samsung). You should still keep a backup of your data, but going with RAID in a normal user environment is overkill.
if you are using SSD drive do you need RAID
Replying with the same thing I wrote above, since it appears the same question is being asked a second time:
You can use RAID with SSDs, but the question of whether it is "needed" or not is the same as with hard drives. If you are in a critical server environment, and you need to avoid downtime, then a redundant array of some sort is a good idea. If you need more speed than a single SSD can offer, then an array that boosts speed (like RAID 0, or 0+1 / 10 / 5 if you also need redundancy) is fine.
However, for the vast majority of users, a single SSD will provide plenty of speed and is also a very reliable storage solution (when using quality brands like Intel or Samsung). You should still keep a backup of your data, but going with RAID in a normal user environment is overkill.
Thanks ...