RAID Guide

[Hitman]

Hitman's RAID Guide

Questions about RAID come up all the time in this forum. I would like to give some general information about RAID, and some links where you can obtain more in depth information about RAID. I would also like to clear up some common misconceptions about RAID and a brief overview of the different RAID Levels available.

What is RAID?

Short for Redundant Array of Independent (or Inexpensive) Disks, a category of disk drives that employ two or more drives in combination for fault tolerance and performance. RAID disk drives are used frequently on servers but aren't generally necessary for personal computers.

There are number of different RAID levels:

Level 0 -- Striped Disk Array without Fault Tolerance: Provides data striping (spreading out blocks of each file across multiple disk drives) but no redundancy. This improves performance but does not deliver fault tolerance. If one drive fails then all data in the array is lost.
Level 1 -- Mirroring and Duplexing: Provides disk mirroring. Level 1 provides twice the read transaction rate of single disks and the same write transaction rate as single disks.
Level 2 -- Error-Correcting Coding: Not a typical implementation and rarely used, Level 2 stripes data at the bit level rather than the block level.
Level 3 -- Bit-Interleaved Parity: Provides byte-level striping with a dedicated parity disk. Level 3, which cannot service simultaneous multiple requests, also is rarely used.
Level 4 -- Dedicated Parity Drive: A commonly used implementation of RAID, Level 4 provides block-level striping (like Level 0) with a parity disk. If a data disk fails, the parity data is used to create a replacement disk. A disadvantage to Level 4 is that the parity disk can create write bottlenecks.
Level 5 -- Block Interleaved Distributed Parity: Provides data striping at the byte level and also stripe error correction information. This results in excellent performance and good fault tolerance. Level 5 is one of the most popular implementations of RAID.
Level 6 -- Independent Data Disks with Double Parity: Provides block-level striping with parity data distributed across all disks.
Level 0+1 – A Mirror of Stripes: Not one of the original RAID levels, two RAID 0 stripes are created, and a RAID 1 mirror is created over them. Used for both replicating and sharing data among disks.
Level 10 – A Stripe of Mirrors: Not one of the original RAID levels, multiple RAID 1 mirrors are created, and a RAID 0 stripe is created over these.
Level 7: A trademark of Storage Computer Corporation that adds caching to Levels 3 or 4.
RAID S: EMC Corporation's proprietary striped parity RAID system used in its Symmetrix storage systems.

http://en.wikipedia.org/wiki/Redunda...ependent_disks
http://www.webopedia.com/TERM/R/RAID.html
http://www.acnc.com/raid.html

History of RAID

RAID has been around for a long time on the server/workstation side of the house. You can read a more detailed history of RAID in the links below. In 2000, and 2001 IDE Raid controllers started to be seen in Home Computers. Company's like ABIT and MSI started to put IDE RAID controllers on their motherboards. IDE RAID quickly became popular with Enthusiast and Gamers. RAID in Home PC's became even more popular with the introduction of SATA and the WD Raptors. Home users were able to have the benefits of Fast 10K RPM Hard Drives, without having to invest in expensive and complex SCSI drives and controllers.

http://www.staff.uni-mainz.de/neuffe...t_is_raid.html
http://searchstorage.techtarget.com/...214332,00.html

RAID 0 vs Single Drive

The argument is that RAID 0 is not faster then a Single Drive. In most real world applications the speed increase from RAID 0 is so minimal that most people wont even notice a difference. RAID 0, biggest fall back is that it does not have fault tolerance, so if one drive fails all data is lost. The more Hard Drives you add the higher your chance of drive failure. The difference between RAID 0 and a Single drive depends on the Hard drives you use. For example two 36GB WD Raptors in RAID 0 are not faster then a Single 74GB Raptor, but 2 36GB WD Raptors in RAID 0 are faster then a single 36GB Raptor. More information about RAID 0 Vs Single Drive can be found in the links below.

http://www.techreport.com/reviews/20.../index.x?pg=27
http://www.anandtech.com/storage/sho...px?i=2101&p=11
http://www.devhardware.com/c/a/Stora...-Your-Home-PC/
http://faq.storagereview.com/SingleDriveVsRaid0

Hardware RAID vs Software RAID

Software RAID is less expensive then Hardware RAID. Windows XP for example offers RAID 0, RAID 1, and RAID 0+1 in the PRO version and with a hack you can enable RAID 5 in XP Pro. Software RAID does not require a RAID controller, the drives can be connected to any IDE or SATA controller. Software RAID can be transferred from one computer to another. If you take your Hard Drives that are in a RAID configuration and put them in another computer, XP will recognize that they are in RAID, and you can import them with out loss of data. The major drawback to Software RAID is performance. Software RAID is dependent on CPU resources, so performance take a major hit. Its also less stable then Hardware RAID.

Hardware RAID is more expensive, and requires a RAID controller. RAID is setup in the controllers BIOS, the RAID Levels available depend on the controller that you are using. Hardware RAID is typically more stable, and uses minimal CPU resources. Most newer motherboards have onboard RAID controllers. The higher end motherboards onboard RAID controllers, offer more RAID Levels. Hardware RAID is not dependent on software. You may be required to install drivers (F6) for your Controller during XP setup for all your RAID array to be seen by windows. Hardware RAID is the preferred method.

http://searchstorage.techtarget.com/...tml?bucket=ETA
http://www.gamepc.com/labs/view_cont...=r52005&page=1
http://www.tomshardware.com/storage/20041119/index.html
http://www.software-raid.tk/

Is RAID Right for You?

Well that depends. Its not right for everyone. There are risks, and frustrations associated with RAID. If RAID fits what you need it for then its right for you. Keep in mind that RAID is not a substitution for Backing Up. Even if you are using RAID 1, you should still to regular backups. Fault tolerance is meant for quick recovery in case of drive failure, it does not protect you from catastrophic disasters.

If you have more questions or comments about RAID feel free to post them in this thread.


Hitman

[Tyreal]

Awesome guide for anyone who wants to create a hard disk raid. I especially like the different raid descriptions. I used to always confuse the raid 0 and raid 1.

[gvblake22]

Excellent info Hitman, thank you! :thup:
Do you run any RAID setups?

[Hitman]

Thanks guys. I actually wrote this for the MaxPC forum. There seemed to be a lot of confusion about RAID in that forum. I wanted to give out some basic info and provide everyone with some good links to find information specific to what they wanted RAID for.

One4u2c and Simple_Inhibition both told me I should also post it here as well.

Yes I do run RAID. I have ran RAID 0, RAID 1, and I now run RAID 5. In my Main Computer I run Four 250GB HD in RAID 5, using the ASUS A8N-SLI Premium Onboard RAID Controller. I used to use Software RAID 5, and it sucked ass. It was dog slow.

[gvblake22]

Cool. What do you think of RAID 1? I'm about ready to buy another 250GB drive to put in RAID 1 for backups/mass storage. I guess I'm just a little put off by setting it up and the possibility of rebuilding it in case of a failure. Mostly just because I've never done a RAID setup before...

Also, would you recommend RAID 5 over a single Raptor and two 7200rpm RAID 1 drives?

[sbohdan]

nice guide. cleared up some things with the latest raid setups. I would be really interested in a speed comparison test between new HDD's and also SATA and SATA2. also a question: is there any difference in speed between connecting the HDD to the onboard SATA or silicon image? :?

[gvblake22]

Originally Posted by sbohdan

also a question: is there any difference in speed between connecting the HDD to the onboard SATA or silicon image? :?

Well, every controller is going to have slightly different performance characteristics, but many boards have onboard silicon image SATA controllers, so I guess you need to specify exactly which controllers you are referring to.

[sbohdan]

I was referring to the onboard silicon image controller on my P4N diamond. when I checked with fresh diagnose, it said 29MB/s for both read and write. this seems way to slow for a new HD with 16MB cache. what you think :?: any other benchie that would show accurate transfer rates :?:

[gvblake22]

Try HDTach

[sbohdan]

thanks blake :P