RAID FAQ V1.3.1

BASIC RAID FAQ
Rev 1.3.1, 26 November 2002

1. What is RAID?
RAID is an acronym for Redundant Array of Inexpensive Disks (as opposed to SLED - Single Large Expensive Disk). Today, most drives are relatively inexpensive and the meaning of the 'i' is changing into 'independent'. The purpose of RAID is to use 2 or more drives together in order to obtain increased performance and/or data security.

2. What types of RAID exists? and how do they differ?
The different types of RAID is typically referred to as 'levels'. This FAQ will focus on level 0, 1 and 0+1 since these are what is most often supported by embedded RAID controllers.

Level 0 : Striping.
Level 0 provides increased performance by writing alternating blocks of data (referred to as the stripesize) to 2 or more drives simultaneously. Read performance is also improved since data is read from all drives at the same time. No redundant information is stored and failure of a SINGLE drive will cause all data to be lost. The number of drives in a level 0 array is sometimes also referred to as the stripe width.

Level 1 : Mirroring.
Level 1 provides redundancy by writing all data to 2 or more drives. Level 1 provides no increase in write performance (it may even be a bit slower). Read performance tend to be faster than a single drive, but not as fast as level 0. Level 1 provides excellent data security since ALL drives has to fail before any data is lost.

Level 2 : Bit interleaving with Hamming error correction code.
Data is striped across multiple disks at the BIT level. Dedicated drives are used for Hamming error correction. Hamming error correction is a forward error correction code capable of correcting any single bit error or detecting any double bit error within the code word.

Level 3 : Striping with parity.
Data is striped across 2 or more disks and parity is written to a dedicated drive. Level 3 is typically implemented at the BYTE level.

Level 4 : Striping with parity.
Data is striped across 2 or more disks and parity is written to a dedicated drive. Level 4 is typically implemented at the block (stripesize) level.

Level 5 : Striping with distributed parity.
Data and parity is striped across 3 or more drives. Parity is distributed to each drive. Level 5 is the most widely used RAID for servers and other highperformance storage solutions. Any single drive can fail without data loss, ie. at least two drives must fail before any data is lost.

Level 6 : Striping with dual distributed parity.
Essentially the same as level 5, but two sets of parity is calculated in order to improve data security.

Level X+Y
It is possible to combine various RAID levels to optimise data security and/or performance. E.g. Level 0+1 and 1+0 as explained below.

Level 0+1 : Striping and Mirroring.
Level 0+1 combines level 0 and level 1 by mirroring a striped volume. Level 0+1 provides read and write performance very close (or equal) to level 0. Level 0+1 should not be confused with level 1+0. If there is 1 mirror set, a single drive failure will cause the whole array to become, in essence, a level 0 array. Level 0+1 requires an even number of drives and minimum 4.

Level 1+0 : Striping and Mirroring.
Level 1+0 (sometimes referred to as level 10) combines level 0 and level 1 by striping a mirrored volume. Level 1+0 has better data security than level 0+1. The reason for this is that the level 1+0 controller can take advantage of a partial mirror set, but the level 0+1 controller cannot take advantage of a partial stripe set.

JBOD : Just a Bunch Of Drives.
Not actually RAID, but some RAID controllers support this. In JBOD, 2 or more drives, which can be of any size, are put together so it appear as a single drive whose capacity is the sum of the individual drives. Since JBOD provides no performance increase and reduced data security, it is seldomly used.

3. Can I use different sized/typed disks for my array?
Yes, but for all levels (except JBOD) you will lose some capacity on the largest drives.
For level 0, total capacity is equal to the stripe width times the smallest drive.
For level 1, total capacity is equal to the smallest drive.
For level 0+1, total capacity is equal to the stripe width times the smallest drive.
For level 5, total capacity is equal to the number of drives minus 1 times the smallest drive.

4. Can I change my array after I have put data on it?
The stripesize or stripewidth of a level 0 or level 0+1 array can not be changed without rebuilding the array. This will cause all data to be lost. For level 1 and level 0+1 additional mirror drives can added to provide additional data security. This will not cause any data to be lost.

5. Software RAID vs. hardware RAID. Which is better ?
For the most part deffinitely hardware RAID. However, software RAID has some few advantages, but its beyond the scope of this FAQ to discuss this further. For further details on advantages/disadvantages of software RAID, check PCGuide.

6. I want to setup a level 0 RAID. Which stripe and clustersize should I use?
It depends on what the array is going to be used for. In general if the array is used for very large files (Video streaming etc.) a larger stripesize is better. For mainstream usage (office, gaming etc.) a stripe and clustersize in the 8-32 kB range is a common choice. To some extend the optimum stripe and clustersize combination also depends on the RAID controller and drives.

7. How do I setup/partition a level 0 RAID array, and install my OS on it?
The easy way:

1) Attach the drives to the RAID controller. Each drive should be master on its own channel (separate cable) for maximum performance.
2) Enter the RAID controller bios (usually you press CTRL+H after powering on the PC). Setup the RAID0 array with your preferred stripesize. The exact way of doing this depends on the controller. Note: Some controllers (e.g. the Promise-lite) does not allow you to change the stripesize.
3) Make sure you have a floppy with the RAID drivers. Boot from the OS installation CD, and when prompted press 'F6' to install third party RAID or SCSI drivers. Insert the floppy.
4) Using the installation program partition and format the drive.
5) Proceed with installing the OS on the boot partition.

The problem with the above method is that you can not specify the wanted clustersize when formatting (For NTFS the default clustersize is 4kB). If you choose to use NTFS it is not possible to change the clustersize without reformatting the drive. For FAT32, the clustersize can be changed at a later time with programs like Partition Magic.
Update: Partition Magic V8.0 is able to change the clustersize of an NTFS partition. I haven't tested this yet, but I expect it to be a slow procedure. So if you wan't to test different clustersizes, the method below should be used.

If you want to use NTFS, or do some benchmarks with different stripe and clustersize combinations the recommended method requires a third temporary drive:

1) Attach the drives to the RAID controller. Each drive should be master on its own channel (separate cable) for maximum performance.
2) Attach the temporary drive to the normal IDE controller.
3) Enter the RAID controller bios. Setup the RAID0 array with your preferred stripesize.
4) Install the OS on the temporary drive.
5) Boot on the temporary drive. When the OS is up and running, install the RAID drivers.
6) Partition and format the RAID array with the preferred clustersize. In Windows XP, Disk Management provides the means to partition drives and formatting with a custom clustersize.
7) Optionally perform benchmarks on the array. Reformat the drive with a different clustersize or rebuild the array with a different stripesize. When the array is partitioned and formatted, the temporary drive can be removed.
8) Make sure you have a floppy with the RAID drivers. Boot from the OS installation CD, and when prompted press 'F6' to install third party RAID or SCSI drivers. Insert the floppy.
9) Install the OS on the boot partition of the RAID array. Make sure you do not format the array during installation, since this will reset the clustersize to the default value.

Additional Stuff
AC&NC on RAID
PCGuide on RAID
Hamming error correction

Acknowledgments
This FAQ would not be as good as it is without the kind help of the following individuals:
LHGPooBaa
FatBurger
Scotty35 - For pointing out the difference between RAID 0+1 and RAID 1+0
sjonnie - For convincing me about the difference between RAID 0+1 and RAID 1+0

FAQ History
Ver 1.0 - Initial draft, 28 October 2002
Ver 1.1 - Revised draft, 31 October 2002
Ver 1.2 - Added RAID 6 and 1+0, 11 November 2002
Ver 1.3 - Added comment about Partition magic V8.0 and NTFS clustersize.
Ver 1.3.1 - Fixed formatting because it has been broken ages. Removed non-working links. 27 April 2009 (Moderator edit)

Engineering is the fine art of making what you want from things you can get

Thanks HammerBot. Look. I'm trying to figureout whether I'd get any meaningfull increase in HD write performance if Ihad RAID0 with two HD's connected to a single SATA port. (compared to a single HD connected to the same port)

The reason for this is here:
http://forumz.tomshardware.com/har [...] 81492.html

thanks, musiclover

Yeah this was a nice FAQ but anyone considering a 7200RPM Raid 0 array 2 drives, just go with a Raptor 10K 74GB as your main drive, you'll be very pleased and they ahve a long long life span without errors...Raid 0 is too risky for those of us that keep family pictures and music stored...i lost mine on a Raid 0 yeah it was fast but so is the Raptor and i dont worry about my data anymore...plus i keep a backup.

1: It varies. You can't really judge performance gains on a % basis, since one WD Raptor may differ slightly from another Raptor. Motherboards with different RAID controllers will perform differently. Also depends on your CPU and RAM.

2: It increases the chance that if there is an error, it will be a serious one. It doesn't make the drives more fragile or susceptible to damage, but if something does happen, all is lost.

3: You need 4 drives. It takes two RAID 1 arrays with 2 drives each and stripes them.

Good luck.

RAID 0 does not sacrifice any data space, right? For example, if I stripe two 160gb drives, I will have 320gb of usable space?

Hi-- I was just wondering if RAID 0 is worth it. How much of a performance boost do I get with it when I play games? Also, lets say I want to add a file to a directory.( lets say a new map for a game). Since the data would be spread around, where could I find the directory?

Never take the advice of a member of the "Geek Squad"! They don't know Jacksh*t. You should have stuck to your original plan and got 1 10,000 RPM drive. AID-0 (there is nothing Redundant in AID-0) is not worth the time or trouble and yes, you would need 4 drives for RAID-0+1.

Grumpy

Just wondering if anyone knows what happens when a RAID1 setup is decommissioned. As in, if I take apart the HDDs and set the BIOS to non-raid settings (Asus P5W DH), is the data on those HDDs still accessible just like two separate drives or if I just plugged them straight into another computer? I have read conflicting information as to whether data is lost or preserved when a RAID setup is decommissioned.

Thanks.

I'm still confussed on the difference between Raid 0+1 and Raid 1+0. Raid 0+1 seems to make sence...Raid 1+0 does not.

1) Is it possible to have 2 HDD with 3 partitions each (a-b-c)
And make a RAID 0 on A, RAID 1 on B and leave C alone?
2) How?
3)Is there a guide to do so?
4)How good does it perform?
THANKS IN ADVANCE...

Raid controllers don't see individual partitions, they only see the entire physical drive.
So the answer is no.

Concerning Raid 0:

This is scary, so if a small word document is corrupt, will it permanantly delete ALL files on the computer, or make that single file unrecoverable (through means such as Windows System Restore), or can you still have hope that the single corrupt word document will be recoverable with System Restore?



P.S. is it any more dangerous/safer than having one giant disk.

answers:

1. if a small document is corrupt then the small document is corrupt..
2. if your whole hd is corrupt/died then everything on your hd is dead ( unless the raid god has mercy on you )
3. the more drives you run in raid the more likely to have a hard drive failure and lose all of your data. ( but i am pretty sure having S>M>A>R>T will let you know whenyou need to backup before that usually happens)
4. having one big drive is better than having 4 smaller drives ( in raid 0 of course).

haha very,very true. i have first hand knowledge on this

its missing raid levels 50 and 60.

which are stripped raid 5s or 6s

Hello.

a. Does this mean that if I wanted to raid-0 an OS and application fast drive (like a raptor 150gb) and a mass storage drive (like a 400gb 7200rpm samsung) I would loose 250gb off the large drive?
b. If not, does anyone have a link to a performance test on a similar setup?
c. If yes, would a raid-0 setup of the raptor + 2 (or 3) other 150gb 7200rpm drives increase performance without loosing capacity? (again, similar performance tests are welcome).

Thanks.

Hi, since I don't have a floppy disk unit and Will have 3 HDs (2 for RAID0 and 1 for important data including OS backup in case something happens to the RAID) is it possible to install the OS in the single HD, then install the RAID controllers, format the other HDs, then create an image of that single HD in the RAID and start booting from it.

If you didn't get me, try this: Is it possible to follow steps 1 trough 6 of second part of the quote, then making an image of the "temporary drive" in your new RAID and config BIOS to start booting from that image on RAID0

In other words, is there a possibility to install Windows in a RAID0 without a floppy disc unit?

no, the os that you are booting from must have raid drivers on it or else the computer wont understand what you are trying to boot from because it does not see the drive.

oh lol. well then of course you can. you are basically throwing the partition to another drive with the raid controller drivers also. you are good to go.

Hope you are right... Thanks a lot

no problem.

I am pretty sure it will work. If it doesn't you could always get a cheap floppy drive that is about 10 dollars and then return it the same day.

Ewww, you need a floppy drive for RAID?

In regards to RAID 0, once it's set and everything works, do you think the chance of corruption is high?
The thing is I've got three 10 years old Quantum Hard drive in my overclocked ol' PC, one two years ol' Seagate HD, two 2 years ol' 2.5" Fujistu & Seagate HD for my Notebook (one of them is lightly dropped occasionally) but I never have any issue with HDD... no bad sector, no corruption, etc....

Is it me being lucky or the risk of corruption in RAID 0 is not that 'Great' anyway?

To AP-123: If you have a 750 GB drive and two 320 GB drives, then it might just be possible to setup the two 320s in hardware RAID 0, and then setup the resulting logical device and your 750 in O/S RAID 1. However, you'll lose the write performance benefits of RAID 0 (because you'll need to wait for all data to be written to the single 750 drive, in addition to your RAID 0 array), and for that matter you may not gain much in terms of read performance if the O/S tries to compare the output of both logical devices in the RAID 1 array. So, not a good idea. Better idea: Use RAID 0 on the 320s if you must, and use a backup utility to periodically backup everything to the 750. Even better, put the 750 in a different place so that it isn't susceptible to a catastrophic hardware failure together with your 320s, so you won't lose your data.

Raptors are better because of reduced seek time. The spot on the disk your computer is looking for gets around to the R/W head quicker (axial velocity 10k vs. 7.2k rpm), and (usually) the R/W head also moves across the disk quicker (radial velocity = ?). RAID 0 is better for sustained, serial access. Unless you're watching movies or listening to music, most things your disks do aren't all that serial.

As for partitioning: If you create three partitions on a disk (or 2 disks, for that matter), the R/W head will need to move back-and-forth across the disk surface to access the different partitions. This will slow down your access. You are better off putting the different partitions (O/S and programs, swap and temp space, data) on three different drives if you can do it. Use one 320 for O/S and programs, a second for temp/swap and data backup, and the third for permanent data.

is it possible to setup RAID with one SATA HDD and i IDE drive??

You can buy one of the SATA/PATA converters for ~$15 to use the IDE drive as SATA. It's not officially supported by a number of raid solutions, but I've had no problems with it using Intel's RAID solution.

As a note, the only real issues I've seen with the SATA/PATA converters are when you're using them with an optical drive. That doesn't always work too well.

What about two 7200 speed hard drives on raid 0? I read somewhere that the threshold was almost reached with the 7200 speed drives in raid 0. So would two raptors be a waste of money?

Fix the HTML.

Not true, nForce4 boards let you RAID IDE and SATA drives together.

A lot of people become confused by this because they usually only think of a RAID array as containing a small number of drives. For example, RAID0 or RAID1 arrays are usually built using two drives, no more. However, consider a system using 8 drives for a RAID array. We are after redundancy here, because it's a RAID array after all, and RAID5 is too slow for what we want to do, plus drives are cheap these days, 8 drives, no problem. So we're going to make a RAID array that consists of mirroring and striping. Our newbie tech guys things, RAID0+1, no problem, solves all our worries.

In this configuration of drives we have the following
M I R R O R
Array1 => Array2 = Drive
RAID 0 + RAID0 = RAID0+1
S
T 1 5
R 2 6
I 3 7
P 4 8
E
Two separate striped arrays of 4 drives each, mirrored, RAID0+1. Okay, consider, our techie is feeling pretty smug about his data all nice and safe except 6pm Firday evening, drive 3 decides to go down. Now what do we have?

DOWN + RAID0 = RAID0
1 5
2 6
(3) 7
4 8

A non-functional RAID0 array and it's mirror, RAID0 array 2. So now all our mission critical data is stored on a RAID0 array!!! That's BAAAD, suddenly what our techie thought was a nice secure RAID0+1 array has become a super scary RAID0 array. Unfortunately for our techie the RAID0+1 arrray is situated offsite and he can't get to it until Monday morning. In the meantime, disaster strikes and drive 5 in array 2 decides to go down. End of array!! Data lots, customers pissed etc.

So why does our techie wish he'd made a RAID10 array? Take a look, same situation, drive 3 and 5 have done down, but look the array is still functioning perfectly.

MIRROR S
Array1 RAID1 1 => (5) T
Array2 RAID1 2 => 6 R = DRIVE
Array3 RAID1 (3) => 7 I
Array4 RAID1 4 => 8 P
E

Although there's no difference between RAID0+1 and RAID10 with regard to the data on the drives, that is, in the RAID0+1 array the data on drives 1 and 5 is exactly the same as the data in the RAID10 array, the essential difference is the way in which the controller access the data. Because a RAID0+1 array consists of two RAID0 sub-arrays the controller only sees the two separate sub-arrays, it isn't set up to read between them (then it would be RAID10), hence it only requires one drive in each of those sub-arrays to go down to cause the whole array to fail. In a RAID10 array you need two drives in the same mirror sub-array to go down. If you do the math you'll realize that, particularly as array size increases, the chances of a RAID0+1 array failing increase with the more drives you add, whereas the chances of a RAID10 array actually decrease (given that there is no measure for the chance of a particular drive failing, drive failures are reported in MTBF for groups of drives).

Hope that makes sense.