<b><font color=blue>BASIC RAID FAQ</font color=blue>
Rev 1.3, 26 November 2002</b>
 
<b>1. What is RAID?</b>
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.
 
 
<b>2. What types of RAID exists? and how do they differ?</b>
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.
 
<i>Level 0 : Striping.</i>
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.
 
<i>Level 1 : Mirroring.</i>
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.
 
<i>Level 2 : Bit interleaving with <A HREF="http://www-gap.dcs.st-and.ac.uk/~history/Mathematicians/Hamming.html" target="_new">Hamming</A> error correction code.</i>
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.
 
<i>Level 3 : Striping with parity.</i>
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.
 
<i>Level 4 : Striping with parity.</i>
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.
 
<i>Level 5 : Striping with distributed parity.</i>
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.
 
<i>Level 6 : Striping with dual distributed parity.</i>
Essentially the same as level 5, but two sets of parity is calculated in order to improve data security.
 
<i>Level X+Y</i>
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.
 
<i>Level 0+1 : Striping and Mirroring.</i>
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.
 
<i>Level 1+0 : Striping and Mirroring.</i>
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.
 
<i>JBOD : Just a Bunch Of Drives.</i>
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.
 
 
<b>3. Can I use different sized/typed disks for my array?</b>
Yes, but for all levels (except JBOD) you will loose 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 <i>minus 1</i> times the smallest drive.
 
 
<b>4. Can I change my array after I have put data on it?</b>
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.
 
 
<b>5. Software RAID vs. hardware RAID. Which is better ?</b>
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 <A HREF="http://www.pcguide.com/ref/hdd/perf/raid/conf/ctrlSoftware-c.html" target="_new">PCGuide</A>.
 
 
<b>6. I want to setup a level 0 RAID. Which stripe and clustersize should I use?</b>
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.
 
 
<b>7. How do I setup/partition a level 0 RAID array, and install my OS on it?</b>
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.
<i>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.</i>
 
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.
 
 
<b>Additional Stuff</b>
<A HREF="http://www.acnc.com/04_01_00.html" target="_new">AC&NC on RAID</A>
<A HREF="http://www.pcguide.com/ref/hdd/perf/raid/index.htm" target="_new">PCGuide on RAID</A>
<A HREF="http://www.rad.com/networks/1994/err_con/hamming.htm" target="_new">Hamming error correction</A>
<A HREF="http://www.storagereview.com/guide2000/ref/hdd/perf/raid" target="_new">Storagereview on RAID</A>
 
<b>Acknowledgments</b>
This FAQ would not be as good as it is without the kind help of the following individuals:
<b><font color=orange>LHGPooBaa</font color=orange>
<font color=green>FatBurger</font color=green></b>
<b>Scotty35</b> - For pointing out the difference between RAID 0+1 and RAID 1+0
<b>sjonnie</b> - For convincing me about the difference between RAID 0+1 and RAID 1+0    
 
 
<b>FAQ History</b>
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.
 
<i><b>Engineering is the fine art of making what you want from things you can get</b></i>
<A HREF="http://www.btvillarin.com/phpBB/viewtopic.php?t=655" target="_new">My systems</A>

Thanks HammerBot. What a great tutorial.

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

Always good to read a good tutorial. I look forward to trying some of these things out at some point.
 
DDay

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.

Great post. Thanx a lot.
But I have some questions :
1. How much % will RAID 0 increase performance?
2. Will RAID 0 increase the potential of error?
3. How many minimum drives required for RAID 1+0?

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.

hi friends
 
I am using Dell dimension 5150,in that i have removed
dell hard disk,and i am connecting new samung hard disk (power cable & ribbon cable)
(and i have removed master pin also from hard disk).since CD-rom is acting as master.
And in boot sequence i have changed the order  
1. Onboard Or USB CD-ROM drive  
2.Onboard IDE hard drive.
3. Onboard or USB Floppy drive.
4. Onboard SATA drive.
 
now i will place the cd in CD-rom and restart
but i am unable to get ...boot from CD why?
where is the mistake...

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

Your correct.

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?

Okay now I'm confused...
 
Went to Best Buy to purchase 10,000 drive for System drive.  "GeeK" Squad seemed knowledgeable and recomended I use a Raid 0 configuration would be better for price and performance.
 
that is fine - I bought it.  I got two 320 gb drives and intended to do a 50 gig partition for System and Applications, 8 gigs for Swap file and the rest for Data on a windows Vista system.  Here is where I lost it.
 
Then the guy said I can take my 750 gb drive and mirror the 320s in a raid 0+1 configuration.  This sounded very interesting as I would have a stripe raid 0 (performance) and Raid 1 (mirror) for backup - but now when i read up on it , Raid 0+1 looks like you need 4 drives.  Am I missing something.
 
Any recommendatiosn would be greatly appreciated with my now 3 drives.

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

Thanks Grumpy - Please see my recent post of my components here.
 
Now that I have the 2 320 gig drives...

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 would think that you could break the mirror through the RAID management utility and still have the first drive boot Windows.
 
Grumpy

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.

 
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.