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RAID uses multiple hard drives to "look like" a single drive increasing either speed, reliability or both. RAID 0 is a configuration with serious risk.

I regularly get questions from people experiencing problems with their hard drives, and one configuration I've seen more and more of is something called "RAID 0". It's such a bad idea, in my opinion, I wanted to draw attention to it and detail why.

Of course that means I also need to touch on just what RAID is, since other variations of RAID are actually quite valuable.

"RAID" stands for "Redundant Array of Inexpensive Disks", and refers to several different ways to use multiple hard drives to either increase performance, reliability or both. RAID is typically transparent to you or the operating system, other than the fact that there is a dedicated piece of hardware, known as a RAID controller, that manages the distribution of data across the drives.

Various standard configurations of RAID drives, typically referred to as "RAID arrays", have been labeled with numbers to differentiate the differing arrangements.

"In a RAID 0 configuration, you've doubled your chances of failure..."

RAID 0 uses two (or more) drives to increase performance.

The approach is this: when you write data to the logical (say, "C:") drive, the data is interweaved in some way such that half of it is written to one physical drive, and half on the other. Effectively, this doubles the speed of your hard disk, since the computer can essentially be writing to both at the same time, instead of writing the same amount of data in two steps to a single drive.

Sounds like a nifty approach, right? You get the same amount of space as you would with two individual drives, but you get it twice as fast.

The problem is that you get twice as much of something else as well: chance of failure and catastrophic data loss.

If one drive has, say, a 1 in 1,000 chance of failure, then two drives have a 2 in 1,000 chance.

Now, if you have two separate drives you still have that same 2 in 1,000 chance of one of them failing - but all you lose is what's on that one drive that fails. In a RAID 0 configuration, you've doubled your chances of failure, and when failure happens, all of your data on both drives is lost.

Think of it this way: if we take a simple sentence like:

RAID 0 configurations are a bad idea.

and sliced it in half such that every other letter was "on alternate drives", this might be what appears on each drive:

Drive 1: R I     o f g r t o s a e a b d i e .
Drive 2:  A D 0 c n i u a i n   r     a   d a

Together they're easy (and quick!) to combine into the original sentence. Take away either one and the other is so much gibberish.

RAID 0 is valuable as a speed enhancement only if you're willing to take double the risk of failure. That means either that it's OK if the failure happens and you lose the data, or you're conscientious about making sure that it's backed up regularly and thoroughly.

I'm not really sure why I'm seeing this configuration more and more, but it does concern me that people are taking it on without understanding the risks.

What's frustrating, in part, is that once you have a RAID controller anyway, with the addition of a third (inexpensive) drive you can get all that speed as well as fault tolerance using different RAID configuration.

Using RAID 5, a third drive contains "parity" information - kind of the "C" in an "A + B = C" type of equation - lose any of A, B or C and it can be recalculated from the other two. In the world of hard disk drives, that means a drive can fail completely and your system continues to operate while the disk is replaced. RAID controllers often include "hot swap" ability, so that could even be literal: the drive could be replaced without stopping or restarting the system.

That's just one example - RAID as a concept actually includes several different arrangements that you can use to balance performance, fault tolerance and risk.

Just make sure that if you're using RAID 0, you know what you're getting into.

Article C3806 - July 12, 2009 « »

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Leo Leo A. Notenboom has been playing with computers since he was required to take a programming class in 1976. An 18 year career as a programmer at Microsoft soon followed. After "retiring" in 2001, Leo started Ask Leo! in 2003 as a place for answers to common computer and technical questions. More about Leo.

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29 Comments
Simon
July 12, 2009 3:50 PM

> If one drive has, say, a 1 in 1,000 chance of failure, then two drives have a 1.9989(9) in 1,000 chance.

As a maths geek I feel obliged to make an unnecessary, pointless, and anal correction: If one drive has a 1 in 1,000 chance of failure (in some time period), then two drives have a 1.9989(9) in 1,000 chance of either failing (where brackets denote recursion).

(Yes, I know you know that and were just approximating it as 2, Leo, but it's good not to get your readers into the habit of assuming probabilities are additive: otherwise they end up thinking things like "So if each drive has a 75% chance of having failed by the end of 20 years, then two drives have a 150% chance of one of them having done so" (the actual figure would be 93.75%).)

Steve
July 12, 2009 10:08 PM

Simon, I know that the combined probability isn't 2, but how did you get 1.9989(9) in 1,000? I get 1.999.

P(no failure in single disk) = 0.999
P(no failure in either disk) = 0.999 * 0.999 = 0.998001
P(failure in either disk) = 1 - 0.998001 = 0.001999 (or 1.999 in 1,000)

Did I forget something? I wouldn't be shocked.

Greg Bulmash
July 12, 2009 10:44 PM

I've heard that RAID 5 doesn't give you the speed of 0 or the security of 1, just the illusion of both.

Just do RAID 10. All the redundancy of RAID 1 with all the speed of RAID 0.

Of course, redundancy or not, there's no substitute for backing up your data. Having a single drive in a RAID 0 array go kablooey is no different than having the one drive in a single drive system go kablooey. You're still going to have to restore from a backup.

JIM E
July 14, 2009 8:59 AM

RAID - SPLIT PERSONALITY- NEVER WORKS

Nick
July 14, 2009 9:13 AM

O.K.,
So I'm a victim of Raid 0. My old MB had a built-in controller and I foolishly used it. Sure enough one of my WD drives failed. (I think it's in the PCB and not the platters, but anyway)
My question is: Can the data from one drive be recovered professionally to be synced with the remaining drive, or do both drives need to be "sent in"? Of course that's double the GB's for recovery cost.
RAID 0 - never again!

Both drives would be required. Further, the service may need the raid controller as well, since there's no standard about how the data should be split across the drive. This type of data recovery on Raid drives gets very expensive very quickly, if they'll even do it. It's much more involved than just recovering two drives worth of data.
- Leo
15-Jul-2009

Simcha Wachtel
July 14, 2009 9:58 AM

I'd go a bit farther and suggest that RAID 1 or 5 are also bad ideas for the ordinary home user. One of the more common problems I've seen in servicing Windows computers is file system corruption that is easily resolved by running scandisk from the Recovery Console. But if the boot volume is on a RAID, the user will need to load drivers in order to access it from the Recovery Console. It can become a mess. On occassion when I didn't have access to the correct drivers or the user had no floppy drive, I've had to break the RAID, fix the file system and then rebuild the RAID. Not for the faint of heart and NOT for the ordinary user.

RAID is also sometimes seen as a substitute for backup and it shouldn't be.

As far as the speed or storage space benefits, I think they are minor for most users.

snail
July 14, 2009 12:08 PM

I'll throw out a speculation at improving performance and a possible backup scheme/arrangement: If you buy into a computer with multiple drives, use the fastest one(DMA and spindle speed and latency) for either or the system and cache(swap file); or, if you have enough fast disks, perhaps have the larger(GB) of the two(or more) be used to install the OS and applications. Use the other one(s) for user-created documents which you might: A)save copies of for distribution, B) delete or modifiy, C)backup for archive purposes(like entries corresponding to calendar dates...
In this set-up, from time to time make copies of all your: Documents and System files.
Perhaps one easy way would be(I haven't tried it. any recommendations or suggestions?) using a hard drive duplicator - - or boot into the system with another OS(LiveBoot Linux distro's make sense) and access the Hard Drives(system install and software install, and documents) to copy them to media(CD's, DVD's, Blu-ray... other hard drives(used for backup only) or best of all, UFD's(flash drives).
Optical disks have been a pain for me as there are stupid naming conventions and 'folder depths' which interfere with copying...I don't think I've had any problems copying from NTFS to a UFD(I think the flash drive was FAT format...)
side note: if possible(without affecting performance or stability) upon a clean install consider installing applications onto a separate drive... many programs are highly customizable and are used in a way which their file creation takes place in the installation folder. In any event, my 'program files' folder contains many hundreds of MB of data, so much of which I depend on for my software's use.

Ideal(under my wild speculations) disk arrangement:
.)Fastest disk, size probably not more than 10GB?: Cache(swap file)

.)Faster disk(or same speed as above), size 4 - 15 GB for system only(maybe you have a higher GB demand than I...

.)Fast disk(or same as above two), size 10- 100GB, depending upon how many apps you install(maybe less than 10GB?).

.)'slower' disk(s), size 100 + GB(depending upon usage): storage of user files(backed up manually, loaded into/from such folders as "My Documents," "My Pictures," et cetera...

.) additional disk for more storage of documents, perhaps additional disks can be used for organization(one disk for videos, one for text; each disk tailored to the size needs for its content).
+) at the document level it might be better to have one 100GB hard drive for all document storage and from time to time(or daily or more frequently for the paranoid with a NAS) back up to other media from this HD

Wookie
July 14, 2009 12:09 PM

Leo villanizes RAID 0 saying it is a "bad idea" and a "serious risk."

WHY do we think a 0.2% chance of failure is so much worse than a 0.1% chance? Either way, we're talking about a fraction of a percent.

Odds are much better that a person will lose data due to user error, malfunctioning software, or malware.

The real bad idea here is to not have any backup plan.

Robin Clay
July 14, 2009 12:23 PM

Simcha Wachtel wrote:"file system corruption that is easily resolved by running scandisk from the Recovery Console"

[Cough, Splutter !]

My hard drive managed to acquire a crosslink, and when I used ScanDisk, I ended up with - wait for it - seventy THOUSAND "new" files ! The cross-link had destroyed my "Pictures" folder. So I still have the data - but as far as I know, I can't re-assemble it !

Corneliu Coterbic
July 14, 2009 1:05 PM

I use RAID 0 for 4 years and I am very pleased with the performance (speed) BUT I use a external HDD for weekly backup. If you think only at bad things you do not dare to cross the street.

C. W. Joiner
July 14, 2009 2:45 PM

Your analysis of increasing your failure rate from 1 out 1000 to 2 out of 1000 is even worse. Using an MYBF calculation, the failure chances increase to much more than 2 out of 1000. But your thought, as usual, is great.

I did not mean to imply that the actual failure rate was 1 in 1000. Just using easy-to-understand numbers for comparisons purpose.
- Leo
15-Jul-2009

Chris Hingee
July 14, 2009 5:03 PM

RAID 0 is NOT a bad idea if you need the speed and if you have fault tolerance system.
If I needed RAID 0, I would also mirror the drives. This would double the cost of hard drives, but it is worth protecting your data.

David
July 14, 2009 6:20 PM

Not really RAID related but addressing a comment by Corneliu. I'm impressed that at least some people bother to do a backup. However, I do a lot of work in one day; six to seven times that in one week. I can't afford to lose a week's work on the day before doing a weekly backup. That's why I do a daily backup - with a sensible protocol and good backup software, it literally takes less than 10 minutes to do an incremental backup.

For the record, I use RAID 5.

Finally Leo, you mentioned a "piece of hardware" - a RAID controller - which can be either a discrete card OR built in to the Mother Board. However (and I guess you omitted it to avoid confusion) there is also Software RAID if your readers care to investigate.

Chris Awad
July 14, 2009 7:27 PM

The "software" raid you are talking about David can be created using windows dynamic disks where disk manager in windows can create stripped partitions/etc. I honestly dont like dynamic disks.. I rather have it done the "right way" during the boot process so partitioning tools can work from bootable CDs, etc, otherwise if windows fails, the stripped partitions might be in trouble...

I honestly like the idea of RAID5 or RAID 1+0.

RAID5 requires at least 3 drives to work and its basically mirroring between 3 drives with a parity section on each drive with information from at least 1 other drive dedicated. If 1 drive dies, the RAID will continue to work flawlessly with a small performance drop (still faster than just 1 drive). If a 2nd drive dies, your in trouble. This raid is n-1 drives in disk space.

RAID 1+0 would require at least 4 drives and is most definitely the fastest and most protected. It would be n-2 drives in disk space, but this RAID would only fail if all the drives fail on a single strip (highly unlikely).

SalemCat
July 15, 2009 11:47 AM

Like RAID 5? Don't forget your MOTHERBOARD can fail, too! If it does (a very frequent occurance with the recent rash of Bad Capacitors) you'll need to go out and buy another Motherboard with the same RAID CONTROLLER to recover your Data. I use only RAID 1, so I can remove a Mirrored Hard Drive and read it on any Windows PC.

Leo Hasbrouck
July 15, 2009 4:23 PM

How do I get rid of RAID? I keep getting a message that it has stopped working. I do not want it.

That depends entirely on your system (and the exact error message you're getting). Typically it means a BIOS/Raid controller change and a reformat/reinstall of your system from scratch.
- Leo
16-Jul-2009

Alan
July 17, 2009 6:22 AM

As a tech, I saw one of the big mistakes that a computer company can do and it was the Now gone MPC computer. In the end I got some PCs from them that had Vista on them and my company wanted XP on them. Well I tried to install the XP software and it would not. I emailed their tech support and after the run around one of their final techs said the RAID in the BIOS was on. I went in and turned it off and was able to install XP. Now RAID is a bad thing when you have one drive. If you really need a RAid system in your house I hope your related to the edisons for your electric power. I know there are people out there running a server at their house, but ask why risk it all just to use RAID on either a server or a home PC. In the early days of RAID at companies you were risking allot I saw RAID not work I know these days they are much better and are working now. I've been working on computers for over 30 years and still don't see people keeping it simple. Home users with drives so cheap you can pick up a stand alone drive turn it off and on when you want, save time, save electric, and save money. If you have files save them there and then if you don't use the files then put them on a cd, dvd, or blue-ray disk. If your a a home computer user and your spouting I have a RAID on your home system then your just wasting your own time. Rule of Thumb RAID bad for home users, RAID good for Bussiness servers at companies. Drives are going to die and your going to have to replace them, same with motherboards, and other computer parts they don't last forever. Keep your computing simple thats what computing was supposed to be all about.

Sandy Smith
July 25, 2009 8:53 PM

I was always told RAID meant, "Redundant array of independent drives." Weird...

Art
August 5, 2009 12:25 PM

You are both sort of right, RAID stands for "Redundant Array of Independent Disks". It was "Redundant Array of Inexpensive Disks" but the companies that make RAID equipment did not like the word "Inexpensive" since it made RAID sound cheap. And Sandy is sort of right because "Drives" are also called disks. But I am still back in the time when URL stood for "Universal Resource Locator"

Jim de Graff
April 11, 2010 1:41 PM

RAID 5 is not for the typical home user. We used this setup for all of our servers at the office. Unlike your usual home user, we had a stack of spare drives on site. Also, our drive arrays were hot-swappable. What this means is that when any single drive fails we are able to pull that drive out of the array without powering down the server. As soon as the replacement drive is inserted the RAID controller immediately starts to rebuild onto the new disk. Because a RAID 5 array can function as normal (to the user) with one drive missing, the system continues functioning with no observable outage.

Carl Graf
March 10, 2012 7:45 PM

My WD Drive Manager icon for my external USB HDD in the lower right corner of screen is flashing red BAD. I think I see what you mean. Computer says it sees the unit but won't let me access it because, I guess, there's no data there to access.

taran
June 9, 2012 1:42 PM

I bought a computer from dell it's came with raid 0. Is there any way I can reverse raid 0 to two disk drive, instead of raid 0.

Larkspur
June 24, 2012 10:13 AM

Leo - RAID 0 is not a "bad idea". If it wasn't for RAID-0 there would be no such thing as a 'poor video editor'. RAID-5 isn't as fast and is more expensive both in money and CPU resources. And unless you are an idiot, you don't put data that you can't replace on a RAID-0 volume. If the volume goes down, you refer to your backup, duh. Saying something is a "bad idea" just because it has specific uses, is simply ignorant. Very disappointing Leo.

I stand by my statement - Raid 0 is a bad idea. Particularly when it comes as a default configuration on some new machines, and particularly given the large numbers of people that still don't back up.
Leo
26-Jun-2012
Larkspur
June 27, 2012 7:34 PM

Alright, and I respect your difference of opinion and realize your target audience. But your statement is a blatant generalization of a technology that is very useful when used for its intended purpose (which has nothing to do with data redundancy unlike every other RAID mode).

I guess it doesn't really matter, just a matter of semantics really, but I feel you would have better served the computing community by using a title (and subsequent conclusion) along the lines of "Why RAID-0 is a bad, bad idea for most people". Not everyone uses a computer just for web surfing and storing their family photos. Also btw - thanks for running this site - you are doing many people a service here : )

Ed
July 26, 2012 7:28 AM

Nice article but I have to correct you as you have made a very common mistake in your definition of the term RAID.

RAID stands for Redundant Array of Independent Disks, not Inexpensive Disks.

This is a very common mistake and I cannot tell you how many people believe that it actually stands for Inexpensive. But the term used is Independent since back when raid started, especially in the medical imaging industry, the disks were anything but inexpensive.

Great Article otherwise

Ian
August 3, 2012 12:05 AM

Backups are easy these days, I use an auto syncing free software to keep my RAID0 flash drive backed up.

Without RAID0, the USB 3.0 flash drive I use everyday would be much, much slower.

RAID0 splitting data across two solid state drives gives me awesome data speeds.

Without RAID0 a benchmark of:
3 x 1000MB
Read / Write (MB/s)
Seq 288 / 102
512k 232 / 54
4k 15 / 15
would be impossible.

RAID0 allows me to quickly store and move 6 GB high definition movies, plus play those HD movies stutter free straight from that flash drive, even via USB 2.0 ports.

AC
January 3, 2013 2:02 PM

I have zero risk increased of failure than normal HDD configuration when using RAID 0. I have write back cache disabled so if my power to the device is interrupted, i.e switch power of completely and this means unplugging AC cable then yes, I will risk complete data loss and this includes any chances of recovering data using software recovery options !!!!!!!!!ONLY IF WRITE BACK CACHE WAS ENABLED!!!!!!!!!

To avoid this disable write back cache and you will never lose data. You only risk having data loss like any other HDD configuration! Anyone that states using RAID arrays (other than RAID 0) to secure or backup data or to make it safer is an imbecile or knows nothing and is merely repeating diatribe that is prevalent over the Internet. Just disable any write back cache, it does not improve performance as much as is claimed and enjoy a 100% - 200% increase in performance over normal HDD speeds (depending on your HDD and chipset) including random access! I have experimented and I proved I do not have any reason to upgrade to an SSD. I use latest generation of VelociRaptor HDDs.

I promise you, no one else knows anything enough to tell you this online! I found out myself and have proven it to myself by trial and error. My system runs on RAID 0 and would have it no other way and it is bombproof. Not even Windows can kill it! make backups as you normally would do and have no more risk than normal HDD setup. The problem with RAID now as I see it is limited bandwidth but this is not RAID standard at fault or HDDs but more so the hardware manufacturers because of limited ability to transfer data. SATA 2.3 already allows very improved speeds of 16GB/ps of speed or more, if you want to compete with this you need to move to DRAM or XDR using Infiniband! SSD has not yet made HDDs obsolete, not by a far shot and SSD have limited write life. So HDDs are still #1 choice and even HDDs that are not green are very energy efficient now, no worse than SSD and some cases better! If my PS3 had more XDR then it would be so much better than my Desktop Workstation and I would install Linux on it. it is better in all other regards and can do 256 billion calculations per second.

HDD technology is only just getting started, we now have 1x1 inch HDDs for embedded boards and can also use them for other uses with large capacities. The possibilities are endless if we had the bandwidth to exploit but we don't and SATA is not moving on fast enough either so SSDs cannot perform as well as they should and this holds back development of SSDs. XDR over Infiniband or Thunderbolt is probably good if it was a low cost option. Experiment and run some clusters if you know how. Do anything you want to test things, but do not ignore RAID 0 for everyday computing needs. People make RAID seem overcomplicated than it needs to be. It is simple to do when you know how and need not over complicate the process. My main point being, there is still a lot that can be done with RAID 0 and HDDs. I don't mean to name call anyone or anything but it has to be said that RAID 0 is very safe and no worse than normal SATA setup when configured correctly! I can easily get 300-500 MB/s using RAID 0 with VelociRaptor and if you want more than this go for Infinibandor Tunderbolt.

I'll stick by my statement: RAID 0 doubles the chances of logical drive failure. If drive "C:" is spread out over two drives, then the failure rate of each individual drive comes into play. I'm not talking about write caching, I'm just talking about normal drive failure that can happen to any drive. If either drive fails, then the entire "C:" drive contents are lost. Absolutely backups are an important way to protect yourself from that increased possibility.

The statement "disable write back cache and you will never lose data." is simply wrong. There are too many other ways the drives can fail and data can be lost.
Leo
05-Jan-2013

AC
January 3, 2013 2:04 PM

I made a few typos and meant to say SATA v3.2 not 2.3. Sorry about that if it caused any confusion.

jojoba
January 20, 2013 10:54 AM

I agree with Leo - RAID0, without FULL, REGULAR backups, is a BAD IDEA! Sure, if you have rather 'static data,' and you have full, regular backups, RAID0 is not that much more risky than 'non-raid,' but still, 2 drives could fail - then you have to get 2 new drives and restore your backups.

Also, PLEASE LEO - *ALWAYS* mention the common names of the RAID - i.e., RAID 0 is 'Striping' - aka 'Volume Striping;' RAID 1 is 'Mirroring;' (you didn't even mention RAID 1, and you should have); i.e., "If you have 2 drives, and want some limited redundancy, use RAID 1 (Mirroring) - sure, you lose the space of that 2nd drive, but you get a 2nd drive that immediately can be booted and used, if the 1st drive dies. Still, RAID 5 and 10 (i.e., 1+0) are better for redundancy. but, for them, you need 3 or more drives. Thanks.

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