Disaster Recovery optimize

Pop Quiz: RAID basics

Do you know the difference between RAID 1 and RAID 5? Take this short quiz and test your knowledge of RAID basics.

Answers to our RAID basics pop quiz:

  1. Redundant array of inexpensive/independent devices
  2. RAID 0
  3. RAID 1
  4. RAID 5
  5. RAID 10
  6. 2
  7. 2
  8. 3
  9. 4
  10. Disk duplexing

For more information on the pros and cons of different RAID levels, including those not covered on this quiz, check out the following TechRepublic resources:

About

Bill Detwiler is Managing Editor of TechRepublic and Tech Pro Research and the host of Cracking Open, CNET and TechRepublic's popular online show. Prior to joining TechRepublic in 2000, Bill was an IT manager, database administrator, and desktop supp...

69 comments
nihilus65
nihilus65

I never use the term RAID10 (maybe I'm also to old!). Because 10 never tells you how the setup is. That is why I always use RAID1+0 or 0+1. That is a BIG difference in my eyes. But hey, I could be wrong and it wouldn't be the first time either. I really enjoyed this one, as always.

david
david

This quiz has numerous mistakes. First, duplexing can be done with a single, dual-ported controller. It does not require 2 controllers. Secondly, the biggie, you can build a RAID0,1 ... RAIDn array with just one disk. I've done it many times. It is just in the world of this author, he obviously has no experience with software/host-based RAID like md, or zfs.

Kerry_Miller
Kerry_Miller

You need to fix question 5 as well; there is no such thing as disk stripping, just disk striping. However, it was a great fundies quiz, thanks for posting!

PVSSR
PVSSR

Thanks for the info. S Rao |01/19/11

Charles Bundy
Charles Bundy

I enjoy your pop quizzes! BTW what about RAID 6? For the most part as a home user I stay away from RAID 5...

john3347
john3347

"Inexpensive" is such a subjective term, or quantity, that it is not reasonable to refer to a hard disk drive as either expensive or inexpensive. Inexpensive relative to what??? Whose definition of inexpensive???? Drop the meaningless (undefined) "inexpensive" relative to what RAID is an acronym of. It involves multiple independent disks (or drives), each of which is an independent drive by itself when not used in a RAID configuration regardless of cost or someone's perception of expensive or inexpensive.

USBPort1
USBPort1

to see that there are some techs (I'm guessing we're all techs answering these questions) that don't know these RAID basics (I'm going by the percentages listed in the wrong answers). This is like Tech knowledge 101 stuff.

BlackKris
BlackKris

I always thought RAID stood for "redundant array of IDENTICAL drives."

cdnjay
cdnjay

From Wikipedia: "RAID 10 can be implemented with as few as two disks." That's what I thought, but apparently I'm wrong? It seems to work better with 4 disks certainly but will it not work at all without at least 4?

Tony_Scarpelli
Tony_Scarpelli

What is the point of building a single disc raid?

Bill Detwiler
Bill Detwiler

Whether you're talking about physical or logical "disks", specific RAID levels still require a minimum number. Perhaps using terms such as "spaces" or "places" would be more accurate, but most definitions I've seen still use the terms "disks". Also, if your goal is to eliminate the physical hard drive as a single point of failure, wouldn't you want two separate devices. Or at the very least, two logical spaces on separate physical devices. Likewise, the goal of disk duplexing is to eliminate the controller as a single point of failure. And, all of the definitions I've read describe it as such. As with most tech subjects, RAID terminology is constantly evolving and there is often disagreement or confusion about the exact meaning of a given term. Just look at the switch from "inexpensive" to "independent". When developing these quizzes, I always try to use the most-widely-accepted terms and definitions.

dtracy1197
dtracy1197

If you check,a dual ported controller is two controllers. Secondly, although you can do it, why would you build a mirror(?) on one drive? That would defeat the purpose of the mirror!

Bill Detwiler
Bill Detwiler

Thanks for the note. That extra "p" just hopped in there.

blarman
blarman

Drop the extra "p". It's even worse than the perpetual use of "loose" instead of "lose" on so many forum posts.

Bill Detwiler
Bill Detwiler

I really wanted to include questions on RAID 6, RAID 50, and RAID 60, but I like to keep these quick quizzes down to 10 questions.

stevebuck
stevebuck

is probably in order here for those that didn't work around mainframe computers 30 years ago. At that time, a 'disk drive' was the size of a washing machine and capacity was measured in only hundreds of megabytes. When a disk started to develop errors or crashed, the HDA (hard disk assembly) needed replacing at the tune of about 10K USD. When personal computers came out and with them the first 5.25" and then 3.5" drives at the end of the 80's, these disks were considered "inexpensive". Eventually mainframes and minis moved to using arrays of these inexpensive drives. So the first time I ever heard the term RAID, it was coined as "Redundant Array of Inexpensive Disks". A new generation can call it whatever they want but it still has the same meaning for me today.

oldbaritone
oldbaritone

the original concept behind RAID was to improve the throughput performance and MTBF of a storage system by using several "lesser" drives in an array. High-speed and high-reliability drives were available, but significantly more expensive (orders of magnitude more expensive) than standard off-the-shelf drives, which were considered "inexpensive." By using several drives in the array, most of the benefits of the high-performance high-reliability systems could be achieved at a much lower cost. Hence, RAID. Of course, "inexpensive" is a relative term. RAID has grown far beyond its original concept (evidence RAID 0/1/5/10) and enhancements have increased the cost. Still, RAID is a less-expensive way to improve performance and reliability, now to a level that was unachievable at any price when RAID was originally conceived.

sserwe
sserwe

I don't believe the term inexpensive in the acronym is used anymore and it really doesn't apply. Most companies spend a lot of money on 10-15k RPM drives then RAID them up. So while RAID was originally intended to speed up slower, cheaper drives, it is now used more to make fast, expensive drives even faster and for fault tolerance.

Tony_Scarpelli
Tony_Scarpelli

I got them all right but that is only because they didn't give 6 drives as an answer for Raid10. For some reason I thought it was two 5's. Everything I have read and used since 1992 says Raid is Redundant array of inexpensive drives. My first HD was 5mb's and cost $400, funny as hell. But then that was cheaper than the stuff the big Iron of the day used.

NickNielsen
NickNielsen

for whom the application is more important than the theory. I know the basic basics, but only as general tech knowledge. My RAID experience these days is pretty much limited to 1) identify bad drive, 2) remove bad drive, 3) install replacement, 4) verify rebuild initiated, 5) go to next call.

digital riverrat
digital riverrat

There's a number of us that aren't able to be practicing techs in the corporate arena anymore. I mostly write freelance articles now and do tech stuff on the side for friends and family; people who don't really need RAID setups. Thus, I've let that part of the skill set stagnate over the years.

Joanne Lowery
Joanne Lowery

I've been RAIDing for some time (20 plus years) and I had always understood that RAID 0 could start with only one drive and extend the stripe from there on. So, I had one question wrong. Equally, a RAID controller should also be able to support JBOD, even though these aren't redundant, (just like RAID 0 isn't redundant).

douglas.gernat
douglas.gernat

Yes, the understanding of what RAID is seems basic, but how many senior level techs don't need to brush up on the basics from time to time? Whether it be storage technology, or customer service? :)

Charles Bundy
Charles Bundy

Because you can have different size drives (not recommended) and the drive array size is determined by the smallest drive. Addendum - Should have read Zen's post before replying :)

gechurch
gechurch

I have heard quite a few different acronyms, but consistently Redundant Array of Independent/Inexpensive Disks is what comes up, and generally that is what is used in authorative sources (although I remember reading a book on the subject that had a different definition). I've never heard of Redundant Array of Identical Drives though. And if you think about it, it doesn't make sense. RAID1 is the only level where every drive is identical - with every other level the contents on each disk are different.

usagihotaru
usagihotaru

RAID 10 can actually work with a minimum of 2 disks, it's just preferred with 4 disks for speed.

Jasonjb1222
Jasonjb1222

Raid 10 or Raid 1+0 Raid 1 = Mirroring. Raid 0 = Striping. You need 2 disks at least to Span a striped volume. You need the equivalent in drives (with at least the smallest amount of required disk space) in order to mirror. I.E Mirror a 250 GB onto a 320 GB. The opposite would not be possible or true. * * * * * * So 2 striped drives would require 2 mirror drives for them to be equivalent. Total of 4 disks. ********* Although, I have not tested it myself, I would think that a striped (2 disk) 500 GB volume could in theory be mirrored to a single equivalent 500 GB Drive. If anyone knows an answer or has tested would be nice to know.

mfa
mfa

If you keep track of head movement, you'll notice that after a read operation, the heads of the mirrored drives are no longer necessarily in the same position. A RAID controller can take that into account on a read operation, and solicit the data from the drive whose head is closest to the desired data, thereby reducing seek time. So, it's arguable that RAID 1 offers both redundancy AND improved performance. Of course, the downside is decreased write performance, but I'll bet most non-paging hard drive operations are reads.

GreatZen
GreatZen

Raid 10 is a stripe of mirrors. That is to say, start with two mirrored pairs (a base RAID1 implementation requiring 4 drives, which will function as two Arrays), and then stripe those two RAID1 arrays to form a RAID10 array. Each RAID1 array sees two drives formed into one array, and the RAID0 array sees two arrays formed into one array.

randy_scadden
randy_scadden

RAID, an acronym for Redundant Array of Independent Disks (formerly Redundant Array of Inexpensive Disks) http://en.wikipedia.org/wiki/RAID So it's DISKS not DEVICES so the correct answer to question 1 is none of the above.

jacobus57
jacobus57

None, but you knew that already ;-) A single ANYTHING cannot, by definition, be an array!

Charles Bundy
Charles Bundy

I suppose you could be redundant with regards to sector based UREs but take a terrible hit on R/W performance. Since you are hammering the drive mechanically I'd say MTBF is met quickly. :)

Tony_Scarpelli
Tony_Scarpelli

Since the drive now has to write twice on the same physical disk it would cut performance considerably. Make no sense at all.

SgtPappy
SgtPappy

I got a stack of one dollar bills.

mfa
mfa

I'll see your washing machine and raise you a refrigerator or two. The first disk storage unit I worked on was an IBM 1301, a successor to the IBM 350 (part of the IBM 305 RAMAC system), in the mid 1960s. It cost about $1/4M (list), weighed close to a ton, and used disk "platters" about 2' in diameter. The neat thing about it was that the entire array was housed in a clear glass enclosure, so you could see the heads moving around as it worked. And when the heads moved, it was pretty exciting. I heard that a programmer once wrote a program to find the seek rate that resonated with the mass of the 2301, and managed to tip the whole thing over. I have never been able to confirm it, but it sure sounds plausible. IT was Fun back then. "HDA" really stands for "Head Disk Assembly". Early disk storage units had disk arrays separate from the read/write head assemblies, which led to contamination and alignment problems. In the mid 70s, IBM announced the 3340 ("Winchester"), combining the disks and heads in sealed HDAs. It was a major advance, and today's drives are all made that way, I'm pretty sure. But they're pretty boring to watch.

boomchuck1
boomchuck1

oldbaritone has got it right. When we first started doing RAID 5 the SCSI disks that it required we very expensive, especially the larger ones like 300mb drives! Use of the inexpensive (relatively speaking) drives was a good benefit, but having redundancy so that a drive failure wouldn't knock you out of business was the big motive for going RAID 5.

DNSB
DNSB

Nah, that's a RAID 50 array. I'd go for a RAID 5E array instead (RAID 5 with a hot spare).

boomchuck1
boomchuck1

I'd suppose that some errors on RAID 0 or 1 can be excused since there hardly seems a reason why anyone would ever use them. I go with RAID for the redundancy which is totally non-existant in 0 and dates back to our old practice of mirroring in 1. In business I can see no reason to use these instead of RAID 5 or better.

GreatZen
GreatZen

Not only are the contents of each drive not identical (the context to which gechurch seemed to be referring), the drives themselves don't need to be identical. You can use ANY make/model/size of drive that you wish and performance and size are limited by the least common denominator. Certainly, using mismatched disks is highly frowned upon for a wide variety of reasons but it's not a technical limitation.

neilb
neilb

Then it isn't really RAID 10. If you started with a proper 4-disk, 2+2 mirror and two have gone, you either have a very large paperweight (one dead from each 0 stripe set) or you've lost the 'R'. :)

nick.ferrar
nick.ferrar

I can assure you RAID10 can be done on 2 drives, there's just a common misconception (and possible limitation on some vendor implementations) that it needs 4 drives. EMC Clariion for example supports 2 drive RAID10. I think the main advantage to using it on 2 drives (over RAID1) is that it's easier to expand at a later date (if required).

Bill Detwiler
Bill Detwiler

It should have indeed been disks instead of devices. Too much copying and pasting from documents into small text boxes. It's been fixed.

Charles Bundy
Charles Bundy

Wikipedia also says you can do RAID 10 with 2 disks. So how much trust do you put in your reference source?

gechurch
gechurch

The brain works in a funny way - I didn't even notice the word "devices" until after I answered and saw a reasonable percentage of people had chosen None of The Above. I read "Redundant Array of Independant/Inexpensive" and glanced at the other options quickly and saw that none of them were even close to correct, so I chose option 1.

GreatZen
GreatZen

It's true that the D is generally accepted to mean "Disks", but since RAID is possible with "devices" that aren't "disks", "devices" is a more accurate acronym. Although SSDs can vaguely be considered "disk drives" (even though the term Hard Disk Drive seems to have been largely forgotten), I've certainly never heard of a RevoDrive or USB thumb drive referred to as a "Disk drive."

dave
dave

We are talking drives here, but maybe solid state drives would be thought of as devices.

DNSB
DNSB

Strictly speaking, an array with a single member is mathematically valid as would an array with no members. Violates the heck out of the first part of the name though -- how do you get redundancy from a single member. But then I have to remember that RAID 0 was a later addition to the definitions of RAID levels.

homebrewmike
homebrewmike

Back in the 90's, I seem to recall working on a SCSI RAID card that would allow RAID 10 on 3 disks. Seemed a little weird, but this is how we assumed it worked. (we never did anything with RAID 10 on 3 disks - we always chose RAID 5 for the app we needed.) D0 D1 D2 0 1 2 2' 0' 1' It's striped, and it's mirrored. With 2 disks, I suppose this is possible: D0 D1 0 1 2 3 1' 0' 3' 2' Both aren't optimal - a full stripe write would cause 2 writes per disk. Same thing as the 3 disk RAID 10. Fun to think about, but who would want to do something like that in the field. (it might give a slight edge on a read - if the read doesn't read the mirror data for accuracy, it would probably be as efficient as a RAID 0.) So, being the experimentalist that I am, I decided to try this on a the $30K disk array sitting in front of me (the vendor shall remain nameless.) And, lo and behold, to make a RAID 10 stripe, one needs 4 disks on that array. So, I'll ask the question a little differently - if this D0 D1 0 1 2 3 1' 0' 3' 2' isn't RAID 10, what is it?

mfa
mfa

According to Wikipedia, it was the ST-506, made by (then) Shugart Technology (soon renamed Seagate Technology) in 1980. I haven't been able to find any link with Olathe, KS. Alan Shugart worked with IBM on the RAMAC project and had his professional roots in Silicon Valley. Seagate is now incorporated in Ireland.

mfa
mfa

I'd kill to be 55 again...

Tony_Scarpelli
Tony_Scarpelli

I remember the Winchester name. I never worked in Big Iron but am familiar with the term. I remember reading about a company in Olathe Kansas that made the first PC drive. Was it Western Digital? or something like Digital Research....its hell to be 55. I can't remember.

Joanne Lowery
Joanne Lowery

Choosing a RAID type is determined by the IOPS requirement and the level of redundancy and recovery required. RAID 1 gives very good read / write performance. Recovery is also quick. RAID 5 improves the hardware utilisation, but write speeds are slower and IOPS can then also be slower. Recovery of a failed disk takes much longer on RAID 5 than RAID 1. Courses for horses. You can't just dismiss RAID 1 and say RAID 5 is better.

DNSB
DNSB

On one manufacturer's site, the only difference I found between their RAID 1 and 2 disk RAID 1+0 implementation was the maximum stripe size. Why there was a difference between the maximum stripe sizes was something that was not clarified.

neilb
neilb

It's like language morphing over time. Until the A in RAID means something other than array then it *is* just a marketing exercise. All of the vendor-independent storage sites that I've been on (and most of the specific vendor storage information sites) seem to still believe that four disks is the minimum starting point for RAID 10. Still, not worth arguing about until someone can produce some concrete eveidence that there is a true difference between RAID 1 and RAID 1+0 using two disks either as measured performance difference or (and I'll go with this one if someone can show me the money) improved ability to scale up the array size. :)

neilb
neilb

"it could as easily be claimed that writing data to a single drive is RAID 0" is, indeed, the claim by some. EMC among them, apparently, can stretch their imagination to an "array" of one. :)

nick.ferrar
nick.ferrar

Whilst I concede it's more of a pseudo RAID10 as you don't get the multiple drive failure etc. the fact is RAID10 on two drives is starting to be pushed (by multiple vendors) and, as I understand it, there are differences over RAID1 in how the blocks are placed (rather than it just being a marketing exercise).

DNSB
DNSB

From IBM's documentation, you need two striped arrays which are then mirrored to produce a RAID 10 or 1+0/0+1 array. I suppose someone might claim that writing data on 1 drive and then mirroring it to a second drive is RAID 10. On the basis of that logic, it could as easily be claimed that writing data to a single drive is RAID 0.

neilb
neilb

Which makes it true...

neilb
neilb

and that you so emphatically say it's so, doesn't make it true. It's just a bit of semantics. The point about the two digit RAID codes is that they signify nested arrays and, however much EMC might want to stretch it, each of the "0" in a two-disk RAID 10 isn't an "Array". It's a disk. So it's RAID 1. Just because EMC systems can subsequently expand those single disks into an underlying RAID 0 stripe set and so [b]achieve[/b] a RAID 10 doesn't really mean that you start from there. :)

Charles Bundy
Charles Bundy

If RAID 1+0 (or 10) is a combination of RAID0 and RAID1 which both require a minimum of 2 drives how do you arrive at 2 + 2 = 2 I could swear the axiom was 2 + 2 = 4

Charles Bundy
Charles Bundy

RAID 10 overall has better redundancy and performance than RAID5. Just not so good on how much capacity you "throw" away. Having had several RAID5 arrays go unrecoverable on me I haven't done distributed parity in about 5 years as the price/bit just got ridiculously affordable.

nick.ferrar
nick.ferrar

I don't agree RAID5 is poor man's RAID, there are many situations where it will satisfy the IOP and redundancy requirements so why use additional drives for RAID6, 10 or 50? It's certainly not the right solution every time but then no RAID type is.

RechTepublic
RechTepublic

...after all, RAID can be used for more than disks these days. We can't stay stuck in old tech definitions. Perhaps, in the back of your mind, you knew "devices" was more accurate so it slipped into your post. Now, let's get the "R" out of RAID 0 and get rid of Poor Man's RAID (RAID 5) now that we can all afford big disks and make RAID 1 and RAID 10 arrays.

Dzmitry Z
Dzmitry Z

A lot of articles on Wiki are written by people like you and me and are only as good as their level of knowledge, which often is not backed by anything. There are tons of great facts on it, but I wouldn't put too much trust in Wikipedia as the oficial source. IMHO, the only way to make RAID 10 with 4 disks. Dividing HDDs into partitions will allow you to have a configuration similar to RAID 10, but it will not be called that since you won't have separate disks/devices.

info
info

A single disk (or device, for those arguing the difference) is not an array in and of itself. That's like owning a single car and saying that you own a 'fleet'. I've never tried it, but you COULD split each of the two drives into two partitions, then mirror and stripe them to technically have a RAID10, but it would be useless from a performance point of view. And you'd still fail the definition drive or device-wise.

vjeran.novak
vjeran.novak

Each stanalone drive (disk) is in fact in RAID0. If it fails, you "loose" (recovery, etc..) data and RAID! So the answer to the question how many disk you need for RAID0 is 1, and from there you can say that for RAID10 you also need only 2 disk drives. But that is then only RAID1 - because you are using RAID10 when there is need for speed.

BernieLyons
BernieLyons

The term RAID was coined long before it involve anything but Disks as we know them. SSD's, Flash drives etc. weren't around either so the answer has to be #1.