March 4, 2011 at 9:21 am
Steve Jones - SSC Editor (3/4/2011)
I would have sworn that the RAID 1+0 was 4 in Wikipedia, and I have submitted a correction.
The really funny part about it? It seems like most of us who ended up answering '2' knew the correct answer, but were double checking ourselves - and, like me, figured that what we knew must be incorrect after finding that article (the pre-edited version) - and therefore answered it incorrectly.
I enjoy doing the research to check my answers on the QotD, because I learn a lot just from the process of double-checking what I think is correct.
Next time I find myself raising an eyebrow and trying to figure out how on earth you mirror and stripe on 2 disks, I'll stop for a minute, remember this, and decide that just because I disagree with the 'expert' opinion doesn't necessarily mean that I am wrong.
-Ki
-Ki
March 4, 2011 at 10:24 am
Kiara (3/4/2011)
Steve Jones - SSC Editor (3/4/2011)
I would have sworn that the RAID 1+0 was 4 in Wikipedia, and I have submitted a correction.The really funny part about it? It seems like most of us who ended up answering '2' knew the correct answer, but were double checking ourselves - and, like me, figured that what we knew must be incorrect after finding that article (the pre-edited version) - and therefore answered it incorrectly.
I enjoy doing the research to check my answers on the QotD, because I learn a lot just from the process of double-checking what I think is correct.
Next time I find myself raising an eyebrow and trying to figure out how on earth you mirror and stripe on 2 disks, I'll stop for a minute, remember this, and decide that just because I disagree with the 'expert' opinion doesn't necessarily mean that I am wrong.
-Ki
... and part two to that is that wikipedia is not an expert, it's merely the most recent entry, reviewed, you hope, by other experts.
I'm one of those people that doesn't do the research before I answer the question... and then learn from the answers, so my points are all over the place. In this case I got it right mostly because I've had to bang my head on RAID for a long while.
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March 4, 2011 at 10:38 am
Craig Farrell (3/4/2011)
Kiara (3/4/2011)
Steve Jones - SSC Editor (3/4/2011)
I would have sworn that the RAID 1+0 was 4 in Wikipedia, and I have submitted a correction.The really funny part about it? It seems like most of us who ended up answering '2' knew the correct answer, but were double checking ourselves - and, like me, figured that what we knew must be incorrect after finding that article (the pre-edited version) - and therefore answered it incorrectly.
I enjoy doing the research to check my answers on the QotD, because I learn a lot just from the process of double-checking what I think is correct.
Next time I find myself raising an eyebrow and trying to figure out how on earth you mirror and stripe on 2 disks, I'll stop for a minute, remember this, and decide that just because I disagree with the 'expert' opinion doesn't necessarily mean that I am wrong.
-Ki
... and part two to that is that wikipedia is not an expert, it's merely the most recent entry, reviewed, you hope, by other experts.
I'm one of those people that doesn't do the research before I answer the question... and then learn from the answers, so my points are all over the place. In this case I got it right mostly because I've had to bang my head on RAID for a long while.
Oh, I totally agree that wikipedia is NOT an expert. I was just too pressed for time this morning to actually chase the discrepancies down. No matter how much fun I have over here on SQL Central, my job comes first. 🙂 And if I don't get a point on a question, so what? I already toasted my score by guessing on the question about Steve's Hawaiian shirts over the holidays and getting it wrong. 🙂
-Ki
-Ki
March 4, 2011 at 10:59 am
I selected 2 because that's what our RAID 10 arrays actually are, 2 disks, on our SAN, oh well 😉
March 4, 2011 at 12:18 pm
Thanks for the question.
I would say that 4 is the correct minimum, otherwise you just have RAID1. (You are just mirroring two drives.) You don't get the +0 until you add two more drives to stripe over.
I suppose you could call a two drive RAID1 array RAID10, but I think that is pushing it.
March 4, 2011 at 3:44 pm
Thanks for the question
Jason...AKA CirqueDeSQLeil
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March 5, 2011 at 5:53 am
UMG Developer (3/4/2011)
Thanks for the question.I would say that 4 is the correct minimum, otherwise you just have RAID1. (You are just mirroring two drives.) You don't get the +0 until you add two more drives to stripe over.
I suppose you could call a two drive RAID1 array RAID10, but I think that is pushing it.
But you do have it striped with a 2-drive mirror. When reading the data, you can read from both drives at the same time reading different stripes. It just so happens that each drive contains all the stripes.
March 7, 2011 at 2:59 am
cengland0 (3/4/2011)
I was about to answer 4 from my own knowledge but then looked it up to be sure and found this:http://en.wikipedia.org/wiki/Nested_RAID_levels
It quotes:
RAID 10 can be implemented with as few as two disks. Implementations supporting two disks such as Linux RAID10 offer a choice of layouts, including one in which copies of a block of data are "near" each other or at the same address on different devices or predictably offset: Each disk access is split into full-speed disk accesses to different drives, yielding read and write performance like RAID0 but without necessarily guaranteeing every stripe is on both drives.
Therefore, the correct answer should be 2 like I answered. Usually it's 4 but there is this possibility of 2 and the question asked for the minimum not the "usual."
No, 2 is a very wrong answer.
I would dearly love to see how LINUX RAID 10 can be claimed to be raid 10 if it doesn't guarantee that every stripe is on both drives.
The standard for control data describing that data layout for each of the RAID configurations for which a definition is agreed are in Common RAID Disk Data Format Specification. Look at Figure 23, which is the illuustration for a nested RAID array with outer RAID level 0, and all the associated text, and see if you you think it permits an arangement where some stripes omit some BVDs. Alternatively, maybe the suggestion is that the inner level of RAID 10, ie RAID 1, allows for some of the data not to be mirrored? Doesn't seem to be permitted by either the description of simple mirroring (Figure 3 and surrounding text) or the description of multi-mirroring (figure 4 and surrounding text).
To put it bluntly: either Wikipedia has got this horribly wrong or "Linux RAID 10" is not any sort of RAID 10 but instead a misleading misappropriation of what is after all standard terminology with a definition agreed by the industry association.
Tom
March 7, 2011 at 3:03 am
Steve Jones - SSC Editor (3/4/2011)
I would have sworn that the RAID 1+0 was 4 in Wikipedia, and I have submitted a correction.
That'll teach you not to rely on Wikipedia in your explanations. :hehe::-P
Avernar fixed this four years ago - I guess the usual Wikipedia process (someone who knows corrects it, someone who doesn't then changes it back) has been going on. I've made some changes that point out the Linux MD Driver thing is NOT standard RAID 10 (changes which may or may not last).
Tom
March 7, 2011 at 9:07 am
Tom.Thomson (3/7/2011)
Steve Jones - SSC Editor (3/4/2011)
I would have sworn that the RAID 1+0 was 4 in Wikipedia, and I have submitted a correction.That'll teach you not to rely on Wikipedia in your explanations. :hehe::-P
Avernar fixed this four years ago - I guess the usual Wikipedia process (someone who knows corrects it, someone who doesn't then changes it back) has been going on. I've made some changes that point out the Linux MD Driver thing is NOT standard RAID 10 (changes which may or may not last).
Yeah, I was hoping for a BOL one, but couldn't find a good definition. I think that some people are assuming "drive" is physical drive in some cases, but not in others. A "drive" could be a logical drive. By definition, you must have two drives, physical or logical, for a RAID 0 stripe. You must have two drives, p or l, for RAID 1. To combine those together means 4 drives at a minimum. Whether that's 4 logical on one physical, 4 physicals, or some combination, you need 4 drives. You won't get the protection if it's logical drives, but I suppose you could meet the definition that way if you want.
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