RAID 10 is not the same as RAID 01.
This article explains the difference between the two with a simple diagram.
I’m going to keep this explanation very simple for you to understand the basic concepts well. In the following diagrams A, B, C, D, E and F represents blocks.
RAID 10
- RAID 10 is also called as RAID 1+0
- It is also called as “stripe of mirrors”
- It requires minimum of 4 disks
- To understand this better, group the disks in pair of two (for mirror). For example, if you have a total of 6 disks in RAID 10, there will be three groups–Group 1, Group 2, Group 3 as shown in the above diagram.
- Within the group, the data is mirrored. In the above example, Disk 1 and Disk 2 belongs to Group 1. The data on Disk 1 will be exactly same as the data on Disk 2. So, block A written on Disk 1 will be mirroed on Disk 2. Block B written on Disk 3 will be mirrored on Disk 4.
- Across the group, the data is striped. i.e Block A is written to Group 1, Block B is written to Group 2, Block C is written to Group 3.
- This is why it is called “stripe of mirrors”. i.e the disks within the group are mirrored. But, the groups themselves are striped.
If you are new to this, make sure you understand how RAID 0, RAID 1 and RAID 5 works.
RAID 01
- RAID 01 is also called as RAID 0+1
- It is also called as “mirror of stripes”
- It requires minimum of 3 disks. But in most cases this will be implemented as minimum of 4 disks.
- To understand this better, create two groups. For example, if you have total of 6 disks, create two groups with 3 disks each as shown below. In the above example, Group 1 has 3 disks and Group 2 has 3 disks.
- Within the group, the data is striped. i.e In the Group 1 which contains three disks, the 1st block will be written to 1st disk, 2nd block to 2nd disk, and the 3rd block to 3rd disk. So, block A is written to Disk 1, block B to Disk 2, block C to Disk 3.
- Across the group, the data is mirrored. i.e The Group 1 and Group 2 will look exactly the same. i.e Disk 1 is mirrored to Disk 4, Disk 2 to Disk 5, Disk 3 to Disk 6.
- This is why it is called “mirror of stripes”. i.e the disks within the groups are striped. But, the groups are mirrored.
Main difference between RAID 10 vs RAID 01
- Performance on both RAID 10 and RAID 01 will be the same.
- The storage capacity on these will be the same.
- The main difference is the fault tolerance level. On most implememntations of RAID controllers, RAID 01 fault tolerance is less. On RAID 01, since we have only two groups of RAID 0, if two drives (one in each group) fails, the entire RAID 01 will fail. In the above RAID 01 diagram, if Disk 1 and Disk 4 fails, both the groups will be down. So, the whole RAID 01 will fail.
- RAID 10 fault tolerance is more. On RAID 10, since there are many groups (as the individual group is only two disks), even if three disks fails (one in each group), the RAID 10 is still functional. In the above RAID 10 example, even if Disk 1, Disk 3, Disk 5 fails, the RAID 10 will still be functional.
- So, given a choice between RAID 10 and RAID 01, always choose RAID 10.
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My name is Ramesh Natarajan. I will be posting instruction guides, how-to, troubleshooting tips and tricks on Linux, database, hardware, security and web. My focus is to write articles that will either teach you or help you resolve a problem. Read more about
{ 17 comments… read them below or add one }
Something is still out of my sight… if performance, sorage and # of disks (=~cost) are the same, and RAID10 is better in fault tolerance than RAID01, why should any vendor offer RAID01? Is someone offering it?
Or there is some topic where RAID01 is better than RAID10?
oh, I almost forget: YOUR BLOG IS EXCELLENT. Thank you.
Hi Ramesh,
If disk1 and disk2 both failed in RAID10, then what will be happend?
I’m sorry, but your logic doesn’t hold here.
You wrote – “In the above RAID 01 diagram, if Disk 1 and Disk 4 fails, both the groups will be down. So, the whole RAID 01 will fail.” Well, that’s also true for the RAID 10 diagram you presented. If disks 1 and 2 fail, your entire RAID is down.
Likewise you wrote – “In the above RAID 10 example, even if Disk 1, Disk 3, Disk 5 fails, the RAID 10 will still be functional.” Again, that also holds true for your RAID 01 diagram. Disks 1, 2 and 3 can all fail in RAID 01 and your system will remain functional.
It’s an issue of probabilities. In both cases, each block exists on two physical drives. The system will fail any time a block becomes unavailable, regardless of grouping. Likewise, the system will remain functional if all blocks are available, regardless of how many drives have failed – assuming your structure can retrieve the unavailable blocks from the other drive on which it resides.
Hi,
thanks a lot
i think raid 5 is the best one…
Hi !
I made the maths with my wife (safety expert) :
- ‘f’ is the failure probability of one single disk.
- Gx is the name of the groups
- Dx is the name of the disks
RAID 10 :
For loosing your file, you need to loose G1 OR G2 OR G3. To loose G1, you need to loose D1 AND D2, to loose G2 you need to loose D3 AND D4 and to loose G3 you need to loose D5 AND D6.
=> probability of loosing your file : (f*f)+(f*f)+(f*f) = f²+f²+f² = 3f²
RAID 01 :
For loosing your file, you need to loose G1 AND G2. To loose G1 you need to loose D1 OR D2 OR D3, and to loose G2 you need to loose D4 OR D5 OR D6.
=> probability of loosing your file : (f+f+f)*(f+f+f)=3f*3f=9f²
In this particular case (6 blocks, 6 disks), you have 3 times more chances to loose your file on RAID01 than on RAID10.
More basically, you can think like that :
- on RAID 10, if one disk fails, when the second failure appears, I have 1 possibility between 5 that this makes my entire system fail (the other disk in the group)
- on RAID 01, if one disk fails, when the second failure appears, I have 3 possibilities between 5 that this makes my entire system fail (any disk in the other group)
The Intel RAID controller in my system (Gigabyte X58A-UD7 motherboard) shows the RAID as RAID 10 and then (RAID0+1) shortly there after while the machine is booting up, and then the Intel RST software shows RAID10 while in Windows. I’m honestly not sure which I have at this point. I’ll just assume I have RAID10 and hope for the best. =)
Ken, I agree the logic is flawed.
gUI, you may have done the math but you are following the flawed logic.
In either of the setups if disk 1 fails you lose the redundancy on blocks A and D, so in order to have a complete failure you would have to lose the other disk that contains the A and D blocks.
In the case of RAID 10 it would be losing disk 1 and 2, in the case of RAID 01 it would be losing disks 1 and 4.
Both have the same probability after loss of the first disk. 1 out of 5.
Hi Ramesh,
In Raid10 section, the last bullet says:
This is why it is called “stripe or mirrors”. i.e the disks within the group are mirrored. But, the groups themselves are striped.
But I think you try to say “stripe of mirror”.
@ramsee,
Thanks for pointing it out. It is corrected now.
@weebl
No, in 01, the problem is the group. If you loose disk 1, group1 is dead, it won’t help you even for getting pieces C,D,E or F. So if the next failure is on the group 2, whatever the disk, the group 2 also is dead, and you won’t get any file.
The disks are not independants.
@gUi thanks for clarifying. It makes sense now, and re-reading the article i see the auhor also states the same thing. I agree RAID 10 is better.
Comparing raid10 and raid01 seems easier with
the same number of disks (4) for each raid:
- raid10: 2 disks in each group for mirroring,
2 groups for stripping
(D1G1 D2G1, D1G2 D2G2)
- raid01: 2 disks in each group for stripping,
2 groups for mirroring
(D1G1 D1G2, D1G1 D2G2).
Then, complete failure if:
- raid 10: if 2 disks of one or more group failed
((D1G1 and D2G1) and/or (D1G2 and D2G2))
- raid 01: if 2 disks of one or more mirror failed
((D1G1 and D1G2) and/or (D2G1 and D2G2)).
But probability of “((D1G1 and D2G1) and/or (D1G2 and D2G2)) failed”
is equal to
probability of “((D1G1 and D1G2) and/or (D2G1 and D2G2)) failed”.
And then raid10 == raid01.
@Jidifi
For 2 disks, maybe it’s the same (don’t want to redo the maths), but for more, it’s no more equivalent. Please re-read carefully all comments.
@gUI
In your first comment, you write:
“RAID 10 :
For loosing your file, you need to loose G1 OR G2 OR G3. To loose G1, you need to loose D1 AND D2 …”.
Question:
“RAID 10 :
If you lose D1 only (or D2 only), have you lost your file ? “
@gUI
In your first comment, you write:
<
< – ‘f’ is the failure probability of one single disk.
< RAID 10 :
probability of loosing your file : (f*f)+(f*f)+(f*f) = f²+f²+f² = 3f²
< RAID 01 :
probability of loosing your file : (f+f+f)*(f+f+f)=3f*3f=9f²
probability of loosing your file : 3 x 0.8 x 0.8 = 1.92 !!
RAID 01 : => probability of loosing your file : 9 x 0.8 x 0.8 = 5.76 !!
gUI, probabilities greater than one do not exist in our world.
(Please re-read carefully all your first mathematical books)
I know, thank you very much… but why did you choose f=1 ? I think you also should have a look at your maths books…
Oups, no, you did not choose f=1, you choose 0.8, yes… But the formula still true. It just means that with a hard drive that have 80% of probability to fail in an hour, you are sure that you will loose your system. Not very interesting.
Now check with other value, more realistic, like 1/10e-4 (around one failure in one year).