What’s better, RAID 1+0 or RAID 0+1?

Because I’m a geek, I’m basically reposting what I’ve found on another site. Some RAID designs call this RAID 10, instead of RAID 1+0, I’m not quite sure why, as they don’t call RAID 0+1 -> RAID 01. For those that are unfamiliar with RAID, it stands for Redundant Array of Independent Disks ( now let me check to see if my memory is correct ) Woo, I remembered it, however the I can also mean inexpensive.

One site that I’ve always gone to to figure out what type of RAID to use is Advanced Computer & Network Corporation [ ACNC ]. I first found their site many many months ago, and it always comes in handy. Here is an example of what RAID 0 is. It’s basically where each disk receives a piece of your data. A really watered down version of it is to think that if you have a 1MB file, 500k goes to one disk, 500k goes to another disk, in a two disk RAID 0 config. If you had a 10 disk RAID 0, then 100k would be written to each disk. This means that your overall speed and performance is greatly increased. The big downside to this is that if you lose one disk, then all of your data is gone.

Well, how do you save your data? You can use RAID 1, which is where you mirror each disk. You get the same write performance, meaning the whole amount of data is written to each disk seperately, but it’s not broken down in to chunks like above. However, you do read the data at twice the rate. Also, if you lose a disk, it’s ok, because you have the other disk as backup.

All RAID 0+1 and RAID 10 ( henceforth RAID 1+0 ) are a combination of the two RAIDs put together, it’s only the implementation that has changed.

Found on http://www.ofb.net/~jheiss/raid10/ explains the difference between the two setups. Reposted here for posterities sake, as well as my own in case I ever need to reference it again 😀

Of late I’ve heard much talk about RAID 1+0 being better than RAID 0+1, but never got a good answer why. Leah and I started talking about this over dinner one night and did a little math (literally on the back of a napkin) to calculate how much better. Here’s what we figured out.

RAID 0+1 configuration where multiple disks are striped together into sets (sets A & B in the diagram, each set being as large as the resulting final volume), and then two or more sets are mirrored together.

RAID 1+0 configuration where two or more drives are mirrored together (mirrors 1-4 in the diagram), and then the mirrors (as many as are needed to result in the desired amount of space) are striped together.

In either case (0+1 or 1+0), the loss of a single drive does not result in failure of the RAID system. The difference comes in the chance that the loss of a second drive from the system will result in the failure of the whole system. In RAID 0+1, you have to lose one drive from each disk set to result in the failure of the whole system. In my diagram that would be one drive from set A and one drive from set B. In RAID 1+0, you have to lose all drives in a mirror. This would be both drives in any numbered pair in the diagram.

Mathematically, the difference is that the chance of system failure with two drive failures in a RAID 0+1 system with two sets of drives is n/(2n-2) where n is the total number of drives in the system. The chance of system failure in a RAID 1+0 system with two drives per mirror is 1/(n-1). So, using the 8 drive systems shown in the diagrams, the chance that losing two drives would bring down the RAID system is 4/7 with a RAID 0+1 system and 1/7 with a RAID 1+0 system.

The math gets more complicated when you have more than two elements to a mirror. Since that’s a rare configuration, I haven’t bothered to figure out the equations. If someone else would like to, I’ll be glad to post them here.

Another difference between the two RAID configurations is performance when the system is in a degraded state, i.e. after it has lost one or more drives but has not lost the right combination of drives to completely fail. In a RAID 0+1 configuration, the loss of any drive in a set causes the failure of that entire set and the set is removed from the RAID system. Generally (in the two set case) this means you are left with a RAID 0 system made up of the remaining set of disks. This probably slightly improves write performance and slightly degrades read performance (but that’s just a WAG, I haven’t done any testing). In a RAID 1+0 system, you would see the same effect on each mirror that loses a drive, but not the whole system. In other words, a RAID 1+0 configuration will tend to show similar, but less dramatic, changes in performance when in a degraded mode than RAID 0+1. However, the changes will likely be slight in any case.

One more difference that was recently pointed out to me is the speed at which the RAID system recovers once the failed disk is replaced. RAID 1+0 only has to re-mirror one drive, whereas RAID 0+1 has to re-mirror the entire failed set. So RAID 1+0 will recover significantly faster.

So, what it seems like to me is if you have the cash, and at least 4 drives, and your platform, motherboard or OS support it, RAID 1+0 seems like a good winner to me. However, there are different RAID configs out there as can be seen on the AC&NC website that I mentioned earlier, that may better suit your needs.

One Response to “What’s better, RAID 1+0 or RAID 0+1?”

Leave a Reply

You must be logged in to post a comment.