Can RAID 1 have more than two drives?

Question

Recently I had a discussion with a teacher of mine. He was claiming that you could set up RAID 1 with five drives and that the data would be mirrored over all of these drives.

I told him a RAID 1 with 5 drives wouldn't work like that. It would be a RAID 1 with two drives and would use the other three drives as hot spare.

He also said that RAID 6 is identical to RAID 5 but you can place all the parity checks on the same drive. I thought RAID 6 was a RAID 5-like solution where two drives where used for parity.

Who's right, then?

Answer 1

You can use as many drives as you want for RAID1. They will all be mirrored, and written on at the same time, and be exact copies of each other. The fact that there isn't a card that do more than x drives doesn't meant anything about the concept. RAID1 is just mirroring your disks, and you can have as many mirrors as you want.

Also, your view of RAID5/6 is erroneous. The parity is distributed on all the drives, there isn't a dedicated drive for that. Compared to raid5, raid6 adds an additional parity block, which is also distributed.

You can find more info on wikipedia.

Answer 2

I've worked with some LenovoEMC PX4-something NAS which had 4 or 12 disks. The first 50 GB of each drive was used as a raid1 for the OS, and the rest of each disk was for user data.

So it has a 4 or 12-way raid1 for the root drive, and a small swap file on this drive. So yes its totally possible and workable, and used in production by commercial solutions.

As long as at least one disk still worked then it would boot and network. The NAS needed to boot off a USB drive if you changed all the disks, to reinstall the base OS.

Here's the 4 bay NAS rebuilding after a disk swap, so no sdd

root@px4-300r-THYAQ42E9:/nfs/# cat /proc/mdstat
Personalities : [linear] [raid0] [raid1] [raid10] [raid6] [raid5] [raid4]
md0 : active raid1 sde1[4] sdc1[1] sda1[3] sdb1[2]
      20964480 blocks super 1.1 [4/3] [UUU_]
      [===========>.........]  recovery = 58.1% (12188416/20964480) finish=7.2min speed=21337K/sec

md1 : active raid5 sde2[4] sdc2[1] sda2[3] sdb2[2]
      5797200384 blocks super 1.1 level 5, 512k chunk, algorithm 2 [4/3] [_UUU]

Answer 3

There is a lot of misunderstanding of RAID levels.

JBoD is Just a Bunch of Drives, where you can see multiple drives in the same box, this is a most confused non-raid term.

Years ago, some RAID manufacturers could not make a truly JBOD with their RAID engine, they call SPAN (BIG) as JBoD.

RAID1 is a Mirror RAID and it needs TWO HDDs to mirror each other. Whereas CLONE is a Multiple Duplicate HDD with the same volume, for example DAT Optic's eBOX, sBOX (hardware RAID). Hardware RAID boxes generally offer RAID 0, 1, 5, CLONE, Large, and Hot spare.

As for RAID 5/6, both have the parity space portion equal to one drive for RAID5 and two drives for RAID6.

The most common mistaken knowledge is that parity data is located in a dedicated drive(s). That is incorrect. The party space is divided equally among the RAID member HDDs.

Example: RAID5 from five HDD, each of the drives will have 1/5 of space allocated for parity, whereas for RAID6, each drive will have 2/5 of space allocated for parity.

For those who want to argue, if there is a dedicated parity drive(s), let's assume there is, what happens to the RAID if the dedicated parity drive fails? The RAID can not be rebuilt because the data needed to rebuild is no longer there.

Answer 4

There are two possibilities:

use

• all 5 drives for the raid 1, with every drive as exact copy of other drives
• mirror (example) 3 drives and use the other two disks as spare (if one of the first 3 disks fails, the 4th will take his place)

I prefer the 2nd solution (with 2+1 drives or 3+1)

your assumption about raid 6 is wrong :)