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(I've posted this on Serverfault as I suspect it a bit to technical for Superuser. If I'm wrong please move it where appropriate)

I am remotely looking at a device which identifies as ReadyNAS ProUltra2 V1.4. It appears this device has 2 disks in it and runs a variant of Debian. I have noticed something concerning about the RAID array - It reports as follows

ReadyNAS:~# cat /proc/mdstat 
Personalities : [raid0] [raid1] [raid10] [raid6] [raid5] [raid4] 
md2 : active raid5 sda3[0] sdb3[1]
      972040704 blocks super 1.2 level 5, 64k chunk, algorithm 2 [2/2] [UU]

md1 : active raid1 sda2[0] sdb2[1]
      524276 blocks super 1.2 [2/2] [UU]

md0 : active raid1 sda1[0] sdb1[1]
      4193268 blocks super 1.2 [2/2] [UU]

Of concern is that md2 (which appears to be most of the space) is configured as RAID5 but with only 2 disks. From doing some reading I understood that mdadm requires a minimum of 3 disks for redundancy in a RAID5 configuration. I have no idea if this understanding is wrong, if the ReadyNAS is doing something non-standard or if this was incorrectly set up by whoever set it up - although I don't know why RAID5 would be enabled on a box like this one appears to be, and the person who set it up must have used a gui based setup process of some sort as they are not technical.

Anyone know if MD2 actually has redundancy or how this setup may have come into being ?

share|improve this question
RAID5 with two disk is possible and has the advantage that it will allow you to expand the capacity by adding a single additional drive without having to do multiple steps, which is easy on the GUI.... It is a lot more inefficient then RAID1 which is why normally you don't want to do that. – HBruijn Mar 7 '14 at 9:07
It also might identify it as RAID 5, but it might be Netgear's X-RAID2. Which like @HBruijn said, allows for volume expansion on the fly. – MikeAWood Mar 8 '14 at 1:16
up vote 3 down vote accepted

I discovered my answer here. - "With mdadm, a 2 drive RAID 5 is binary identical to a RAID1, not RAID 0"

share|improve this answer
If that's the concise answer, accept your own answer :) – Basil Mar 8 '14 at 3:26
This is the correct answer, as I have learned many times with dealing with similar situations left over by previous admins. Here's hoping he accepts when he has the chance, I don't believe the timer has run out. For anyone curious, there's another possibility when encountering a 2-disk Raid5 via MDADM, which is a "3 disk" array operating in "degraded" mode. Because of how the raid is built in the software, it initially appears as two active disks and one inactive disk, if the inactive disk is removed, it can sometimes appear as two active disks, despite operating in degraded mode. – George Spiceland Mar 8 '14 at 5:11

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