Hi there.

I'm working on a server and noticed that the to RAID5 setup is showing 4 Raid devices but only 3 Total devices. It's on a fully updated CentOS 5 system that only has three SATA drives, as it can not hold anymore. I've done some researching but am unable to remove the fourth device, which is listed as removed. The full output of `mdadm -D /dev/md2` can be see below. I've never run into this situation before.

Anyone have any pointers on how I can reduced the Raid Devices from 4 to 3? I have tried

mdadm /dev/md2 -r failed
mdadm /dev/md2 -r detached

but neither work and since there is no block device listed I'm not quite sure how to get things back in sync so it's only seeing the three drives. Any tips or help is greatly appreciated.

Thanks all.

~Dan

/dev/md2:
Version : 0.90
Creation Time : Tue May 25 11:07:04 2010
Raid Level : raid5
Array Size : 2800803840 (2671.05 GiB 2868.02 GB)
Used Dev Size : 933601280 (890.35 GiB 956.01 GB)
Raid Devices : 4
Total Devices : 3
Preferred Minor : 2
Persistence : Superblock is persistent

Update Time : Tue Dec 7 08:28:38 2010
State : clean, degraded
Active Devices : 3
Working Devices : 3
Failed Devices : 0
Spare Devices : 0

Layout : left-symmetric
Chunk Size : 256K

UUID : e65047be:98023718:ba85fc6d:d9fd475c
Events : 0.1165610

Number Major Minor RaidDevice State
0 8 5 0 active sync /dev/sda5
1 8 21 1 active sync /dev/sdb5
2 8 37 2 active sync /dev/sdc5
3 0 0 3 removed

Hi Dan,

You might want to try:

mdadm /dev/md2 --fail detached --remove detached

As per the man page in mdadm: Any devices which are components of /dev/md2 will be marked as faulty and then remove from the array.

It seems that at some point there was a 4th device added to this array, I've never reduced an array, as per mdadm man page you can do it using --grow and --array-size:

When decreasing the number of devices, the size of the array will also decrease. If there was data in the array, it could get destroyed and this is not reversible. To help prevent accidents, mdadm requires that the size of the array be decreased first with mdadm --grow --array-size. This is a reversible change which simply makes the end of the array inaccessible. The integrity of any data can then be checked before the non-reversible reduction in the number of devices is request.


Can you check your /etc/mdadm.conf file to see if it shows the extra device in the configuration.
Before looking into those options I would have a full backup just in case.

Well, turns out the motherboard failed. Performed a reboot at one point and it failed to POST. Thanks for your input, axelsallright.