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Rebuilding Failed Linux Software RAID

A.P. Lawrence
Expert Author
Published: 2004-10-22

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Recently I had a hard drive fail. It was part of a Linux software RAID 1 (mirrored drives), so we lost no data, and just needed to replace hardware. However, the raid does requires rebuilding. A hardware array would usually automatically rebuild upon drive replacement, but this needed some help.

When you look at a "normal" array, you see something like this:

That's the normal state - what you want it to look like. When a drive has failed and been replaced, it looks like this:

Notice that it doesn't list the failed drive parts, and that an underscore appears beside each U. This shows that only one drive is active in these arrays - we have no mirror.

Another command that will show us the state of the raid drives is "mdadm"

As this shows, we presently only have one drive in the array.

Although I already knew that /dev/hdb was the other part of the raid array, you can look at /etc/raidtab to see how the raid was defined:

To get the mirrored drives working properly again, we need to run fdisk to see what partitions are on the working drive:

Duplicate that on /dev/hdb. Use "n" to create the parttions, and "t" to change their type to "fd" to match. Once this is done, use "raidhotadd":

The rebuilding can be seen in /proc/mdstat:

The md0, a small array, has already completed rebuilding (UU), while md1 has only begun. After it finishes, it will show:

I was a little surprised that this process wasn't entirely automatic. There's no reason it couldn't be. This is an older Linux install; I don't know if more modern versions will just automatically rebuild.

*Originally published at http://www.aplawrence.com

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About the Author:
A.P. Lawrence provides SCO Unix and Linux consulting services http://www.pcunix.com

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