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Linux RAID One-Liners

Here are some useful one-liner recipes to help manage Linux Device Mapper (sometimes also known as ‘software RAID’) devices. These are too short to warrant separate recipes. They're here because I need them for myself: no matter how many times I've done this, I always check again.

Disclaimera good craftsman knows their tools. These particular tools are powerful but highly dangerous. If you rely on these recipes with no knowledge of what they're doing, you will lose data, your job, your street cred, et cetera. Consider yourself warned.

Create MD With Devices Initially Missing

mdadm --create /dev/md0 --level=1 --raid-devices=2 /dev/sdb3 missing

That is, add the missing keyword in place of any and all missing parts of the array.

Change /dev/md0 to the RAID device name you wish to create, and /dev/sdb3 to the partition you wish to add to the newly created array. This will obviously be razed in the process.

Re-Insert a Previously Removed Device

Sometimes a disk is dropped from an array even though it's healthy. There are good reasons for this, such as a dead host adaptor or a failed power supply on an external disk enclosure. When connectivity is restored, or should you wish to force the disk to be re-inserted, say this:

mdadm --manage /dev/md0 --re-add /dev/sda3

The disk must have previously been part of the array, of course. This is preferable to the next recipe, which registers the disk as an altogether new device.

Hot-add New Devices to an MD Array

mdadm --manage /dev/md0 --add /dev/sda3

In this way, a RAID array may be assembled piecemeal, and the system allowed to synchronise it.

View Status Detail on an MD Array

mdadm --misc -D /dev/md0

Displays essential information on the RAID array in question.

cat /proc/mdstats

Displays less information in a more compact form (but includes more rebuild details).