Swapping a Linux Software RAID Drive

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These are instructions on how I swap out a RAID disk. I use Linux md software RAID1.

Informational commands

These commands gather information on the state of the system.

  • cat /proc/mdstat: Shows the status of all md arrays and any updates.
  • lshw -short -class disk: Shows all attached disks with model numbers and device names (e.g., /dev/sda1). Omit -short to see more information, including serial numbers.
  • smartctl -i [device]: Show SMART information on a disk device.

Removing the Old Disk

We first need to instruct md that the old disk is to be removed. The following commands are useful, where [array] is the name of the md array (e.g., /dev/md#), [partition] is the partition that underlies the array (e.g., /dev/sda1), and [device] is the name of the device (e.g., /dev/sda).

  • mdadm [array] --fail [partition]: “Fail” the device from the md array in preparation for removing it.
  • mdadm [array] --remove [partition]: Remove the device from the md array; the device must be failed first.
  • mdadm --grow [array] --raid-devices=[#]: Reconfigure the RAID array to expect the given number of devices. (Despite the name, this command can also shrink an array.) An array “expects” a number of devices to exist and is also associated with a number of actually present devices. These two numbers need not match up, and md will reconcile any difference either by designating extra devices as “spares” or by leaving blank slots for missing devices (designating the array as degraded).

My procedure for removing a drive was: (1) unmount the filesystem to avoid changes; (2) fail and remove the partition using mdadm; and (3) shut down the computer, physically pull out the disk, and install the new one. Upon starting the computer up again, cat /proc/mdstat will show a degraded array.

Perhaps a better procedure, if extra SATA slots are available, would be to install the new disk first, grow the array for the new disk, set up the new disk, remove the old disk, and then shrink the array.

Setting Up a New Disk

The following are steps for setting up a new disk.

  • Find the device with lshw.
  • Check the new disk: smartctl -t short [device] and then smartctl -a [device] to view the results
  • Partition the device with parted. It is possible to make an md array with the raw disk, but many people seem to recommend partitioning it. I use the following settings: mklabel gpt; mkpart from 0% to 100%; set the partition's raid flag to on.
  • mdadm [array] --add [partition] to add the device.
  • Mount the array onto the filesystem.