Linux – How to Convert RAID1 to RAID0

You can force a check of (eg) md0 with

echo "check" > /sys/block/md0/md/sync_action

You can check the state of the test with

cat /sys/block/md0/md/sync_action

while it returns check the check is running, once it returns idle you can do a

cat /sys/block/$dev/md/mismatch_cnt

to see if the mismatch count is zero or not. Many distros automate this check to run eg weekly for you anyway, just as most industrial hardware RAIDs continually run this in the background (they often call it "RAID scrubbing") while the array is otherwise idle. Note that according to the comments in fedora's automated check file, RAID1 writes in the kernel are unbuffered and therefore mismatch counts can be non-zero even for a healthy array if the array is mounted.

So quiescing the arrays by doing this check while the VM is down, if at all possible, is probably a good idea.

I'd add that I agree with the docs when they say that

RAID cannot and is not supposed to guard against data corruption on the media

RAID is supposed to guard against complete failure of a device; guarding against incremental random failures in elements of a storage device is a job for error-checking and block-remapping, which is probably best done in the controller itself. I'm happy that the docs warn people of the limitations of RAID, especially if it's implemented on top of flaky devices. I find that frequent smartctl health checks of my drives help me to stay on top of drives which are starting to show the sort of errors that lead to out-of-sync mirrors.

Yes, you need to start the array in degraded mode using sdc drive. Then, you can re-add the failed drive sdd.

I usually use the command mdadm with option --assemble to assemble and start the array in degraded mode using one disk only. Something like this:

$ mdadm --assemble /dev/md1 /dev/sda2

You may need to use -f option to force starting degraded array.

If that works fine, you can proceed by re-adding the replaced drive.

$ mdadm --re-add /dev/md1 /dev/sdb2

I used these commands many times before and did not experience any data loss. They worked smoothly when the drives are OK.

Precaution: Please backup your disks before running such commands to avoid any possible data loss.