I find mdadm --detail <device> very useful when I need to know what is going on with the device (/proc/mdadm too). Example: /dev/md0: Version : 1.2 Creation Time : Mon Mar 4 ...
I've read that BTRFS requires at least 4 disks (https://btrfs.wiki.kernel.org/index.php/Using_Btrfs_with_Multiple_Devices). Indeed, Linux mdadm raid10 requires only 2 disks ...
I have a RAID10 managed by the mdadm and I have EXT4 filesystem on top of it. However, I like BTRFS and would like to convert the EXT4 filesystem to BTRFS, but I was thinking about performance and ...