My goal is to shrink an mdadm RAID 1 partition while the system is running. I have to disks sda and sdb, which build my two RAID partitions md0 and md1. On md1 I use LVM, md0 is the /boot partition. I already reduced the size of the LVM physical volume (and all its logical volumes/file systems) AND the md1 without a problem. But the actual partitions sda2 and sdb2 are not updated automatically. Can I just fdisk them while the system is running? Maybe reboot afterwards? Will that suffice? Or do I have to take the system offline? Are there other ways to shrink the partitions?
EDIT: I just tried it, it didn't work. After changing the partition sizes, the machine refused to reboot, giving me just a black screen with a blinking line. I changed the partitions back using a Live CD. Now it's booting again. Can anyone tell me what the heck I'm doing wrong?
EDIT 2: By the way, I used
mdadm --grow /dev/md1 --size=<smaller size> to reduce the size of md1.