How to resize two RAID-1 partitions?

Question

I have an Ubuntu 12.04 server with two 3TB disks and Software-RAID 1. Currently they are partitioned as follows:

# discus Mount Total Used Avail Prcnt Graph / 1.00 TB 8.55 GB 0.99 TB 0.8% [----------] +onnections 0 KB 0 KB 0 KB 0.0% [----------] +rnel/debug 0 KB 0 KB 0 KB 0.0% [----------] +l/security 0 KB 0 KB 0 KB 0.0% [----------] /run 3.12 GB 304 KB 3.12 GB 0.0% [----------] /run/lock 5.0 MB 0 KB 5.0 MB 0.0% [----------] /run/shm 7.79 GB 0 KB 7.79 GB 0.0% [----------] /home 1.70 TB 13.06 GB 1.68 TB 0.8% [----------] /boot 495.8 MB 38.0 MB 457.9 MB 7.7% [*---------] # df -h Filesystem Size Used Avail Use% Mounted on /dev/md2 1.0T 8.6G 964G 1% / udev 7.8G 4.0K 7.8G 1% /dev tmpfs 3.2G 304K 3.2G 1% /run none 5.0M 0 5.0M 0% /run/lock none 7.8G 0 7.8G 0% /run/shm /dev/md3 1.7T 14G 1.6T 1% /home /dev/md1 496M 38M 433M 9% /boot # cat /proc/partitions major minor #blocks name 8 0 2930266584 sda 8 1 33554432 sda1 8 2 524288 sda2 8 3 1073741824 sda3 8 4 1822443975 sda4 8 5 1024 sda5 8 16 2930266584 sdb 8 17 33554432 sdb1 8 18 524288 sdb2 8 19 1073741824 sdb3 8 20 1822443975 sdb4 8 21 1024 sdb5 9 0 33553336 md0 9 1 524276 md1 9 2 1073740664 md2 9 3 1822442815 md3 

I would like to reduce / to 100Gb and grow /home to 2.6Tb. What's the best way to do that? Server has no important data so I don't care about possible data loss.

Answer 1

The trick is using your second drive in RAID as spare one:

1. Break the mirror(s) you have (except /boot one, if you have it);
2. Create one big single partition for RAID on the disengaged drive (in addition to /boot's);
3. Create RAID-1 using missing device (see man mdadm for that) and the partition;
4. Make new mirror to be LVM-2 disk (pvcreate);
5. Create volume group (vgcreate) and needed logical volumes (lvcreate), format them, mount temporary;
6. Copy your stuff;
7. Correct new /etc/fstab, make sure initrd would handle root filesystem on LVM-2 ok;
8. Boot in;
9. Stop old (intentionally broken) RAID;
10. Partition its disk as replica of disengaged drive;
11. Add it to new RAID so it would be based on missing drive no more.