I've blogged about using md with lvm before. This weekend I came to appreciate it even more when I migrated a live system from single disk to RAID-1 without needing to unmount or reboot. Here's the overview of the steps I used:
- Create a RAID-1 on the second disk (sdb). It's initially degraded, meaning that even though we're accessing it as md0, there's only one disk in the array.
- Migrate data from the first disk (sda) to the newly-created md0.
- Now that sda is freed up, add it to the array to make it a mirror.
Reviewing my system inventory, I have two 500G disks but I'm only using one of them, hosting logical volumes for root, swap, four nfsroots, and two (running) KVM virtual machines. Apart from the large partition holding the logical volumes, I also have a couple small partitions for Windows 7 (I don't actually use it but it's there in case I need to call for warranty support). The other disk is blank and idle. I installed it some time back when I had the machine powered off, though I could have attached it with eSATA to avoid even that interruption.
Here's the partition table from that first disk:
$ sudo parted /dev/sda (parted) unit kB (parted) print Model: ATA WDC WD5000AAKS-6 (scsi) Disk /dev/sda: 500107862kB Sector size (logical/physical): 512B/512B Partition Table: msdos Number Start End Size Type File system Flags 1 1049kB 6443500kB 6442451kB primary ntfs boot 2 6443500kB 46443500kB 40000000kB primary ntfs 3 46448156kB 47443415kB 995259kB primary ext3 4 47443415kB 500105249kB 452661834kB extended 5 47443447kB 500105249kB 452661802kB logical lvm
and here's the volume groups and logical volumes, annotated a bit:
$ sudo vgs VG #PV #LV #SN Attr VSize VFree vg0 1 10 0 wz--n- 421.57G 250.40G $ sudo lvs LV VG Attr LSize jabberwock vg0 -wi-ao 9.31G # host root filesystem swap vg0 -wi-ao 1.86G # host swap jubjub vg0 -wi-ao 20.00G # jubjub VM disk agriffis vg0 -wi-ao 50.00G # my $HOME, jubjub VM second disk oliva vg0 -wi-a- 20.00G # oliva VM disk amg vg0 -wi-a- 50.00G # Amy's $HOME, oliva VM second disk nord vg0 -wi-ao 5.00G # thin client nfsroot sud vg0 -wi-ao 5.00G # thin client nfsroot parmigiano vg0 -wi-ao 5.00G # thin client nfsroot romano vg0 -wi-ao 5.00G # thin client nfsroot
For the partition listing, I used units of kB because that makes it easier to be sure the partitions are exactly the same size when I apply them to the second disk. I'll initially create the RAID-1 with only sdb5 (a degraded array), then later I'll add sda5 to md0 to make it a mirror. For this reason, it's important that the two partitions that will eventually make up md0 are exactly the same size.
Here's the application of that partition table to the second disk. Note I'm not going to actually use the first few partitions there, but I create them anyway for the sake of symmetry:
$ sudo parted /dev/sdb (parted) unit kB (parted) mklabel msdos (parted) mkpart primary 1049kB 6443500kB (parted) mkpart primary 6443500kB 46443500kB (parted) mkpart primary 46448156kB 47443415kB (parted) mkpart extended 47443415kB 500105249kB (parted) mkpart logical 47443447kB 500105249kB (parted) set 5 raid on (parted) print Model: ATA ST3500418AS (scsi) Disk /dev/sdb: 500107862kB Sector size (logical/physical): 512B/512B Partition Table: msdos Number Start End Size Type File system Flags 1 1049kB 6443500kB 6442451kB primary 2 6443500kB 46443500kB 40000000kB primary 3 46448156kB 47443415kB 995259kB primary 4 47443415kB 500105249kB 452661834kB extended lba 5 47443447kB 500105249kB 452661802kB logical raid
Now create the RAID-1 and extend the volume group to it:
$ sudo mdadm --create /dev/md0 --level=1 --raid-devices=2 /dev/sdb5 missing $ sudo pvcreate /dev/md0 $ sudo vgextend vg0 /dev/md0
Move the data hosted on sda5 to md0. Note pvmove reports an error, but when I try to continue the operation, it reports that it's already done, so I don't think this is a real problem:
$ sudo pvmove /dev/sda5 /dev/md0 /dev/sda5: Moved: 99.9% ABORTING: Can't find mirror LV in vg0 for /dev/sda5 $ sudo pvmove /dev/sda5 /dev/md0 No data to move for vg0
Now remove sda5 from the volume group and add it to md0:
$ sudo vgreduce vg0 /dev/sda5 $ sudo pvremove /dev/sda5 $ sudo parted /dev/sda set 5 lvm off set 5 raid on $ sudo mdadm /dev/md0 --add /dev/sda5
And see that it's working:
$ cat /proc/mdstat
Personalities : [raid1]
md0 : active raid1 sda5[2] sdb5[0]
442052416 blocks [2/1] [U_]
[>....................] recovery = 0.0% (288512/442052416)
finish=153.1min speed=48085K/sec
Finally, add the info to the mdadm config and regenerate the initrd to make sure md0 is found when the system eventually reboots. I found that I had to remove the "metadata" tag from the generated config line, YMMV:
$ sudo mdadm --detail --scan | sudo tee -a /etc/mdadm/mdadm.conf ARRAY /dev/md0 level=raid1 num-devices=2 metadata=00.90 UUID=4aa66439:62c76598:cb06150d:d44aeb51 $ sudo vim /etc/mdadm/mdadm.conf # to remove "metadata=00.90" $ sudo update-initramfs -u -k all
All this without any interruption to the system operation!
