In this tutorial, just right after adding physically 2 new HDD to a server (/dev/sdc and /dev/sdd), we will create a new /dev/md2 RAID1 software device and a new volume group (VG) running over it.
1 Partition the hard drives
I like using cfdisk, and I used it to create sdc1 and sdd1. For example:
cfdisk /dev/sdc
Then go in "NEW", add a partition of the size of the HDD minus a reasonable 5 GB (because if it fails, and the replacement HDD is just few megs smaller, you'll be fucked...). Like, if you have a 1.5 TB hdd, then make a partition of 1495 GB, and leave 5 GB of free space.
Then go in "TYPE" and select "linux raid autodetect" (type FD).
Redo the work with cfdisk /dev/sdd
2 Create a new RAID1 devices
With mdadm, something like this will do:
mdadm --create /dev/md2 --level=1 --raid-devices=2 /dev/sdc1 /dev/sdd1
As this doesn't start the resync, I remove then re-add one of the partitions:
mdadm /dev/md2 -f /dev/sdc1
mdadm /dev/md2 --remove /dev/sdc1
mdadm /dev/md2 --add /dev/sdc1
then I add the UUID of /dev/md2. To see it, I do:
mdadm --detail /dev/md2
then I add the below line in /etc/mdadm/mdadm.conf:
ARRAY /dev/md2 level=raid1 num-devices=2 UUID=<UUID that you just saw with mdadm --detail /dev/md2>
so that at next reboot it the RAID1 device is started automatically. You can check the state of the RAID1 device that is now rebuilding using:
cat /proc/mdstats
3 Creating a new volume group (VG) over the new RAID1
Everything here is taken from what's written here: http://tldp.org/HOWTO/LVM-HOWTO/commontask.html(approve sites) just browse it, it's a very good documentation.
We start by initializing the PV on /dev/md2:
pvcreate /dev/md2
In return the system will tell:
Physical volume "/dev/md2" successfully created
Then we create the volume group:
vgcreate vgcreate HOSTNAMEvg1 /dev/md2
It's my administrator habits to prefix each vg name with the hostname, to make sure all of my servers have unique vgnames (which is more easy for maintenance). The system will in return tell:
Volume group "HOSTNAMEvg1" successfully created
which you can check with:
vgdisplay
You can now create new partitions on it:
lvcreate -L20G -ndata HOSTNAMEvg1
will create a 20G partition on /dev/mapper/HOSTNAMEvg1-data. There's normally nothing to do in the lvm.conf, and at next reboot the volume group will be activated.
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