Tag Archives: mount

LVM – How to


LVM is a very powerful technology, and can really help the Sysadmin’s life.
However, this is something that we generally setup at the beginning (most of the time now it’s automatically setup during the installation process), and it’s well know… when we stop using something, we tend to forget how to use it.

This is why I’m writing this how to, mostly to keep track of the major features and commands, in case I will need them again in the future ūüėČ

Before proceeding, please digest the following journey of this poor physical device that gets abstracted up to usable pieces.



Prepare partions

First of all, we need to find which device(s) we want to setup for LVM

fdisk -l

We can see 3 md devices, probably RAID devices. These are the ones that we are going to use for our LVM exercise.

Now, let’s create an LVM partition.

fdisk <device> => n , p , 1 , (enter) , (enter) , t , 8e , w

Do the same for all the devices that you want to use for LVM. In my example, I’ve done this for /dev/md1, /dev/md2 and /dev/md3.

Shortcut (risky but quicker) ūüôā

All seems now good to go: we have Linux LVM partitions!

Time to start to configure LVM

Configure LVM

First of all, we need to make these Linux LVM partition able to be part of a group (vg). I always find tricky to remember the logic behind. Let’s imagine that the device itself now is just labelled “Linux LVM” but we need to¬†initiate¬†it in somehow.

pvcreate <dev>

Now these guys are ready to be part of a group. In this case a Virtual Group (vg).
Let’s check that it’s actually true:

Time to create a group with these devices (this could be done also with just a single device):

vgcreate <lvmgroupname> <dev> <dev> …

Now, let’s check again with pvs and vgs

Now pvs shows the VG group no longer empty but with mylvmvg. And vgs tells us that the VG is about 14GB in size, fully free with no LV in it.

Good! Now, let’s make some LVs (logical volumes). These will be the new¬†“partitions/disks” that we will be actually able to format, mount and use! ūüôā

lvcreate -n <name> -L xGB <vg_group_name>

Some checks to verify:

A new LV appears in vgs and lvs shows the 2GB volume that we have created.

Let’s create another one, but this time, using the full remaining space (using -l 100%VG¬†option instead of -L xGB)


Now, we have two devices, both ‘a’ -> active and ready to be formatted:
mkfs.ext4 <device>

I’ve done this for /dev/mylvmvg/part1 and /dev/mylvmvg/part2.

Let’s create the mount points and mount them:

As you can see, the devices are appearing now as /dev/mapper/mylvmvg-partX. You can use either /dev/mylvmvg/partX or /dev/mapper/mylvmvg-partX. Theoretically, the mapper one is recommended (my bad!).

Now the 2 devices are ready to be used as a typical disk/partition formatted with ext4 filesystem.

Resize Logical Volume

Now, imagine that¬†part1 is too small, and you need more space. And luckily, your¬†part2 volume has plenty. Is there any way to “steal” some space from¬†part2¬†and give it to¬†part1?
Ooohh yesss! ūüôā


  1. shrink part2 logical volume AND its filesystem
  2. expand part1 logical volume AND its filesystem

Here the comments inline:


Move logical volume onto a new RAID array

Now, let’s imagine that one of the 3 initial¬†md devices are having problems, or simply we want to move on a faster/bigger raid array.
The magic of LVM is that we can actually do this with NO DOWNTIME!


In this example we assume that a new /dev/md10 device is attached to our server and we want to remove /dev/md2 device.

  1. We need to take the new device and go through all the previous steps:
    1. fdisk
    2. pvcreate
  2. After that, we need to add this initialised device in the existing volume group (vg)
  3. Move whatever is stored on the physical device
  4. Shrink the volume group
  5. Remove the device

Now where the new bits are starting:
pvmove, vgreduce, pvremove


In this example we have left LVM to decide where to put the data that was stored on /dev/md2 device.
Just for reference, we could have specified the destination physical device (e.g. if we were thinking to remove more devices and make sure that the data was ending up on the new RAID and not sprat across the other disks):

Or, in case we just wanted to move a specific logical volume, let’s say¬†part1


…happy LVM’ing! ūüėČ

Mac /Users partition on RAID1

This is what I’ve created:

  • Fresh new installation of Mac OS X Lion on a new 128SSD hard drive (this should work with Mavericks too);
  • Create a RAID1 with the 2 TB disks, and mounted in /Users;
  • Restored apps/confs/data from Time Machine using¬†Migration Assistant.

Create RAID1

This is quite handy to have the user’s data safe.
I’ve simply created a RAID1 using¬†Disk Utility¬†and set this new device to auto mount in /Users folder ūüėČ
Call the new device “Users“. I will use this name as reference in this guide.
Simply drop the hard drives in the Raid section and select¬†Mirrored Raid Set¬†–¬†Mac OS Extended (journaled)
Once created, CMD+I on the new device and take note of Universal Unique Identifier the famous UUID

It might be interesting to follow this guide to create an encrypted logical volume on the RAID partition to save your data ūüėČ

How to mount this new drive in /Users?

First of all you need to enable “root” user

Very easy… open¬†Terminal, got superuser privilegies (sudo su -¬†command) and once the prompt shows “#“, type¬†passwd command.
Set a new password (it will ask to insert it twice).
Now, go to Users and Group and in Login Options select Show Input menu in login window (this can be rolled back once finished).
Logout (better if you do a full reboot).
Once the OS is up again you will have an “old fashion” login window, where you need to enter username and password.
Insert¬†root¬†as username and the password is the one you’ve just set via Terminal: you will login as super user.

Second, copy the current data

Now… back to the Terminal!
Open it (and you will be already root user!¬†), have handy the UUID you’ve saved before and digit the following commands:

Now you need to press SHIFT+g
Then press o
Then insert the following, of course replacing the XXX with your UUID

Once finished, to save press these in sequence esc : wq

For more info check any vim editor guide.

What we’ve done is coping the full content of /Users in our new RAID device (called “Users” during the RAID setup).
Created a copy on the local drive as well (just to stay from the safe side ūüėõ )
Removed all the content of /Users and make it ready to get the new RAID1 device mounted inside (sounds a bit xxx but it’s not ūüėÄ )
Edited /etc/fstab to auto mount the RAID1 device automatically at the boot.

Third… reboot and login

Now it’s all ready for the reboot and login with your normal user!
Once confirmed that all works, you can safely remove Users.orig folder and restore the original login window.

Forth (optional): restore from Time Machine

Now you can finally restore your data manually using Time Machine.
Migration Assistant will probably fail if the amount of data is bigger than the size of your main OS disks. The reason is that It’s not able to understand that there are extra 2TB of space under Users.
What I’ve done is restoring ALL my OS except¬†the users, using Migration Assistant, and then, I’ve used Time machine and copied manually all the folders inside /Users … and all worked! ūüėČ