LVM

LVM (Logical Volume Manager) is a software which uses physical devices abstract as PVs (Physical Volumes) in storage pools called VG (Volume Group). Whereas physical volumes could be a partition, whole hard drives grouped as JBOD (Just a Bunch Of Disks) or complete RAID systems.

Installation
First we need to enable the device mapper in our kernel:

is available on Gentoo Linux and supports the following useflags: You can install LVM2 using the following command:
 * clvm = Allow users to build clustered lvm2
 * cman = Cman support for clustered lvm
 * lvm1 = Allow users to build lvm2 with lvm1 support
 * static = Install a statically linked lvm2, useful for e.g. initramfs
 * readline = Enables support for libreadline, a GNU line-editing library that almost everyone wants
 * selinux = Enable Security Enhanced Linux (SELinux) support
 * static-libs = Install static libraries

The package comes with a start-stop-daemon, which we start and add to the boot runlevel:

The configuration files are located in Configuration Files

Usage
LVM organizes storage in three different levels as follows:
 * hard drives, partitions, RAID systems or other means of storage are initialized as PV (Physical Volume)
 * Physical Volumes (PV) are grouped together in Volume Groups (VG)
 * Logical Volumes (LV) are managed in Volume Groups (VG)

PV (Physical Volume)
Physical Volumes are the actual hardware or storage system LVM builds up upon.

Partitioning
The partition type for LVM is 8e(Linux LVM):

In fdisk, you can create MBR partitions using the n key and then change the partition type with the t key to 8e. We will end up with one primary partition /dev/sdX1 of partition type 8e (Linux LVM).

Create PV
The following command creates a Physical Volume (PV) on the two first primary partitions of /dev/sdX and /dev/sdY:

List PV
The folloing command lists all active Physical Volumes (PV) in the system:

You can scan for PV in the system, to troubleshoot not properly initialized or lost storage devices:

Remove PV
LVM automatically distributed the data onto all available PV, if not told otherwise. To make sure there is no data left on our device before we remove it, use the following command:

This might take a long time and once finished, there should be no data left on /dev/sdX1. We first remove the PV from our Volume Group (VG) and then the actual PV:

VG (Volume Group)
Volume Groups (VG) consist of one or more Physical Volumes (PV) and show up as /dev// in the device file system.

Create VG
The following command creates a Volume Group (VG) named vg0 on two previously initialized Physical Volumes (PV) named /dev/sdX1 and /dev/sdY1:

List VG
The folloing command lists all active Volume Groups (VG) in the system:

You can scan for VG in the system, to troubleshoot not properly created or lost VGs:

Extend VG
With the following command, we extend the exisiting Volume Group (VG) vg0 onto the Physical Volume (PV) /dev/sdZ1:

Reduce VG
Before we can remove a Physical Volume (PV), we need to make sure that LVM has no data left on the device. To move all data off that PV and distribute it onto the other available, use the following command:

This might take a while and once finished, we can remove the PV from our VG:

Remove VG
Before we can remove a Volume Group (VG), we have to remove all existing Snapshots, all Logical Volumes (LV) and all Physical Volumes (PV) but one. The following command removes the VG named vg0:

LV (Logical Volume)
Logical Volumes (LV) are created and managed in Volume Groups (VG), once created they show up as /dev// and can be used like normal partitions.

Create LV
With the following command, we create a Logical Volume (LV) named lvol1 in Volume Group (VG) vg0 with a size of 150MB:

There are other useful options to set the size of a new LV like:
 * -l 100%FREE = maximum size of the LV within the VG
 * -l 50%VG = 50% size of the whole VG

List LV
The folloing command lists all Logical Volumes (LV) in the system:

You can scan for LV in the system, to troubleshoot not properly created or lost LVs:

Extend LV
With the following command, we can extend the Logical Volume (LV) named lvol1 in Volume Group (VG) vg0 to 500MB:

Once the LV is extended, we need to grow the file system as well (in this example we used ext4 and the LV is mounted to /mnt/data):

Reduce LV
Before we can reduce the size of our Logical Volume (LV) without corrupting existing data, we have to shrink the file system on it. In this example we used ext4, the LV needs to be unmounted to shrink the file system:

Now we are ready to reduce the size of our LV:

LV Permissions
Logical Volumes (LV) can be set to be read only storage devices.

The LV needs to be remounted for the changes to take affect:

To set the LV to be read/write again:

Remove LV
Before we remove a Logical Volume (LV) we should unmount and deactivate, so no further write activity can take place:

The following command removes the LV named lvol1 from VG named vg0:

Examples
We can create some scenarios using loopback devices, so no real storage devices are used.

Preparation
First we need to make sure the loopback module is loaded. If you want to play around with partitions, use the following option:

Now we need to either tell LVM to not use udev to scan for devices or change the filers in /etc/lvm/lvm.conf. In this case we just temporarely do not use udev:

We create some image files, that will become our storage devices (uses ~6GB of real hard drive space):

Check which loopback devices are available:

We assume all loopback devices are available and create our hard drives:

Now we can use /dev/loop[0-2] as we would use any other hard drive in the system.

Two Hard Drives
In this example, we will initialize two hard drive as PV and then create the VG vg0:

Now lets create the LV lvol1 in our VG vg0 and take the maximum space available:

Create the file system and mount it to /mnt/data:

Now we have the capacity of 2GB from each hard drive available in /mnt/data as one 4GB device.

/etc/fstab
Here is an example of an entry in /etc/fstab (using ext4):

LVM2 MIRROR
We use two hard drives and create our LV lvol1 like in the first example. This time we use 40% of the size of our VG vg0, because we need some space in the VG for the MIRROR and log files:

To create our copy of /dev/vg0/lvol1 on the PV /dev/loop1, use the following command:

LVM will now ensure that a full copy (MIRROR) of /dev/vg0/lvol1 exists on /dev/loop1 and is not distributed between other PVs.

To remove the MIRROR:

If one half of the MIRROR fails, the other one will be automatically converted into a not mirrored LV (loose the mirror atribute). LVM is different from Linux RAID1 that it doesn't read/write from both mirrored images, there is no performace increase.

LVM2 Snapshots
A snapshot is an LV as copy of another LV, which takes in all the changes that were made in the original LV to show the content of that LV in a different state. We once again use our two hard drives and create LV lvol1 this time with 60% of VG vg0:

Now we create a snapshot of lvol1 named 08092011_lvol1 and give it 10% of VG vg0:

Mount our snapshot somewhere else:

We could now access data in lvol1 from a previous state. Since LVM2, snapshots are writeable LV, so we could use them to let a project go on into two different directions:

Now we have three different versions of LV lvol1, the original and two snapshots which can be used parallel and changes are written to the snapshots.

LVM2 Stripeset
The STRIPSET is the same as RAID0, data is written to several devices at the same time to increase performance. In LVM2 it is possible to distribute LV over several PV for the same effect. We create three PV and then VG vg0:

VG vg0 consists of three different hard drives and now we can create our LV and spread it over them:

The option -i 3 indicates that we want to spread it over 3 PV in out VG vg0:

On each PV 400MB got reserved for our LV lvm_stripe in VG vg0

Troubleshooting
LVM has only MIRROR and snapshots to provide some level of redundancy. However there are certain situations where one might be able to restore lost PV or LV.

vgcfgrestore
In /etc/lvm/archive and /etc/lvm/backup are files which contain logs about metadata changes in LVM. To see what states of the VG are available to be restored:

In this example we removed the LV lvol1 by accident and want it back in our VG vg0:

Replace PV
We want to replace a PV and then restore the metadata to a new one, so that we reach the same state as before the device stopped working. To display all PV in a VG (even lost ones) use the following command:

In this example I let /dev/loop1 (unknown device) fail:

Using the UUID, we can tell LVM to restore new hardware and be implemented within the VG as the old one was.

Then we restore the VG to the state before the PV failed:

Now you can replay your file backup if you haven't already restored the PV itself.

Deactivate LV
You can deactivat a LV with the following command:

You will not be able to mount the LV anywhere before it got reactivated:

Links

 * lvm2
 * LVM tldp.org