Logical Volume Management (LVM)¶

What is LVM?¶

Logical Volume Management (LVM) allows flexible management of disk storage by abstracting the physical hardware and creating an easier-to-manage virtual storage layer.

How does LVM work?¶

In a nutshell:

Raw disks are loaded into LVM as physical volumes.
Those physical volumes are then aggregated into a volume group.
That volume group is then used to create logical volumes.
The logical volume is formatted with mkfs and mounted.

From there, you can use the storage space of any of the physical volumes in the volume group(s) to add to a logical volume.

Where Logical Volumes are Stored¶

When a logical volume is created, it's stored in /dev/mapper as /dev/mapper/myvg-mylv.
A symlink to the LV is also created for convenience when it's created with lvcreate. The symlinks are stored in /dev/ as /dev/myvg/mylv.

LVM Tools Cheatsheet¶

Tools for managing Physical Volumes, Volume Groups, and Logical Volumes (PV, VG, LV): For more indepth info on these, see this section.

pvcreate: Creates a PV from 8e type partition
vgcreate: Creates VG using PVs
lvmdiskscan: Displays all storage devices
vgscan: Scans all physical devices, searches for VGs
pvdata: Displays debugging information about PV, reads VGDA
pvscan: Scans PVs and displays active
pvmove: Moves data from one PV to another inside one VG
vgreduce: Removes PV from VG
pvdisplay: Displays information about physical volumes
vgdisplay: Displays information about volume groups
lvdisplay: Displays information about logical volumes
vgchange: Activates or deactivates VG
vgexport: Makes VGs unknown to the system, used prior to importing them on a different system
vgimport: Imports VG from a different system
vgsplit: Splits PV from existing VG into new one
vgmerge: Merges two VGs
lvcreate: Creates LV inside VG
```
lvcreate vg1 -n space -L 5G # Create a logical volume called space, with 5GB of storage space
lvcreate vg1 -n storage -l +100%FREE  # Create a logical volume called storage, with all free space inside the VG.  
```
- -n: Name of the LV
- -L: Size of the LV
- Use -l +100%FREE to use all available space in the VG.
  - man://lvcreate 558
- The -L (--size) and -l (--extents) options are alternate methods of specifying size.
- lvcreate --snapshot: Creates a snapshot of a LV
```
sudo lvcreate --size 1G --snapshot --name my_snapshot /dev/my_volume_group/my_logical_volume
```
- This creates only a 1 gig snapshot of the LV.
- For a snapshot, the size can be expressed with -l as a percentage of the total size of the origin LV with the suffix %ORIGIN (100%ORIGIN provides space for the whole origin).
lvextend: Increases the size of LV
lvreduce: Decreases the size of LV

LVM, Step by Step¶

LVM starts with turning raw disks into physical volumes. Then the physical volumes are aggregated into a volume group.
The storage space of all the disks in the volume group can then be used by a logical volume.

Raw Disks and Physical Volumes¶

Raw disks are unformatted, unpartitioned disks that are available for use in the LVM setup.
- MBR (BIOS-based operating systems) uses the partition type code 8e for LVM, while GPT (UEFI-based operating systems) uses the lvm flag to indicate an LVM partition.
Before raw disks can be used by LVM, they are initialized as physical volumes (PV) using pvcreate.
pvcreate gives permission to LVM to use these raw disks as storage devices.
```
sudo pvcreate /dev/sdb /dev/sdc
```
This initializes /dev/sdb and /dev/sdc as physical volumes.

Checking the Type of a Disk¶

You can check whether a disk is using GPT or MBR by using lsblk -f

lsblk -f

This will have a column called PTTYPE (Partition Type). It will show gpt for GPT, or dos for MBR.

You can also use the blkid command to check the partitioning scheme:

blkid -p /dev/sda

-p: Low-level probing mode.
The output should indicate either PTTYPE=gpt for GPT or PTTYPE=dos for MBR.

To check if the disk partitions are the correct type for LVM (8e or lvm):

For either MBR or GPT, you can use lsblk or blkid to see if partitions are marked for LVM

lsblk -o NAME,TYPE,FSTYPE,SIZE,UUID,MOUNTPOINT
blkid /dev/sdX  # Where X is the disk partition

To check on MBR (Master Boot Record, used by BIOS-based operating systems):
```
fdisk -l /dev/sdX  # Where X is the disk partition
```
There will be a Type column that will show 8e for LVM.
To check on GPT (GUID Partition Tables, used by UEFI-based operating systems): To check if the disks are the correct type for LVM (8e), you can use fdisk -l
```
parted /dev/sdX  # Where X is the disk partition
```
There will be a Flags field that will show lvm for LVM.

Physical Volumes and Volume Groups¶

Once physical volumes are created, they are aggregated (grouped) together into a volume group (VG) using vgcreate.
- A volume group is essentially a pool of storage made up of the physical volumes created with pvcreate.
- You can allocate space from this group when creating logical volumes.
- E.g.:
```
sudo vgcreate my_volume_group /dev/sdb /dev/sdc
```
  This creates a volume group named my_volume_group that includes /dev/sdb and /dev/sdc.

Volume Groups and Logical Volumes¶

The next step is to create logical volumes (LV) from the volume group using lvcreate.
- Logical volumes act like partitions inside a volume group, but they are much more flexible because you can resize them on the fly and span them across multiple physical volumes.
- E.g.:
```
sudo lvcreate -L 50G -n my_logical_volume my_volume_group
```
  This creates a 50GB logical volume named my_logical_volume from the my_volume_group volume group.

Formatting and Mounting Logical Volumes¶

Once the logical volume is created, you need to format it with a filesystem so it can be used to store data. This is done with mkfs.
- The most common filesystems are ext4 and xfs, though there are others like btrfs.
- E.g.:
```
sudo mkfs.ext4 /dev/my_volume_group/my_logical_volume
```
  This formats the logical volume with the ext4 filesystem.
After formatting, the logical volume is ready to be mounted and used like any other filesystem.
- E.g.:
```
sudo mount /dev/my_volume_group/my_logical_volume /mnt/my_mount_point
```
  This mounts the logical volume to /mnt/my_mount_point.

Implementing RAID on Logical Volumes with mdadm¶

If redundancy or performance is a concern, you can configure RAID (Redundant Array of Independent Disks) using a tool like mdadm (Multiple Disk Admin). RAID and LVM are separate technologies.

RAID provides redundancy and performance benefits, while LVM provides flexibility in managing storage. You can use them together, but they serve different purposes.
Example (creating RAID 1 for redundancy):
```
sudo mdadm --create /dev/md0 --level=1 --raid-devices=2 /dev/sdb /dev/sdc
```
This creates a RAID 1 array (mirrored) with /dev/sdb and /dev/sdc. Then you can use md0 as a physical volume in your LVM setup.

Creating a Logical Volume from Raw Disks¶

fdisk -l | grep -i xvd  # See all xvd-type raw disks
vgcreate vg1 /dev/xvdb /dev/xvdc  # Make a volume group called vg1, with the two physical volumes xvdb and xvdc
pvs # Show all physical volumes
vgextend vg1 /dev/xvde  # Add the 3rd physical volume to the volume group
lvcreate vg1 -n space -L 5G # Create a logical volume called space, with 5GB of storage space
lvs
# The logical volume will be stored in /dev/mapper/vg1-space
mkfs.ext4 /dev/mapper/vg1-space 
mount /dev/mapper/vg1-space /space
pvdisplay  # Show more information than pvs
vgdisplay
lvdisplay

Resizing Logical Volumes¶

One of LVM's best features is the ability to resize logical volumes dynamically.
You can both extend or reduce the size of a logical volume.

To extend a logical volume, use lvextend and resize2fs:

sudo lvextend -L +20G /dev/my_volume_group/my_logical_volume
sudo resize2fs /dev/my_volume_group/my_logical_volume

Tools for Managing Physical Volumes, Volume Groups, and Logical Volumes (PV, VG, LV):¶

Physical Volume Management¶

pvcreate: Initializes a physical volume for use by LVM.
```
sudo pvcreate /dev/sdb1
```
This creates a physical volume on partition /dev/sdb1. The partition should be of the type 8e on MBR (BIOS-based OSs), or lvm on GPT (UEFI-based OSs).
pvdisplay: Displays detailed information about physical volumes.
```
sudo pvdisplay /dev/sdb1
```
pvscan: Scans all devices for physical volumes and displays active ones.
```
sudo pvscan
```
pvmove: Moves physical extents (data) from one physical volume to another within the same volume group.
- This is useful when you need to migrate data off a failing disk or redistribute storage.
```
sudo pvmove /dev/sdb1 /dev/sdc1
```
vgreduce: Removes a physical volume from a volume group, but only after its data has been moved or there are no logical volumes on it.
```
sudo vgreduce myvg /dev/sdb1
```

Volume Group Management¶

vgcreate: Creates a volume group using one or more physical volumes.
```
sudo vgcreate myvg /dev/sdb1 /dev/sdc1
```
vgdisplay: Displays information about volume groups.
```
sudo vgdisplay myvg
```
vgscan: Scans all devices for volume groups and displays found VGs.
```
sudo vgscan
```
vgextend: Adds one or more physical volumes to an existing volume group, increasing its capacity.
```
sudo vgextend myvg /dev/sdd1
```
vgreduce: Removes a physical volume from a volume group (already covered above).
vgchange: Activates or deactivates volume groups.
- Activating makes the logical volumes in the VG available for use, while deactivating disables access to the logical volumes.
```
sudo vgchange -a y myvg   # Activates VG
sudo vgchange -a n myvg   # Deactivates VG
```
vgexport: Marks a volume group as inactive and exports it so it can be moved or imported on another system.
```
sudo vgexport myvg
```
vgimport: Imports a volume group that has been exported from another system.
```
sudo vgimport myvg
```
vgsplit: Splits a volume group into two separate volume groups.
- One or more physical volumes from the original VG are moved to a new VG.
```
sudo vgsplit myvg mynewvg /dev/sdd1
```
vgmerge: Merges two volume groups into one.
- The second VG is absorbed into the first.
```
sudo vgmerge myvg1 myvg2
```

Logical Volume Management¶

lvcreate: Creates a logical volume from a volume group. You can specify the size and the name for the logical volume.
- Example:
```
sudo lvcreate -L 10G -n mylv myvg
```

lvextend: Increases the size of a logical volume. You must resize the filesystem afterwards.

Example:

sudo lvextend -L +5G /dev/mapper/myvg-mylv
sudo resize2fs /dev/mapper/myvg-mylv   # If ext4 filesystem

lvreduce: Decreases the size of a logical volume. Be careful when reducing the size of an LV to avoid data loss. Ensure that the filesystem is resized before reducing the LV.
- Example:
```
sudo resize2fs /dev/mapper/myvg-mylv 5G   # Resize filesystem first (e.g., ext4)
sudo lvreduce -L 5G /dev/mapper/myvg-mylv
```
lvresize: Resizes a logical volume, which can be used for both increasing and decreasing the size.
- Example:
```
sudo lvresize -L 15G /dev/mapper/myvg-mylv
```
lvdisplay: Displays information about logical volumes.
- Example:
```
sudo lvdisplay /dev/mapper/myvg-mylv
```

Other Useful LVM Commands¶

lvmdiskscan: Scans for all storage devices and shows their suitability for use as physical volumes in LVM.
- Example:
```
sudo lvmdiskscan
```

pvdata: Deprecated. This has been replaced by the pvdisplay command in most modern distributions of Linux.

* It used to provide detailed debugging information about physical volumes and Volume 
  Group Descriptor Areas (VGDA).

lvremove: Deletes a logical volume.
- Example:
```
sudo lvremove /dev/mapper/myvg-mylv
```
vgremove: Deletes a volume group. You must remove all logical volumes from the VG first.
- Example:
```
sudo vgremove myvg
```
pvremove: Removes a physical volume from LVM control, effectively undoing the pvcreate command.
- Example:
```
sudo pvremove /dev/sdb1
```

More LVM Actions¶

Check available free space in a volume group¶

You can use the vgs command to quickly see the available free space in a volume group.
```
sudo vgs
```

Filesystem resizing¶

lvextend: Increases the size of LV When extending an LV, ensure that the underlying filesystem is resized accordingly.

For ext4 or ext3 filesystems:
```
sudo resize2fs /dev/mapper/myvg-mylv
```
For xfs filesystems:
```
sudo xfs_growfs /mount/point
```

LVM snapshots¶

LVM allows you to create snapshots of Logical Volumes for backup/testing.

sudo lvcreate --size 5G --snapshot --name mysnapshot /dev/mapper/myvg-mylv
# Or, use the symlink if you want (and if it's there)
sudo lvcreate --size 5G --snapshot --name mysnapshot /dev/myvg/mylv

LVM Thin Provisioning¶

Thin provisioning allows you to over-allocate storage to logical volumes, and space is allocated only when data is written.

This is useful for maximizing storage utilization.

sudo lvcreate --thinpool thinpool --size 100G myvg
sudo lvcreate --thin --virtualsize 50G --name thinlv myvg/thinpool

Reverting a Logical Volume back to Raw Disks¶

Backup data. Removing a logical volume is destructive and irreversible.
Identify the LV, VG, and PGs
```
lvdisplay
vgdisplay
pvdisplay
```

Unmount the LV

umount /dev/mapper/myvg-mylv
# or
umount /dev/myvg/mylv

Also remove the entry in /etc/fstab

Remove the LV

lvremove /dev/mapper/myvg-mylv
# or
lvremove /dev/myvg/mylv

Remove the VG
```
vgremove myvg
```

Remove the PVs

pvremove /dev/xda  # Replace with the PVs you want to remove

(Optional) Wipe the disk to return it to a raw state.
wipefs -a will return the disk to a raw, unpartitioned state.
dd can also be used to do this.
```
wipefs -a /dev/xda
```