LVM Deep Dive

Concept

LVM (Logical Volume Manager) adds a flexible layer between physical disks and filesystems:

Physical Volumes (PVs)  →  Volume Group (VG)  →  Logical Volumes (LVs)  →  Filesystem

This lets you resize, snapshot, and move storage more easily than with plain partitions.

Why it matters

Most production Linux servers use LVM.
Resizing a filesystem almost always involves the LV underneath it.
Mistakes in LVM operations can lead to data loss, so a clear mental model is essential.

Mental Model

Disk / Partition
└── Physical Volume (PV)
    └── Volume Group (VG)          ← pool of storage
        ├── Logical Volume 1 (LV)  ← appears as /dev/vgname/lvname
        ├── Logical Volume 2
        └── Free space in VG

The filesystem sits on top of the LV.
You normally extend the LV first, then grow the filesystem.

Key Commands

# Overview
pvs
vgs
lvs
 
# Detailed view
pvdisplay
vgdisplay
lvdisplay
 
# What is mounted where
lsblk
df -hT
 
# Free space in a volume group
vgdisplay <vgname> | grep Free

Common safe operations

# Extend an LV (add 10G from free space in the VG)
lvextend -L +10G /dev/<vg>/<lv>
 
# Extend LV to use all remaining free space
lvextend -l +100%FREE /dev/<vg>/<lv>
 
# Then grow the filesystem (example for XFS and ext4)
xfs_growfs /mountpoint          # XFS
resize2fs /dev/<vg>/<lv>        # ext4

Common Failure Modes & Symptoms

Symptom	Likely cause	First checks
Cannot extend LV	No free space in VG	vgs, vgdisplay
Filesystem still full after lvextend	Forgot to grow the filesystem	df -h, run xfs_growfs/resize2fs
LV not visible after reboot	Activation issue	lvscan, vgchange -ay
“Device is busy” when reducing	Filesystem or mounts still using it	lsof, fuser, unmount first

Investigation Tips

• Always check pvs / vgs / lvs together.
• Never shrink an LV without shrinking the filesystem first (and having a backup).
• Snapshots are useful but consume space from the VG — monitor them.
• On cloud VMs the physical disk may need to be expanded at the provider level before the PV can be grown.

Related Notes

• Block Devices and Partitions
• Filesystems and Mounts
• XFS Operations
• ext4 Operations
• Disk Full Runbook
• Troubleshooting Methodology

Personal Lessons Learned