LVM & Disk Management

Physical volumes, volume groups, logical volumes, snapshots, thin provisioning, and live resize.

Overview

LVM (Logical Volume Manager) adds a layer of abstraction between physical storage and filesystems.

Three-Tier Hierarchy

Physical Volumes (PVs) — raw disks initialised for LVM: pvcreate /dev/sdb
Volume Groups (VGs) — pool of PVs: vgcreate myvg /dev/sdb /dev/sdc
Logical Volumes (LVs) — virtual partitions: lvcreate -L 100G -n data myvg

LVM Snapshots

LVM snapshots are copy-on-write:

Use snapshots for consistent backups of running databases. The snapshot must be large enough to hold all writes during the backup window.

Thin Provisioning

Thin provisioning overcommits storage — allocating real blocks only when written. Useful for dev environments but dangerous in production without monitoring.

Key concepts

Physical Volume (PV): A disk or partition initialised with pvcreate. Contains LVM metadata and Physical Extents (PE, typically 4 MB each).
Volume Group (VG): A pool of one or more PVs. Logical Volumes are allocated from VG free space.
Logical Volume (LV): A virtual partition within a VG. Has its own device path (/dev/VG/LV). Can span multiple PVs.
LVM snapshot: Copy-on-write point-in-time copy of an LV. Used for online backups — the snapshot presents the state at creation time.
pvmove: Migrates physical extents from one PV to another within the same VG. Allows hot removal of a disk without downtime.

When to use it

Growing a volume for a database that is running out of disk space without downtime
Creating a consistent database backup snapshot while the database is running
Migrating data across disks with pvmove without taking services offline

Interview questions

Your /data partition is 95% full. How do you extend it online without restarting the service?