Lvm

Chapter 13: LVM - Logical Volume Manager

Comprehensive Linux Logical Volume Management

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13.1 LVM Architecture Deep Dive

What is LVM?

LVM (Logical Volume Manager) is a device mapper framework that provides logical volume management for the Linux kernel. It allows for flexible disk storage management, including resizing, snapshots, and thin provisioning.

┌────────────────────────────────────────────────────────────────────────┐
│                    LVM ARCHITECTURE LAYERS                             │
├────────────────────────────────────────────────────────────────────────┤
│                                                                        │
│  ┌──────────────────────────────────────────────────────────────────┐  │
│  │                      USER SPACE                                  │  │
│  │  ┌────────────┐ ┌────────────┐ ┌────────────┐ ┌────────────┐  │  │
│  │  │   pvcreate │ │  vgcreate  │ │  lvcreate  │ │   lvm      │  │  │
│  │  │   pvs      │ │    vgs     │ │    lvs     │ │   tools    │  │  │
│  │  └────────────┘ └────────────┘ └────────────┘ └────────────┘  │  │
│  └──────────────────────────────────────────────────────────────────┘  │
│                              │                                          │
│                              ▼                                          │
│  ┌──────────────────────────────────────────────────────────────────┐  │
│  │                    KERNEL MODULES                                 │  │
│  │  ┌──────────────────────────────────────────────────────────┐    │  │
│  │  │  device-mapper (dm.ko)                                 │    │  │
│  │  │  dm-mod.ko (snapshot, thin, cache, mirror, raid)       │    │  │
│  │  └──────────────────────────────────────────────────────────┘    │  │
│  └──────────────────────────────────────────────────────────────────┘  │
│                              │                                          │
│                              ▼                                          │
│  ┌──────────────────────────────────────────────────────────────────┐  │
│  │                    DEVICE MAPPER                                  │  │
│  │                                                                   │  │
│  │  /dev/mapper/vg_name-lv_name  ──► /dev/dm-0, /dev/dm-1          │  │
│  │  /dev/vg_name/lv_name         ──► Symlink to above             │  │
│  │                                                                   │  │
│  └──────────────────────────────────────────────────────────────────┘  │
│                                                                        │
└────────────────────────────────────────────────────────────────────────┘

LVM Components

Component	Abbreviation	Description	Example
Physical Volume	PV	Partition or disk initialized for LVM	/dev/sdb1
Volume Group	VG	Pool of storage from PVs	vg_data
Logical Volume	LV	Virtual partitions from VGs	lv_data
Physical Extent	PE	Smallest allocatable unit (4MB default)	4MB chunk
Logical Extent	LE	Maps to PE for allocation	4MB chunk
Volume Descriptor	VGDA	Metadata at start of PV	Metadata

LVM Workflow

┌────────────────────────────────────────────────────────────────────────┐
│                    LVM DATA FLOW                                       │
├────────────────────────────────────────────────────────────────────────┤
│                                                                        │
│  Physical Disk                                                        │
│  ┌─────────────┐                                                     │
│  │ /dev/sdb    │                                                     │
│  │              │                                                     │
│  │ ┌─────────┐ │     ┌─────────────────┐      ┌──────────────────┐  │
│  │ │/dev/sdb1│ │────►│  Physical Volume │      │   /dev/sdb1      │  │
│  │ │ Partition│ │     │    (PV Header)   │      │  PV Header       │  │
│  │ └─────────┘ │     └─────────────────┘      │  PE Table         │  │
│  └─────────────┘                              └──────────────────┘  │
│                                                       │               │
│                                                       ▼               │
│  ┌─────────────────────────────────────────────────────────────┐     │
│  │                   Volume Group (VG)                         │     │
│  │                                                              │     │
│  │    ┌───────────────────────────────────────────────┐        │     │
│  │    │              Physical Extents                  │        │     │
│  │    │  PE0  PE1  PE2  PE3  PE4  PE5  PE6  PE7    │        │     │
│  │    │  ▓▓▓  ▓▓▓  ░░  ░░  ███  ███  ▓▓▓  ▓▓▓     │        │     │
│  │    │  (used)      (free)   (used)  (used)         │        │     │
│  │    └───────────────────────────────────────────────┘        │     │
│  │                         │                                   │     │
│  │                         ▼                                   │     │
│  │    ┌───────────────────────────────────────────────┐        │     │
│  │    │           Logical Extents                     │        │     │
│  │    │  LE0  LE1  LE2  LE3  LE4  LE5  LE6  LE7    │        │     │
│  │    │  ▓▓▓  ▓▓▓  ▓▓▓  ▓▓▓  ███  ███  ███  ███    │        │     │
│  │    │  (lv_data)       (lv_logs)                    │        │     │
│  │    └───────────────────────────────────────────────┘        │     │
│  │                                                              │     │
│  └─────────────────────────────────────────────────────────────┘     │
│                              │                                          │
│                              ▼                                          │
│  ┌─────────────────────────────────────────────────────────────┐     │
│  │              Logical Volumes (LV)                           │     │
│  │                                                              │     │
│  │  ┌──────────────────────┐   ┌──────────────────────┐       │     │
│  │  │    /dev/vg_data/    │   │    /dev/vg_data/    │       │     │
│  │  │      lv_data         │   │      lv_logs        │       │     │
│  │  │      (100GB)         │   │      (50GB)         │       │     │
│  │  └──────────────────────┘   └──────────────────────┘       │     │
│  │                                                              │     │
│  └─────────────────────────────────────────────────────────────┘     │
│                              │                                          │
│                              ▼                                          │
│  ┌─────────────────────────────────────────────────────────────┐     │
│  │              Filesystem                                      │     │
│  │                                                              │     │
│  │  ┌──────────────────────┐   ┌──────────────────────┐       │     │
│  │  │   /dev/mapper/       │   │   /dev/mapper/      │       │     │
│  │  │  vg_data-lv_data    │   │  vg_data-lv_logs   │       │     │
│  │  │   (ext4/xfs)        │   │   (ext4/xfs)       │       │     │
│  │  └──────────────────────┘   └──────────────────────┘       │     │
│  │                                                              │     │
│  └─────────────────────────────────────────────────────────────┘     │
│                                                                        │
└────────────────────────────────────────────────────────────────────────┘

---

13.2 LVM Installation and Setup

Installing LVM

# Debian/Ubuntu
sudo apt-get install lvm2

# RHEL/CentOS/Fedora
sudo yum install lvm2
# or
sudo dnf install lvm2

# Arch Linux
sudo pacman -S lvm2

# Verify installation
lvm version

Preparing Disks

# Using fdisk to create partition
sudo fdisk /dev/sdb

# In fdisk:
# n - new partition
# p - primary partition
# 1 - partition number
# Enter - default first sector
# Enter - default last sector (use whole disk)
# t - change partition type
# 8e - Linux LVM type
# w - write and quit

# Alternative: Use parted
sudo parted /dev/sdb
(parted) mklabel gpt
(parted) mkpart primary 0% 100%
(parted) set 1 lvm on
(parted) quit

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13.3 Creating LVM Volumes

Step 1: Create Physical Volumes

# Create PV on single disk
sudo pvcreate /dev/sdb1

# Create PV on multiple disks
sudo pvcreate /dev/sdb1 /dev/sdc1 /dev/sdd1

# Force creation (if disk has existing data)
sudo pvcreate -ff /dev/sdb1

# View PV information
sudo pvs                          # Brief summary
sudo pvdisplay                    # Detailed info
sudo pvdisplay /dev/sdb1          # Specific PV

# PV details with specific fields
sudo pvs -o +pv_used,pv_free

Step 2: Create Volume Group

# Create VG from single PV
sudo vgcreate vg_data /dev/sdb1

# Create VG from multiple PVs
sudo vgcreate vg_data /dev/sdb1 /dev/sdc1

# Create VG with specific PE size
sudo vgcreate -s 8M vg_data /dev/sdb1

# Add PV to existing VG
sudo vgextend vg_data /dev/sdd1

# Remove PV from VG
sudo vgreduce vg_data /dev/sdd1

# View VG information
sudo vgs                          # Brief summary
sudo vgs -a                       # Show all VGs
sudo vgdisplay                    # Detailed info
sudo vgdisplay vg_data           # Specific VG

Step 3: Create Logical Volume

# Create LV with fixed size
sudo lvcreate -L 10G -n lv_data vg_data

# Create LV using all free space
sudo lvcreate -l 100%FREE -n lv_backup vg_data

# Create LV using percentage of VG
sudo lvcreate -l 50%VG -n lv_data vg_data

# Create LV with specific PE count
sudo lvcreate -l 2560 -n lv_data vg_data

# Create linear LV (default)
sudo lvcreate -L 20G -n lv_linear vg_data

# Create striped LV (for performance)
sudo lvcreate -L 20G -i 3 -n lv_striped vg_data
# -i 3 = 3 stripes across PVs

# Create mirrored LV
sudo lvcreate -L 10G -m 1 -n lv_mirror vg_data
# -m 1 = 1 mirror copy

# View LV information
sudo lvs                          # Brief summary
sudo lvs -a                       # Show all LVs
sudo lvdisplay                    # Detailed info
sudo lvdisplay /dev/vg_data/lv_data

Step 4: Create Filesystem and Mount

# Create ext4 filesystem
sudo mkfs.ext4 /dev/vg_data/lv_data

# Create XFS filesystem
sudo mkfs.xfs /dev/vg_data/lv_data

# Create with specific block size
sudo mkfs.ext4 -b 4096 /dev/vg_data/lv_data

# Mount temporarily
sudo mount /dev/vg_data/lv_data /mnt/data

# Mount persistently (/etc/fstab)
# /dev/mapper/vg_data-lv_data /mnt/data ext4 defaults 0 2

# Verify mount
df -h /mnt/data

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13.4 LVM Management Operations

Extending Logical Volumes

# Extend LV by specific size
sudo lvextend -L +10G /dev/vg_data/lv_data

# Extend LV to specific size
sudo lvextend -L 50G /dev/vg_data/lv_data

# Extend to use all free space
sudo lvextend -l +100%FREE /dev/vg_data/lv_data

# Extend with specific PE
sudo lvextend -l +2560 /dev/vg_data/lv_data

# Online extension (without unmounting)
# For ext4
sudo resize2fs /dev/vg_data/lv_data

# For XFS (online, no unmount needed)
sudo xfs_growfs /mnt/data

# Check result
sudo lvs
df -h /mnt/data

Reducing Logical Volumes

# WARNING: Reducing LV can cause data loss!
# Always backup first!

# Unmount the filesystem
sudo umount /mnt/data

# Check filesystem
sudo e2fsck -f /dev/vg_data/lv_data

# Resize filesystem to new smaller size
sudo resize2fs /dev/vg_data/lv_data 20G

# Reduce LV
sudo lvreduce -L 20G /dev/vg_data/lv_data

# Remount
sudo mount /dev/vg_data/lv_data /mnt/data

# Alternative: Use lvreduce with -r (auto-resize filesystem)
# This combines the above steps:
sudo lvreduce -r -L 20G /dev/vg_data/lv_data

Removing LVM Components

# Remove LV
sudo lvremove /dev/vg_data/lv_data
# Will ask for confirmation

# Force remove (with -f)
sudo lvremove -f /dev/vg_data/lv_data

# Remove VG
sudo vgremove vg_data
# Must remove all LVs first

# Remove PV
sudo pvremove /dev/sdb1
# Must not be part of any VG

Moving Data Between PVs

# Move all extents from one PV to others
sudo pvmove /dev/sdb1

# Move specific extents
sudo pvmove /dev/sdb1 /dev/sdc1

# Move with progress
sudo pvmove -i 10 /dev/sdb1

# Note: Can take long time for large volumes
# Use screen or tmux for long operations

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13.5 LVM Snapshots

Creating Snapshots

# Create snapshot (read-only by default)
sudo lvcreate -L 5G -s -n lv_snap /dev/vg_data/lv_data

# Create writable snapshot
sudo lvcreate -L 5G -s -n lv_snap_rw /dev/vg_data/lv_data
# Note: Changes won't persist after merge

# Create snapshot with specific extents
sudo lvcreate -l 1000 -s -n lv_snap /dev/vg_data/lv_data

# View snapshots
sudo lvs -a
sudo lvdisplay /dev/vg_data/lv_snap

Mounting and Using Snapshots

# Mount snapshot (read-only)
sudo mkdir /mnt/snap
sudo mount -o ro /dev/vg_data/lv_snap /mnt/snap

# Browse snapshot contents
ls /mnt/snap
sudo cp -a /mnt/snap /backup/snapshot-$(date +%Y%m%d)

# Unmount
sudo umount /mnt/snap

Restoring from Snapshot

# Method 1: Merge snapshot into original
# This reverts the original LV to snapshot state
# All changes since snapshot creation are lost

# Unmount original filesystem
sudo umount /mnt/data

# Merge snapshot
sudo lvconvert --merge /dev/vg_data/lv_snap

# Mount restored volume
sudo mount /dev/vg_data/lv_data /mnt/data

# Note: Snapshot is removed after merge

# Method 2: Create new LV from snapshot
sudo lvcreate -L 10G -n lv_restored -s /dev/vg_data/lv_data

# Mount and copy data
sudo mount /dev/vg_data/lv_restored /mnt/restored
cp -a /mnt/restored/* /new/location/

Snapshot Maintenance

# View snapshot status
sudo lvdisplay /dev/vg_data/lv_snap

# Check snapshot space usage
sudo lvs -o+lv_sn%_size

# Remove snapshot
sudo lvremove /dev/vg_data/lv_snap

# Extend snapshot
sudo lvextend -L +2G /dev/vg_data/lv_snap

# Check what snapshot is full
sudo lvs -a | grep snap

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13.6 LVM Thin Provisioning

Thin Pool Creation

# Create thin pool
sudo lvcreate -L 100G --thinpool vg_data/thin_pool

# Create thin pool with metadata size
sudo lvcreate -L 100G --metadatavol 1G --thinpool vg_data/thin_pool

# View thin pool
sudo lvs -a
sudo lvs -o +thin_pool,thin_size,data_size

Thin Volume Creation

# Create thin volume (virtual size can exceed physical)
sudo lvcreate -V 200G --thin -n thin_data vg_data/thin_pool

# View thin volumes
sudo lvs -a

# Create thin volume with specific virtual size
sudo lvcreate -V 500G --thin -n thin_archive vg_data/thin_pool

# Extend thin pool
sudo lvextend -L +50G vg_data/thin_pool

Thin Pool Monitoring

# Monitor thin pool usage
sudo lvs -o lv_name,thin_size,data_size,metadata_size,thin_percent

# Check pool full percentage
sudo lvs -o lv_name,data_percent

# Example output:
# LV         Thin Size  Data Size  Metadata  %Used
# thin_pool   100.00g   100.00g    1.00g     45.00
# thin_data   500.00g              Pool data

Thin Pool Management

# Enable autoextension
sudo lvchange --autobackup y --thinpool vg_data/thin_pool

# Set autoextend threshold
sudo lvchange --thinpool_autoextend_threshold 80 --thinpool_autoextend_percent 20 vg_data/thin_pool

# Trim unused space (if supported)
sudo fstrim /mnt/thin_data

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13.7 LVM RAID and Mirroring

RAID Levels with LVM

RAID Level	LVM Option	Description
RAID 0	-i	Striping (performance, no redundancy)
RAID 1	-m 1	Mirroring (redundancy)
RAID 4	-m 1 —type raid4	Dedicated parity
RAID 5	-m 1 —type raid5	Distributed parity
RAID 6	-m 2 —type raid6	Double parity
RAID 10	-m 1 -i 2	Striped mirrors

Creating Mirrored LV

# Create 2-way mirror
sudo lvcreate -L 20G -m 1 -n lv_mirror vg_data

# Create 2-way mirror on specific PVs
sudo lvcreate -L 20G -m 1 -n lv_mirror vg_data /dev/sdb1 /dev/sdc1 /dev/sdd1

# Convert existing LV to mirror
sudo lvconvert -m 1 /dev/vg_data/lv_data /dev/sdc1

# Remove mirror (keep data on one disk)
sudo lvconvert -m 0 /dev/vg_data/lv_data /dev/sdb1

Creating Striped LV

# Create 3-stripe RAID0
sudo lvcreate -L 30G -i 3 -n lv_striped vg_data

# Create stripe on specific PVs
sudo lvcreate -L 30G -i 3 -n lv_striped vg_data /dev/sdb1 /dev/sdc1 /dev/sdd1

# Extend striped LV (must add PVs)
sudo vgextend vg_data /dev/sde1
sudo lvextend -i 3 -l +100%FREE /dev/vg_data/lv_striped

RAID Maintenance

# Check RAID status
sudo lvs -a -o lv_name,lv_health_status,devices

# Repair failed mirror
sudo lvconvert --repair /dev/vg_data/lv_mirror

# Replace failed disk
sudo lvconvert --replace /dev/sdb1 /dev/vg_data/lv_mirror /dev/sdc1

# Monitor resync
sudo lvs -o lv_name,sync_percent

---

13.8 LVM Cache

Creating Cache

# Create cache pool from fast SSD
sudo lvcreate -L 20G -n cache_pool vg_data /dev/ssd1

# Create cache volume from pool
sudo lvcreate -L 20G -n cache_data vg_data /dev/ssd1

# Attach cache to LV
sudo lvconvert --type cache-pool /dev/vg_data/lv_data
# Or using cache pool
sudo lvconvert --type cache --cachepool vg_data/cache_pool /dev/vg_data/lv_data

# Verify cache
sudo lvs -o lv_name,cache_mode,cache_policy

Cache Modes

Mode	Description	Best For
writethrough	Write to both, acknowledge after slow	Data safety priority
writeback	Write to fast first, then slow	Performance priority
passthrough	Bypass cache	Migration/safety

Cache Management

# Change cache mode
sudo lvchange --cachemode writethrough /dev/vg_data/lv_data

# Detach cache (split cache pool)
sudo lvconvert --splitcache /dev/vg_data/lv_data

# Remove cache
sudo lvremove /dev/vg_data/cache_pool

---

13.9 LVM Troubleshooting

Common Issues and Solutions

Issue	Symptoms	Solution
PV not found	VG won’t activate	Check device, run pvscan
No free space	LV extension fails	Add PV or remove snapshots
Snapshot full	Read-only snapshot	Extend snapshot or remove
VG activation fails	System won’t boot	Check /etc/lvm/archive
Corrupted metadata	Can’t access VG	Restore from archive
Disk failure	PV missing	Use pvmove, replace disk

Diagnostic Commands

# Scan for all PVs
sudo pvscan

# Scan for VGs
sudo vgscan

# Scan for LVs
sudo lvscan

# Display detailed PV info
sudo pvdisplay -v

# Display detailed VG info
sudo vgdisplay -v

# Display detailed LV info
sudo lvdisplay -v

# Check LVM configuration
sudo lvm dumpconfig

# View LVM logs
sudo less /var/log/lvm.log

# Kernel device mapper info
sudo dmsetup info
sudo dmsetup table

# Check /proc/mdstat for LVM devices
cat /proc/mdstat

Metadata Recovery

# List archived VG metadata
ls /etc/lvm/archive/

# Restore VG from archive
sudo vgcfgrestore -f /etc/lvm/archive/vg_data_00001-123456789.vg vg_data

# List backup files
sudo vgcfgrestore --list vg_data

# Force restore (if needed)
sudo vgcfgrestore -f /etc/lvm/archive/vg_data_*.vg vg_data

Emergency Procedures

# If PV is missing but data intact
sudo vgchange -a n vg_data           # Deactivate
sudo vgchange -a y vg_data           # Reactivate (if fixed)

# Recreate missing PV
sudo pvcreate --restorefile /etc/lvm/backup/vg_data /dev/sdb1
sudo vgcfgrestore vg_data

# If LVM won't start
sudo lvm pvscan
sudo vgscan
sudo lvchange -a y /dev/vg_data/lv_data

# Emergency mount (read-only)
sudo mount -o ro /dev/mapper/vg_data-lv_data /mnt/rescue

---

13.10 Production Examples

Database Server LVM Setup

# Physical volumes on fast storage
sudo pvcreate /dev/nvme0n1p1
sudo pvcreate /dev/nvme0n1p2

# Create volume groups
sudo vgcreate vg_db /dev/nvme0n1p1
sudo vgcreate vg_logs /dev/nvme0n1p2

# Database data (fast SSD/NVMe)
sudo lvcreate -L 200G -n lv_data vg_db
sudo mkfs.xfs /dev/vg_db/lv_data

# Database logs
sudo lvcreate -L 100G -n lv_logs vg_logs
sudo mkfs.xfs /dev/vg_logs/lv_logs

# Archive/backup (slow storage)
sudo pvcreate /dev/sdb1
sudo vgcreate vg_backup /dev/sdb1
sudo lvcreate -L 500G -n lv_backup vg_backup

Web Server LVM Setup

# Create PV
sudo pvcreate /dev/sdb1

# Create VG
sudo vgcreate vg_web /dev/sdb1

# Webroot
sudo lvcreate -L 50G -n lv_www vg_web
sudo mkfs.ext4 /dev/vg_web/lv_www

# Logs (separate to manage space)
sudo lvcreate -L 20G -n lv_log vg_web
sudo mkfs.ext4 /dev/vg_web/lv_log

# Tmp
sudo lvcreate -L 10G -n lv_tmp vg_web
sudo mkfs.ext4 /dev/vg_web/lv_tmp

Backup and Snapshot Strategy

# Daily snapshot of data LV
sudo lvcreate -L 20G -s -n lv_data_daily /dev/vg_data/lv_data

# Weekly snapshot
sudo lvcreate -L 40G -s -n lv_data_weekly /dev/vg_data/lv_data

# Backup script
#!/bin/bash
# /usr/local/bin/backup-lvm.sh
set -e
DATE=$(date +%Y%m%d)
sudo lvcreate -L 20G -s -n lv_backup_$DATE /dev/vg_data/lv_data
sudo mount -o ro /dev/vg_data/lv_backup_$DATE /mnt/snap
sudo tar -czf /backup/lv_data_$DATE.tar.gz -C /mnt/snap .
sudo umount /mnt/snap
sudo lvremove -f /dev/vg_data/lv_backup_$DATE

---

13.11 Interview Questions

Q1: What are the three main components of LVM?

Answer:

1. Physical Volume (PV): A partition or whole disk that has been initialized for use by LVM. Contains LVM headers and physical extents.
2. Volume Group (VG): A collection of one or more PVs. Acts as a pool of storage that can be allocated to logical volumes.
3. Logical Volume (LV): Virtual partitions created from a VG. These are what users and applications interact with, like regular partitions.

Data flow: Physical disks/partitions (PV) → Pooled storage (VG) → Virtual partitions (LV) → Filesystem

Q2: What is the difference between LVM thin provisioning and regular LVM?

Answer:

• Regular LVM: Logical volumes are allocated with a fixed size that uses physical space immediately
• Thin Provisioning: Logical volumes can be created with virtual sizes larger than physically available storage. Space is allocated only as data is written

Benefits of thin provisioning:

• Over-commitment of storage
• More efficient space utilization
• Ability to create many large volumes
• Better for virtual environments

Q3: How do you extend a logical volume online?

Answer:

# Extend the LV
sudo lvextend -L +10G /dev/vg_data/lv_data

# Resize filesystem (ext4)
sudo resize2fs /dev/vg_data/lv_data

# For XFS (online, no unmount needed)
sudo xfs_growfs /mount/point

For ext4, you can extend while mounted in recent kernels. For XFS, always online-extendable.

Q4: What is an LVM snapshot and how does it work?

Answer: An LVM snapshot is a read-only copy of a logical volume at a specific point in time. It works using copy-on-write:

• When created, snapshot uses minimal space
• When data changes on original LV, old data is copied to snapshot before overwrite
• Snapshot grows as changes accumulate
• Can be used for backups, testing, or recovery

Snapshots are temporary - they must be explicitly removed or merged.

Q5: How do you recover from a failed disk in an LVM setup?

Answer:

1. Identify failed disk: sudo pvs shows failed PV
2. If mirror/RAID: LVM automatically recovers if configured
3. For linear/striped:
   • Replace failed disk
   • Create new PV: sudo pvcreate /dev/newdisk
   • Add to VG: sudo vgextend vg_name /dev/newdisk
   • Move data: sudo pvmove /dev/failed_disk /dev/newdisk
   • Remove failed: sudo vgreduce vg_name /dev/failed_disk

Q6: What is the difference between lvreduce and lvresize?

Answer:

• lvreduce: Specifically reduces LV size
• lvresize: Can both increase and decrease LV size
• In practice, lvresize is more commonly used as it handles both directions

Both require:

1. Unmounting the filesystem (for reduction)
2. Running filesystem check (e2fsck)
3. Resizing filesystem before reducing LV

Q7: How do you monitor LVM space usage?

Answer:

# VG free space
sudo vgs

# LV usage
sudo lvs -o lv_name,lv_size,data_percent

# Thin pool usage
sudo lvs -o lv_name,thin_size,data_size,metadata_size

# PV space
sudo pvs

# Detailed info
sudo pvdisplay -m  # shows extent mapping

Q8: What are LVM cache volumes and when would you use them?

Answer: LVM cache uses fast storage (SSD) to cache data from slower storage (HDD):

• Creates a small logical volume on fast storage
• Attaches as cache to slow LV
• Frequently accessed data stays in fast storage
• Improves performance without replacing all storage

Use cases:

• Hybrid storage (SSD + HDD)
• Caching database writes
• Accelerating file servers

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Quick Reference

Essential Commands

# Physical Volume
pvcreate /dev/sdb1
pvs
pvdisplay
pvremove /dev/sdb1

# Volume Group
vgcreate vg_data /dev/sdb1
vgs
vgdisplay
vgextend vg_data /dev/sdc1
vgreduce vg_data /dev/sdb1
vgremove vg_data

# Logical Volume
lvcreate -L 10G -n lv_data vg_data
lvs
lvdisplay
lvextend -L +10G /dev/vg_data/lv_data
lvreduce -L 10G /dev/vg_data/lv_data
lvremove /dev/vg_data/lv_data

# Snapshots
lvcreate -L 5G -s -n lv_snap /dev/vg_data/lv_data
lvconvert --merge /dev/vg_data/lv_snap

# Thin Provisioning
lvcreate -L 100G --thinpool vg_data/thin_pool
lvcreate -V 200G --thin -n thin_vol vg_data/thin_pool

Device Paths

Type	Path
LV (mapper)	/dev/mapper/vg_name-lv_name
LV (by-id)	/dev/vg_name/lv_name
LV (by-uuid)	/dev/disk/by-uuid/uuid

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Summary

In this chapter, you learned:

• ✅ LVM architecture and components (PV, VG, LV)
• ✅ Creating physical volumes, volume groups, and logical volumes
• ✅ Extending and reducing logical volumes
• ✅ LVM snapshots for backups and recovery
• ✅ Thin provisioning for efficient storage
• ✅ LVM RAID and mirroring
• ✅ LVM caching for hybrid storage
• ✅ Troubleshooting and recovery
• ✅ Production configuration examples
• ✅ Interview questions and answers

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Next Chapter

Chapter 14: Mounting and fstab

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Last Updated: February 2026