Operating SystemsStorage Management

Storage Virtualization

LevelAdvanced

Duration90 mins

TopicStorage Management

3 / 5

Logical Volumes

The Application-Facing Layer

Logical Volumes (LVs) are the culmination of LVM's abstraction stack—the layer where virtualized storage becomes usable by file systems and applications. While Physical Volumes provide raw capacity and Volume Groups aggregate that capacity into pools, Logical Volumes are where storage actually gets allocated, formatted, mounted, and used.

From the perspective of a file system or application, a logical volume is indistinguishable from a physical disk partition. It appears as /dev/vg_name/lv_name or /dev/mapper/vg_name-lv_name, supports all standard block device operations, and can host any file system. The magic lies in what happens beneath this abstraction: the freedom to resize, relocate, snapshot, and stripe without the application knowing or caring.

This page provides comprehensive coverage of logical volume types, creation strategies, resizing mechanics, and the critical relationship between LVs and the file systems that inhabit them.

What You Will Learn

By the end of this page, you will understand linear, striped, and mirrored logical volumes; LV creation with all essential options; online and offline resizing operations; the critical difference between LV and file system resize; LV activation and device paths; RAID logical volumes; and best practices for production LV design.

Logical Volume Types

LVM supports multiple logical volume types, each optimized for different use cases. Understanding these types and their trade-offs is essential for effective storage design.

Linear Volumes:

The simplest and most common type. A linear volume maps logical extents to physical extents sequentially. If the volume spans multiple segments (due to non-contiguous allocation or spanning PVs), LVM simply concatenates them.

Striped Volumes:

Data is distributed across multiple physical volumes in an interleaved pattern. Each "stripe" writes to a different PV in round-robin fashion. This improves I/O parallelism and throughput for workloads that can benefit from concurrent disk access.

Mirrored Volumes:

Data is duplicated across multiple physical volumes. Every write goes to all mirrors simultaneously, providing redundancy. If one PV fails, data remains available from surviving mirrors.

RAID Volumes:

LVM implements standard RAID levels (0, 1, 4, 5, 6, 10) natively using dm-raid. These provide various combinations of striping, mirroring, and parity for performance and redundancy.

Thin Volumes:

Thin provisioning volumes allocate physical storage on-demand rather than upfront. Covered in detail on Page 5.

Logical Volume Type Comparison
Type	Performance	Redundancy	Space Efficiency	Use Case
Linear	Good (single disk)	None	100%	General storage, databases
Striped	Excellent (parallel I/O)	None	100%	High-throughput workloads
Mirror (RAID1)	Good read, slower write	Full redundancy	50%	Critical data, boot volumes
RAID5	Good balanced	Single disk fault tolerance	~(n-1)/n	Balanced performance/protection
RAID6	Good balanced	Dual disk fault tolerance	~(n-2)/n	High availability requirements
RAID10	Excellent	Full redundancy	50%	High-performance with protection
Thin	Good with overhead	None (pool-level)	Variable	Over-committed environments

Converting Mermaid diagram...

Linear vs Striped Decision

Default to linear volumes unless you have a specific performance requirement. Striped volumes improve throughput for large sequential I/O but add complexity and require all stripe PVs to extend the volume. For most database and application workloads, the file system and I/O scheduler already optimize access patterns effectively.

Creating Logical Volumes

The lvcreate command creates logical volumes with extensive options for size, type, naming, and allocation. Mastering these options enables precise control over storage allocation.

Size Specification Methods:

LVM offers flexible ways to specify volume size:

-L size: Absolute size (e.g., -L 10G, -L 500M, -L 1T)
-l extents: Size in logical extents (e.g., -l 100 = 100 extents)
-l %FREE: Percentage of free space (e.g., -l 50%FREE = half of remaining)
-l %VG: Percentage of total VG size
-l %PVS: Percentage of specified PVs
-l 100%FREE: All remaining free space

Creating Linear Volumes
Bash
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
#!/bin/bash
# Creating Linear Logical Volumes
 
# === BASIC LINEAR VOLUME CREATION ===
 
# Create 50GB linear volume named lv_data
lvcreate -L 50G -n lv_data vg_storage
 
# Create volume using extent count (assumes 4MB extents)
# 12800 extents × 4MB = 50GB
lvcreate -l 12800 -n lv_data vg_storage
 
# Create volume using all free space
lvcreate -l 100%FREE -n lv_remaining vg_storage
 
# Create volume using 50% of free space
lvcreate -l 50%FREE -n lv_half vg_storage
 
# === SPECIFYING PHYSICAL VOLUME LOCATION ===
 
# Create volume only on specific PV
lvcreate -L 50G -n lv_ssd vg_mixed /dev/nvme0n1p2
 
# Create volume spanning specific PVs (in order)
lvcreate -L 100G -n lv_large vg_storage /dev/sdb /dev/sdc
 
# Create volume on PVs with specific tag
lvcreate -L 50G -n lv_fast vg_mixed @tier_ssd
 
# === CONTIGUOUS ALLOCATION ===
 
# Require physically contiguous extents (best performance)
lvcreate -L 50G -n lv_contiguous --alloc contiguous vg_storage
 
# This will fail if 50GB of contiguous space isn't available
 
# === SPECIFYING PE RANGE ===
 
# Allocate from specific PE range on a PV
# Syntax: PV[:PE-PE]
lvcreate -L 20G -n lv_specific vg_storage /dev/sdb:0-5119
 
# === ZERO INITIALIZATION ===
 
# Create volume and zero all blocks (security, clean state)
lvcreate -L 50G -n lv_zeroed --zero y vg_storage
 
# Skip zeroing (faster, for trusted environments)
lvcreate -L 50G -n lv_fast --zero n vg_storage
 
# === ACTIVATION CONTROL ===
 
# Create but don't activate (useful for scripting)
lvcreate -L 50G -n lv_inactive --activate n vg_storage
 
# Create with specific activation mode
lvcreate -L 50G -n lv_local --activate ay vg_storage  # auto-yes
lvcreate -L 50G -n lv_cluster --activate ey vg_storage  # exclusive
 
# === VERIFY CREATION ===
 
# Display the new volume
lvdisplay /dev/vg_storage/lv_data
 
# Show segment information
lvs -o lv_name,seg_count,seg_start_pe,devices /dev/vg_storage/lv_data

RAID Is Not Backup

RAID protects against hardware failure, not data corruption, accidental deletion, or software bugs. Files corrupted by application errors are corrupted identically across all RAID members. Always maintain proper backups regardless of RAID configuration.

Logical Volume Device Paths

Every logical volume is accessible through multiple device paths. Understanding these paths is essential for scripting, configuration, and troubleshooting.

Device Path Formats:

LV Device Paths
Bash
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
#!/bin/bash
# Logical Volume Device Paths
 
# For a logical volume named 'lv_data' in volume group 'vg_storage':
 
# === TRADITIONAL LVM PATH ===
# Symbolic link maintained by LVM
/dev/vg_storage/lv_data
 
# This is a symlink to the device-mapper device:
ls -la /dev/vg_storage/lv_data
# lrwxrwxrwx 1 root root 7 Jan 15 10:00 /dev/vg_storage/lv_data -> ../dm-3
 
# === DEVICE-MAPPER PATH ===
# The actual dm device created by device-mapper
/dev/mapper/vg_storage-lv_data
 
# Note the naming convention: VG-LV with hyphens
# Hyphens in names are doubled: vg-name/lv-data becomes vg--name-lv--data
 
ls -la /dev/mapper/vg_storage-lv_data
# lrwxrwxrwx 1 root root 7 Jan 15 10:00 /dev/mapper/vg_storage-lv_data -> ../dm-3
 
# === RAW DM DEVICE ===
# The underlying device node (dm-N where N is a number)
/dev/dm-3
 
# Not recommended for configuration files - number can change!
 
# === WHICH PATH TO USE? ===
 
# For /etc/fstab: Use the mapper path or VG/LV path
#   /dev/mapper/vg_storage-lv_data   /data   ext4   defaults   0 2
# Or use UUID (most reliable):
#   UUID=a1b2c3d4-...                /data   ext4   defaults   0 2
 
# For scripts: Use /dev/mapper path (works consistently)
mkfs.ext4 /dev/mapper/vg_storage-lv_data
 
# For documentation: Use /dev/VG/LV format (most readable)
mount /dev/vg_storage/lv_data /mnt/data
 
# === FINDING UUID ===
 
blkid /dev/vg_storage/lv_data
# /dev/vg_storage/lv_data: UUID="a1b2c3d4-5e6f-7a8b-9c0d-e1f2a3b4c5d6" TYPE="ext4"
 
# LVM also assigns its own UUID (different from filesystem UUID)
lvs -o lv_name,lv_uuid /dev/vg_storage/lv_data
 
# === DEVICE-MAPPER INTERNALS ===
 
# List all device-mapper devices
dmsetup ls
# vg_storage-lv_data    (253:3)
 
# Show mapping table for an LV
dmsetup table vg_storage-lv_data
# 0 104857600 linear 8:17 2048
 
# Interpretation:
# 0 = start sector of this segment
# 104857600 = number of sectors (50GB)
# linear = target type
# 8:17 = major:minor of underlying device (/dev/sdb1)
# 2048 = offset on underlying device

UUID for Stability

In /etc/fstab, prefer UUID references over device paths. While /dev/mapper paths are stable, UUIDs survive VG renames and are the most portable. Use 'blkid' to find the filesystem UUID (not the LVM UUID) for fstab entries.

Resizing Logical Volumes

The ability to resize logical volumes—often online, without unmounting—is one of LVM's most valuable features. However, resizing involves two distinct operations that must be performed correctly:

Resizing the Logical Volume: Changes the block device size
Resizing the File System: Changes the usable capacity

These operations have different requirements and constraints depending on whether you're growing or shrinking.

Converting Mermaid diagram...

LV Resize Operations
Bash
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
#!/bin/bash
# Resizing Logical Volumes
 
# ============================================
# EXTENDING (GROWING) LOGICAL VOLUMES
# ============================================
 
# === EXTEND LV, THEN FS ===
 
# Step 1: Extend the logical volume by 20GB
lvextend -L +20G /dev/vg_storage/lv_data
 
# Step 2a: Extend ext4 filesystem (online capable)
resize2fs /dev/vg_storage/lv_data
 
# Step 2b: Extend XFS filesystem (online ONLY)
xfs_growfs /mnt/data   # XFS uses mount point, not device!
 
# === EXTEND LV + FS IN ONE COMMAND ===
 
# The -r flag automatically resizes the filesystem
lvextend -L +20G -r /dev/vg_storage/lv_data
 
# This works for ext4, XFS, and other supported filesystems
# Much safer - no chance of forgetting the FS resize
 
# === SIZE SPECIFICATION OPTIONS ===
 
# Extend to absolute size
lvextend -L 100G -r /dev/vg_storage/lv_data
 
# Extend by specific amount
lvextend -L +50G -r /dev/vg_storage/lv_data
 
# Extend by extent count
lvextend -l +1000 -r /dev/vg_storage/lv_data
 
# Use all remaining free space in VG
lvextend -l +100%FREE -r /dev/vg_storage/lv_data
 
# Use specific percentage of VG
lvextend -l 50%VG -r /dev/vg_storage/lv_data
 
# ============================================
# REDUCING (SHRINKING) LOGICAL VOLUMES
# ============================================
 
# WARNING: Shrinking is DANGEROUS if done incorrectly!
# You MUST shrink the filesystem BEFORE shrinking the LV!
# Otherwise you will DESTROY your filesystem!
 
# === SAFE SHRINK PROCEDURE FOR EXT4 ===
 
# Step 1: Unmount the filesystem
umount /mnt/data
 
# Step 2: Check filesystem integrity
e2fsck -f /dev/vg_storage/lv_data
 
# Step 3: Shrink the filesystem to target size
# MUST be smaller than or equal to LV target size
resize2fs /dev/vg_storage/lv_data 80G
 
# Step 4: Shrink the logical volume
# Use equal or slightly larger size than FS
lvreduce -L 80G /dev/vg_storage/lv_data
 
# Step 5: Remount
mount /dev/vg_storage/lv_data /mnt/data
 
# === COMBINED SHRINK COMMAND ===
 
# The -r flag handles FS resize automatically
# Still requires unmount for ext4 shrinking!
umount /mnt/data
lvreduce -L 80G -r /dev/vg_storage/lv_data
mount /dev/vg_storage/lv_data /mnt/data
 
# === XFS CANNOT BE SHRUNK ===
 
# XFS does not support shrinking!
# To reduce XFS volume:
# 1. Backup data
# 2. Create new smaller volume
# 3. Format with XFS
# 4. Restore data
# 5. Delete old volume
 
# === FORCE REDUCE (DANGEROUS!) ===
 
# Only use if filesystem is already smaller than target LV size
# Or if you're intentionally destroying the filesystem
lvreduce -f -L 50G /dev/vg_storage/lv_data
 
# ============================================
# RESIZE (ALTERNATIVE COMMAND)
# ============================================
 
# lvresize can both grow and shrink
lvresize -L 100G -r /dev/vg_storage/lv_data   # Set absolute size
lvresize -L +20G -r /dev/vg_storage/lv_data   # Grow by 20G
lvresize -L -20G -r /dev/vg_storage/lv_data   # Shrink by 20G

Critical: Shrink Order Matters

When shrinking, ALWAYS shrink the filesystem FIRST, then the LV. Shrinking the LV before the filesystem truncates the filesystem, causing catastrophic data loss. The -r flag handles this automatically, but if doing manual operations, never forget this order. Growing is the opposite: extend LV first, then filesystem.

Filesystem Resize Capabilities
Filesystem	Online Grow	Offline Grow	Online Shrink	Offline Shrink
ext4	✅ Yes	✅ Yes	❌ No	✅ Yes
XFS	✅ Yes	✅ Yes	❌ No	❌ No
Btrfs	✅ Yes	✅ Yes	✅ Yes	✅ Yes
ReiserFS	✅ Yes	✅ Yes	❌ No	✅ Yes
NTFS	❌ No	✅ Yes	❌ No	✅ Yes

LV Activation and State Management

Logical volumes must be activated before they can be mounted or used. Activation loads the device-mapper configuration into the kernel and creates the /dev entries. Understanding activation is crucial for troubleshooting boot issues and managing shared storage.

Activation States:

An LV can be in one of several states:

Active (a): Loaded in device-mapper, accessible for I/O
Inactive (-): Defined in metadata but not accessible
Exclusive (e): Active and locked for single-host access (clustered LVM)

LV Activation Management
Bash
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
#!/bin/bash
# Logical Volume Activation Management
 
# === CHECKING ACTIVATION STATE ===
 
# Show all LVs with activation state
lvs -o lv_name,lv_attr,lv_active
 
# The 5th character in lv_attr shows activation:
# 'a' = active
# '-' = inactive
# 's' = suspended
# 'I' = invalid snapshot
# 'd' = device is present without tables
 
# Example output:
#   LV      Attr       Active
#   lv_data -wi-a----- active
 
# === MANUAL ACTIVATION/DEACTIVATION ===
 
# Activate a single LV
lvchange -ay /dev/vg_storage/lv_data
 
# Deactivate a single LV (must be unmounted first!)
umount /mnt/data
lvchange -an /dev/vg_storage/lv_data
 
# Activate all LVs in a VG
vgchange -ay vg_storage
 
# Deactivate all LVs in a VG
vgchange -an vg_storage
 
# === ACTIVATION MODES ===
 
# Standard activation (local)
lvchange -ay /dev/vg_storage/lv_data
 
# Exclusive activation (for clustered environments)
# Prevents other nodes from activating
lvchange -aey /dev/vg_storage/lv_data
 
# Shared activation (for clustered environments)
# Multiple nodes can activate simultaneously
lvchange -asy /dev/vg_storage/lv_data
 
# === AUTO-ACTIVATION CONTROL ===
 
# Set LV to auto-activate at boot
lvchange --setactivationskip n /dev/vg_storage/lv_data
 
# Prevent LV from auto-activating
lvchange --setactivationskip y /dev/vg_storage/lv_data
 
# Check auto-activation setting (look for 'k' in attr)
lvs -o lv_name,lv_attr vg_storage
# 'k' in position 10 = activation skip enabled
 
# === BOOT-TIME ACTIVATION ===
 
# LVs are typically activated during boot by:
# 1. systemd lvm2-activation-generator
# 2. initramfs with lvm2 hook
# 3. dracut lvm module
 
# Force regeneration of initramfs with LVM support
# Debian/Ubuntu:
update-initramfs -u
 
# RHEL/CentOS/Fedora:
dracut -f
 
# === TROUBLESHOOTING ACTIVATION ===
 
# Scan for all PVs and update cache
pvscan --cache
 
# Activate with verbose output
vgchange -ay -vvvv vg_storage 2>&1 | tee /tmp/lvm_debug.log
 
# Check if device-mapper module is loaded
lsmod | grep dm_mod
 
# Verify dm device was created
ls -la /dev/mapper/vg_storage-lv_data
dmsetup ls | grep vg_storage
 
# Force activation even with partial VG
vgchange -ay --partial vg_storage
 
# === SUSPEND AND RESUME ===
 
# Suspend I/O to an LV (for consistent snapshots)
lvchange --suspend /dev/vg_storage/lv_data
 
# Resume I/O
lvchange --refresh /dev/vg_storage/lv_data
 
# Refresh device-mapper table (after pvmove, etc.)
lvchange --refresh /dev/vg_storage/lv_data

Root LV Activation

If your root filesystem is on LVM, activation happens in the initramfs before the root filesystem is mounted. If you're having boot issues with LVM root volumes, ensure your initramfs includes LVM support and rebuild it with 'update-initramfs -u' or 'dracut -f' as appropriate for your distribution.

Converting Between LV Types

LVM allows converting logical volumes between different types—adding mirrors, converting to RAID, or changing stripe configurations. The lvconvert command handles these transformations, often online.

Common Conversions:

LV Type Conversions
Bash
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
#!/bin/bash
# Converting Between Logical Volume Types
 
# === LINEAR TO MIRRORED ===
 
# Add a mirror to a linear volume (creates RAID1)
lvconvert --type raid1 -m 1 /dev/vg_storage/lv_data
 
# This triggers a synchronization - check progress
lvs -o lv_name,copy_percent /dev/vg_storage/lv_data
 
# Specify which PV for the new mirror leg
lvconvert --type raid1 -m 1 /dev/vg_storage/lv_data /dev/sdc
 
# === MIRRORED TO LINEAR ===
 
# Remove mirror, keeping data on one leg
lvconvert -m 0 /dev/vg_storage/lv_data
 
# Specify which PV's data to keep
lvconvert -m 0 /dev/vg_storage/lv_data /dev/sdb
 
# === LINEAR TO RAID5 ===
 
# Convert linear to RAID5 (requires free space for parity)
lvconvert --type raid5 -i 2 /dev/vg_storage/lv_data
 
# This is a complex operation:
# 1. Allocates space for parity
# 2. Reshapes data layout
# 3. Calculates and writes parity
 
# === RAID1 TO RAID5 ===
 
# Convert RAID1 to RAID5 for space efficiency
lvconvert --type raid5 /dev/vg_storage/lv_mirror
 
# === RAID5 TO RAID6 ===
 
# Add extra parity for dual-failure protection
lvconvert --type raid6 /dev/vg_storage/lv_raid5
 
# === ADD/REMOVE MIRRORS ===
 
# Add another mirror to existing RAID1
lvconvert -m +1 /dev/vg_storage/lv_mirror
 
# Remove a mirror leg
lvconvert -m -1 /dev/vg_storage/lv_mirror
 
# === REPLACE FAILED DEVICE IN RAID ===
 
# Replace a specific PV in a RAID array
lvconvert --replace /dev/failed_disk /dev/vg_storage/lv_raid \
    /dev/replacement_disk
 
# === STRIPE CONVERSION ===
 
# Note: Stripe count changes require reshape support
# Converting between stripe counts is version-dependent
 
# === CONVERSION TO/FROM THIN ===
 
# Convert linear to thin (requires thin pool)
# First create a thin pool if not exists
lvcreate --type thin-pool -L 100G -n thin_pool vg_storage
 
# Convert regular LV to thin LV
lvconvert --type thin --thinpool vg_storage/thin_pool \
    /dev/vg_storage/lv_data
 
# === MONITORING CONVERSION ===
 
# Watch conversion progress
watch -n 1 'lvs -o lv_name,copy_percent,sync_percent vg_storage'
 
# Check RAID synchronization status
lvs -o lv_name,seg_type,data_percent,sync_percent
 
# === CONVERSION OPTIONS ===
 
# Run conversion in background
lvconvert --background --type raid1 -m 1 /dev/vg_storage/lv_data
 
# Set synchronization rate limits
lvconvert --type raid1 -m 1 \
    --minrecoveryrate 1024 \  # KB/s minimum
    --maxrecoveryrate 10240 \ # KB/s maximum
    /dev/vg_storage/lv_data

Conversion Resource Requirements

Type conversions often require significant free space in the VG (for new mirror legs or parity) and can take hours for large volumes. Plan conversions during low-activity periods. While conversions are typically online-safe, system crashes during conversion can leave volumes in intermediate states requiring recovery.

Logical Volume Best Practices

Effective logical volume design requires balancing performance, flexibility, and maintainability. These best practices derive from real-world production experience.

Production Recommendations

•Use -r flag for resize operations: Automatically handles filesystem resize, prevents order-of-operations errors
•Separate data and logs: Different LVs allow independent sizing and performance tuning
•Use meaningful names: lv_mysql_data is self-documenting; lv_vol1 is not
•Leave VG free space: 15-20% for snapshots, emergency growth, pvmove operations
•Monitor growth trends: Set alerts at 80% capacity to allow proactive expansion
•Document stripe and RAID configs: Future admins need to know the design rationale

Common Mistakes

•Forgetting filesystem resize: Volume grows but usable space doesn't increase
•XFS shrink attempts: XFS cannot shrink; plan ahead or use ext4
•Shrinking LV before FS: Catastrophic data loss—always shrink FS first
•Creating too many small LVs: Hard to manage, wastes metadata overhead
•Using raw dm-N paths in configs: These numbers can change; use mapper paths
•Ignoring striped volume extend requirements: All stripe PVs need free space

Filesystem Selection for LV:

Use Case	Recommended FS	Rationale
Database data files	XFS or ext4	XFS for large files, ext4 for general purpose
Web server content	ext4	Good all-around performance, shrink capability
Large media files	XFS	Excellent large file performance
Container storage	XFS or Btrfs	Overlay/devicemapper backend support
Boot volume	ext4	Universal bootloader support
Frequently resized	ext4 or Btrfs	XFS can't shrink

Automation Tip

Create wrapper scripts for common resize operations that include pre-checks (space available, filesystem type), use the -r flag, and log operations. This prevents human error and creates an audit trail. Example: 'extend_lv vg_storage lv_data +10G' that handles all safety checks.

Summary: Logical Volumes

Logical volumes are the user-facing layer of LVM—the virtualized block devices that applications and file systems interact with. Let's consolidate the essential concepts:

Key Takeaways

•Multiple LV types exist: Linear (simple), striped (performance), mirrored/RAID (redundancy), thin (overcommit)
•Size specification is flexible: Absolute (-L), extents (-l), percentages (%FREE, %VG)
•Device paths vary: /dev/vg/lv, /dev/mapper/vg-lv, and dm-N paths all refer to the same device
•Resizing is two operations: LV resize + filesystem resize; use -r flag to combine them
•Grow order: LV then FS: Extend LV first, then grow filesystem
•Shrink order: FS then LV: Shrink filesystem first, then reduce LV—critical for data safety
•Activation is required: LVs must be activated before use; managed at boot via initramfs
•Type conversion is possible: lvconvert can add mirrors, create RAID, or change configurations online

What's Next:

With logical volumes mastered, we turn to one of LVM's most powerful features: Snapshots. The next page explores how LVM creates point-in-time copies of volumes using copy-on-write mechanics, enabling consistent backups, testing, and rollback capabilities without doubling storage consumption.

Page Complete

You now possess comprehensive knowledge of LVM Logical Volumes—from creation through resizing, activation, and type conversion. This foundation prepares you for advanced topics including snapshots and thin provisioning.

3 / 5

Loading learning content...

Operating SystemsStorage Management

Storage Virtualization

LevelAdvanced

Duration90 mins

TopicStorage Management

3 / 5

Logical Volumes

The Application-Facing Layer

This page provides comprehensive coverage of logical volume types, creation strategies, resizing mechanics, and the critical relationship between LVs and the file systems that inhabit them.

What You Will Learn

Logical Volume Types

LVM supports multiple logical volume types, each optimized for different use cases. Understanding these types and their trade-offs is essential for effective storage design.

Linear Volumes:

Striped Volumes:

Mirrored Volumes:

Data is duplicated across multiple physical volumes. Every write goes to all mirrors simultaneously, providing redundancy. If one PV fails, data remains available from surviving mirrors.

RAID Volumes:

LVM implements standard RAID levels (0, 1, 4, 5, 6, 10) natively using dm-raid. These provide various combinations of striping, mirroring, and parity for performance and redundancy.

Thin Volumes:

Thin provisioning volumes allocate physical storage on-demand rather than upfront. Covered in detail on Page 5.

Logical Volume Type Comparison
Type	Performance	Redundancy	Space Efficiency	Use Case
Linear	Good (single disk)	None	100%	General storage, databases
Striped	Excellent (parallel I/O)	None	100%	High-throughput workloads
Mirror (RAID1)	Good read, slower write	Full redundancy	50%	Critical data, boot volumes
RAID5	Good balanced	Single disk fault tolerance	~(n-1)/n	Balanced performance/protection
RAID6	Good balanced	Dual disk fault tolerance	~(n-2)/n	High availability requirements
RAID10	Excellent	Full redundancy	50%	High-performance with protection
Thin	Good with overhead	None (pool-level)	Variable	Over-committed environments

Converting Mermaid diagram...

Linear vs Striped Decision

Creating Logical Volumes

The lvcreate command creates logical volumes with extensive options for size, type, naming, and allocation. Mastering these options enables precise control over storage allocation.

Size Specification Methods:

LVM offers flexible ways to specify volume size:

-L size: Absolute size (e.g., -L 10G, -L 500M, -L 1T)
-l extents: Size in logical extents (e.g., -l 100 = 100 extents)
-l %FREE: Percentage of free space (e.g., -l 50%FREE = half of remaining)
-l %VG: Percentage of total VG size
-l %PVS: Percentage of specified PVs
-l 100%FREE: All remaining free space

Creating Linear Volumes
Bash
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
#!/bin/bash
# Creating Linear Logical Volumes
 
# === BASIC LINEAR VOLUME CREATION ===
 
# Create 50GB linear volume named lv_data
lvcreate -L 50G -n lv_data vg_storage
 
# Create volume using extent count (assumes 4MB extents)
# 12800 extents × 4MB = 50GB
lvcreate -l 12800 -n lv_data vg_storage
 
# Create volume using all free space
lvcreate -l 100%FREE -n lv_remaining vg_storage
 
# Create volume using 50% of free space
lvcreate -l 50%FREE -n lv_half vg_storage
 
# === SPECIFYING PHYSICAL VOLUME LOCATION ===
 
# Create volume only on specific PV
lvcreate -L 50G -n lv_ssd vg_mixed /dev/nvme0n1p2
 
# Create volume spanning specific PVs (in order)
lvcreate -L 100G -n lv_large vg_storage /dev/sdb /dev/sdc
 
# Create volume on PVs with specific tag
lvcreate -L 50G -n lv_fast vg_mixed @tier_ssd
 
# === CONTIGUOUS ALLOCATION ===
 
# Require physically contiguous extents (best performance)
lvcreate -L 50G -n lv_contiguous --alloc contiguous vg_storage
 
# This will fail if 50GB of contiguous space isn't available
 
# === SPECIFYING PE RANGE ===
 
# Allocate from specific PE range on a PV
# Syntax: PV[:PE-PE]
lvcreate -L 20G -n lv_specific vg_storage /dev/sdb:0-5119
 
# === ZERO INITIALIZATION ===
 
# Create volume and zero all blocks (security, clean state)
lvcreate -L 50G -n lv_zeroed --zero y vg_storage
 
# Skip zeroing (faster, for trusted environments)
lvcreate -L 50G -n lv_fast --zero n vg_storage
 
# === ACTIVATION CONTROL ===
 
# Create but don't activate (useful for scripting)
lvcreate -L 50G -n lv_inactive --activate n vg_storage
 
# Create with specific activation mode
lvcreate -L 50G -n lv_local --activate ay vg_storage  # auto-yes
lvcreate -L 50G -n lv_cluster --activate ey vg_storage  # exclusive
 
# === VERIFY CREATION ===
 
# Display the new volume
lvdisplay /dev/vg_storage/lv_data
 
# Show segment information
lvs -o lv_name,seg_count,seg_start_pe,devices /dev/vg_storage/lv_data

RAID Is Not Backup

Logical Volume Device Paths

Every logical volume is accessible through multiple device paths. Understanding these paths is essential for scripting, configuration, and troubleshooting.

Device Path Formats:

LV Device Paths
Bash
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
#!/bin/bash
# Logical Volume Device Paths
 
# For a logical volume named 'lv_data' in volume group 'vg_storage':
 
# === TRADITIONAL LVM PATH ===
# Symbolic link maintained by LVM
/dev/vg_storage/lv_data
 
# This is a symlink to the device-mapper device:
ls -la /dev/vg_storage/lv_data
# lrwxrwxrwx 1 root root 7 Jan 15 10:00 /dev/vg_storage/lv_data -> ../dm-3
 
# === DEVICE-MAPPER PATH ===
# The actual dm device created by device-mapper
/dev/mapper/vg_storage-lv_data
 
# Note the naming convention: VG-LV with hyphens
# Hyphens in names are doubled: vg-name/lv-data becomes vg--name-lv--data
 
ls -la /dev/mapper/vg_storage-lv_data
# lrwxrwxrwx 1 root root 7 Jan 15 10:00 /dev/mapper/vg_storage-lv_data -> ../dm-3
 
# === RAW DM DEVICE ===
# The underlying device node (dm-N where N is a number)
/dev/dm-3
 
# Not recommended for configuration files - number can change!
 
# === WHICH PATH TO USE? ===
 
# For /etc/fstab: Use the mapper path or VG/LV path
#   /dev/mapper/vg_storage-lv_data   /data   ext4   defaults   0 2
# Or use UUID (most reliable):
#   UUID=a1b2c3d4-...                /data   ext4   defaults   0 2
 
# For scripts: Use /dev/mapper path (works consistently)
mkfs.ext4 /dev/mapper/vg_storage-lv_data
 
# For documentation: Use /dev/VG/LV format (most readable)
mount /dev/vg_storage/lv_data /mnt/data
 
# === FINDING UUID ===
 
blkid /dev/vg_storage/lv_data
# /dev/vg_storage/lv_data: UUID="a1b2c3d4-5e6f-7a8b-9c0d-e1f2a3b4c5d6" TYPE="ext4"
 
# LVM also assigns its own UUID (different from filesystem UUID)
lvs -o lv_name,lv_uuid /dev/vg_storage/lv_data
 
# === DEVICE-MAPPER INTERNALS ===
 
# List all device-mapper devices
dmsetup ls
# vg_storage-lv_data    (253:3)
 
# Show mapping table for an LV
dmsetup table vg_storage-lv_data
# 0 104857600 linear 8:17 2048
 
# Interpretation:
# 0 = start sector of this segment
# 104857600 = number of sectors (50GB)
# linear = target type
# 8:17 = major:minor of underlying device (/dev/sdb1)
# 2048 = offset on underlying device

UUID for Stability

Resizing Logical Volumes

The ability to resize logical volumes—often online, without unmounting—is one of LVM's most valuable features. However, resizing involves two distinct operations that must be performed correctly:

Resizing the Logical Volume: Changes the block device size
Resizing the File System: Changes the usable capacity

These operations have different requirements and constraints depending on whether you're growing or shrinking.

Converting Mermaid diagram...

LV Resize Operations
Bash
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
#!/bin/bash
# Resizing Logical Volumes
 
# ============================================
# EXTENDING (GROWING) LOGICAL VOLUMES
# ============================================
 
# === EXTEND LV, THEN FS ===
 
# Step 1: Extend the logical volume by 20GB
lvextend -L +20G /dev/vg_storage/lv_data
 
# Step 2a: Extend ext4 filesystem (online capable)
resize2fs /dev/vg_storage/lv_data
 
# Step 2b: Extend XFS filesystem (online ONLY)
xfs_growfs /mnt/data   # XFS uses mount point, not device!
 
# === EXTEND LV + FS IN ONE COMMAND ===
 
# The -r flag automatically resizes the filesystem
lvextend -L +20G -r /dev/vg_storage/lv_data
 
# This works for ext4, XFS, and other supported filesystems
# Much safer - no chance of forgetting the FS resize
 
# === SIZE SPECIFICATION OPTIONS ===
 
# Extend to absolute size
lvextend -L 100G -r /dev/vg_storage/lv_data
 
# Extend by specific amount
lvextend -L +50G -r /dev/vg_storage/lv_data
 
# Extend by extent count
lvextend -l +1000 -r /dev/vg_storage/lv_data
 
# Use all remaining free space in VG
lvextend -l +100%FREE -r /dev/vg_storage/lv_data
 
# Use specific percentage of VG
lvextend -l 50%VG -r /dev/vg_storage/lv_data
 
# ============================================
# REDUCING (SHRINKING) LOGICAL VOLUMES
# ============================================
 
# WARNING: Shrinking is DANGEROUS if done incorrectly!
# You MUST shrink the filesystem BEFORE shrinking the LV!
# Otherwise you will DESTROY your filesystem!
 
# === SAFE SHRINK PROCEDURE FOR EXT4 ===
 
# Step 1: Unmount the filesystem
umount /mnt/data
 
# Step 2: Check filesystem integrity
e2fsck -f /dev/vg_storage/lv_data
 
# Step 3: Shrink the filesystem to target size
# MUST be smaller than or equal to LV target size
resize2fs /dev/vg_storage/lv_data 80G
 
# Step 4: Shrink the logical volume
# Use equal or slightly larger size than FS
lvreduce -L 80G /dev/vg_storage/lv_data
 
# Step 5: Remount
mount /dev/vg_storage/lv_data /mnt/data
 
# === COMBINED SHRINK COMMAND ===
 
# The -r flag handles FS resize automatically
# Still requires unmount for ext4 shrinking!
umount /mnt/data
lvreduce -L 80G -r /dev/vg_storage/lv_data
mount /dev/vg_storage/lv_data /mnt/data
 
# === XFS CANNOT BE SHRUNK ===
 
# XFS does not support shrinking!
# To reduce XFS volume:
# 1. Backup data
# 2. Create new smaller volume
# 3. Format with XFS
# 4. Restore data
# 5. Delete old volume
 
# === FORCE REDUCE (DANGEROUS!) ===
 
# Only use if filesystem is already smaller than target LV size
# Or if you're intentionally destroying the filesystem
lvreduce -f -L 50G /dev/vg_storage/lv_data
 
# ============================================
# RESIZE (ALTERNATIVE COMMAND)
# ============================================
 
# lvresize can both grow and shrink
lvresize -L 100G -r /dev/vg_storage/lv_data   # Set absolute size
lvresize -L +20G -r /dev/vg_storage/lv_data   # Grow by 20G
lvresize -L -20G -r /dev/vg_storage/lv_data   # Shrink by 20G

Critical: Shrink Order Matters

Filesystem Resize Capabilities
Filesystem	Online Grow	Offline Grow	Online Shrink	Offline Shrink
ext4	✅ Yes	✅ Yes	❌ No	✅ Yes
XFS	✅ Yes	✅ Yes	❌ No	❌ No
Btrfs	✅ Yes	✅ Yes	✅ Yes	✅ Yes
ReiserFS	✅ Yes	✅ Yes	❌ No	✅ Yes
NTFS	❌ No	✅ Yes	❌ No	✅ Yes

LV Activation and State Management

Activation States:

An LV can be in one of several states:

Active (a): Loaded in device-mapper, accessible for I/O
Inactive (-): Defined in metadata but not accessible
Exclusive (e): Active and locked for single-host access (clustered LVM)

LV Activation Management
Bash
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
#!/bin/bash
# Logical Volume Activation Management
 
# === CHECKING ACTIVATION STATE ===
 
# Show all LVs with activation state
lvs -o lv_name,lv_attr,lv_active
 
# The 5th character in lv_attr shows activation:
# 'a' = active
# '-' = inactive
# 's' = suspended
# 'I' = invalid snapshot
# 'd' = device is present without tables
 
# Example output:
#   LV      Attr       Active
#   lv_data -wi-a----- active
 
# === MANUAL ACTIVATION/DEACTIVATION ===
 
# Activate a single LV
lvchange -ay /dev/vg_storage/lv_data
 
# Deactivate a single LV (must be unmounted first!)
umount /mnt/data
lvchange -an /dev/vg_storage/lv_data
 
# Activate all LVs in a VG
vgchange -ay vg_storage
 
# Deactivate all LVs in a VG
vgchange -an vg_storage
 
# === ACTIVATION MODES ===
 
# Standard activation (local)
lvchange -ay /dev/vg_storage/lv_data
 
# Exclusive activation (for clustered environments)
# Prevents other nodes from activating
lvchange -aey /dev/vg_storage/lv_data
 
# Shared activation (for clustered environments)
# Multiple nodes can activate simultaneously
lvchange -asy /dev/vg_storage/lv_data
 
# === AUTO-ACTIVATION CONTROL ===
 
# Set LV to auto-activate at boot
lvchange --setactivationskip n /dev/vg_storage/lv_data
 
# Prevent LV from auto-activating
lvchange --setactivationskip y /dev/vg_storage/lv_data
 
# Check auto-activation setting (look for 'k' in attr)
lvs -o lv_name,lv_attr vg_storage
# 'k' in position 10 = activation skip enabled
 
# === BOOT-TIME ACTIVATION ===
 
# LVs are typically activated during boot by:
# 1. systemd lvm2-activation-generator
# 2. initramfs with lvm2 hook
# 3. dracut lvm module
 
# Force regeneration of initramfs with LVM support
# Debian/Ubuntu:
update-initramfs -u
 
# RHEL/CentOS/Fedora:
dracut -f
 
# === TROUBLESHOOTING ACTIVATION ===
 
# Scan for all PVs and update cache
pvscan --cache
 
# Activate with verbose output
vgchange -ay -vvvv vg_storage 2>&1 | tee /tmp/lvm_debug.log
 
# Check if device-mapper module is loaded
lsmod | grep dm_mod
 
# Verify dm device was created
ls -la /dev/mapper/vg_storage-lv_data
dmsetup ls | grep vg_storage
 
# Force activation even with partial VG
vgchange -ay --partial vg_storage
 
# === SUSPEND AND RESUME ===
 
# Suspend I/O to an LV (for consistent snapshots)
lvchange --suspend /dev/vg_storage/lv_data
 
# Resume I/O
lvchange --refresh /dev/vg_storage/lv_data
 
# Refresh device-mapper table (after pvmove, etc.)
lvchange --refresh /dev/vg_storage/lv_data

Root LV Activation

Converting Between LV Types

Common Conversions:

LV Type Conversions
Bash
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
#!/bin/bash
# Converting Between Logical Volume Types
 
# === LINEAR TO MIRRORED ===
 
# Add a mirror to a linear volume (creates RAID1)
lvconvert --type raid1 -m 1 /dev/vg_storage/lv_data
 
# This triggers a synchronization - check progress
lvs -o lv_name,copy_percent /dev/vg_storage/lv_data
 
# Specify which PV for the new mirror leg
lvconvert --type raid1 -m 1 /dev/vg_storage/lv_data /dev/sdc
 
# === MIRRORED TO LINEAR ===
 
# Remove mirror, keeping data on one leg
lvconvert -m 0 /dev/vg_storage/lv_data
 
# Specify which PV's data to keep
lvconvert -m 0 /dev/vg_storage/lv_data /dev/sdb
 
# === LINEAR TO RAID5 ===
 
# Convert linear to RAID5 (requires free space for parity)
lvconvert --type raid5 -i 2 /dev/vg_storage/lv_data
 
# This is a complex operation:
# 1. Allocates space for parity
# 2. Reshapes data layout
# 3. Calculates and writes parity
 
# === RAID1 TO RAID5 ===
 
# Convert RAID1 to RAID5 for space efficiency
lvconvert --type raid5 /dev/vg_storage/lv_mirror
 
# === RAID5 TO RAID6 ===
 
# Add extra parity for dual-failure protection
lvconvert --type raid6 /dev/vg_storage/lv_raid5
 
# === ADD/REMOVE MIRRORS ===
 
# Add another mirror to existing RAID1
lvconvert -m +1 /dev/vg_storage/lv_mirror
 
# Remove a mirror leg
lvconvert -m -1 /dev/vg_storage/lv_mirror
 
# === REPLACE FAILED DEVICE IN RAID ===
 
# Replace a specific PV in a RAID array
lvconvert --replace /dev/failed_disk /dev/vg_storage/lv_raid \
    /dev/replacement_disk
 
# === STRIPE CONVERSION ===
 
# Note: Stripe count changes require reshape support
# Converting between stripe counts is version-dependent
 
# === CONVERSION TO/FROM THIN ===
 
# Convert linear to thin (requires thin pool)
# First create a thin pool if not exists
lvcreate --type thin-pool -L 100G -n thin_pool vg_storage
 
# Convert regular LV to thin LV
lvconvert --type thin --thinpool vg_storage/thin_pool \
    /dev/vg_storage/lv_data
 
# === MONITORING CONVERSION ===
 
# Watch conversion progress
watch -n 1 'lvs -o lv_name,copy_percent,sync_percent vg_storage'
 
# Check RAID synchronization status
lvs -o lv_name,seg_type,data_percent,sync_percent
 
# === CONVERSION OPTIONS ===
 
# Run conversion in background
lvconvert --background --type raid1 -m 1 /dev/vg_storage/lv_data
 
# Set synchronization rate limits
lvconvert --type raid1 -m 1 \
    --minrecoveryrate 1024 \  # KB/s minimum
    --maxrecoveryrate 10240 \ # KB/s maximum
    /dev/vg_storage/lv_data

Conversion Resource Requirements

Logical Volume Best Practices

Effective logical volume design requires balancing performance, flexibility, and maintainability. These best practices derive from real-world production experience.

Production Recommendations

•Use -r flag for resize operations: Automatically handles filesystem resize, prevents order-of-operations errors
•Separate data and logs: Different LVs allow independent sizing and performance tuning
•Use meaningful names: lv_mysql_data is self-documenting; lv_vol1 is not
•Leave VG free space: 15-20% for snapshots, emergency growth, pvmove operations
•Monitor growth trends: Set alerts at 80% capacity to allow proactive expansion
•Document stripe and RAID configs: Future admins need to know the design rationale

Common Mistakes

•Forgetting filesystem resize: Volume grows but usable space doesn't increase
•XFS shrink attempts: XFS cannot shrink; plan ahead or use ext4
•Shrinking LV before FS: Catastrophic data loss—always shrink FS first
•Creating too many small LVs: Hard to manage, wastes metadata overhead
•Using raw dm-N paths in configs: These numbers can change; use mapper paths
•Ignoring striped volume extend requirements: All stripe PVs need free space

Filesystem Selection for LV:

Use Case	Recommended FS	Rationale
Database data files	XFS or ext4	XFS for large files, ext4 for general purpose
Web server content	ext4	Good all-around performance, shrink capability
Large media files	XFS	Excellent large file performance
Container storage	XFS or Btrfs	Overlay/devicemapper backend support
Boot volume	ext4	Universal bootloader support
Frequently resized	ext4 or Btrfs	XFS can't shrink

Automation Tip

Summary: Logical Volumes

Logical volumes are the user-facing layer of LVM—the virtualized block devices that applications and file systems interact with. Let's consolidate the essential concepts:

Key Takeaways

•Multiple LV types exist: Linear (simple), striped (performance), mirrored/RAID (redundancy), thin (overcommit)
•Size specification is flexible: Absolute (-L), extents (-l), percentages (%FREE, %VG)
•Device paths vary: /dev/vg/lv, /dev/mapper/vg-lv, and dm-N paths all refer to the same device
•Resizing is two operations: LV resize + filesystem resize; use -r flag to combine them
•Grow order: LV then FS: Extend LV first, then grow filesystem
•Shrink order: FS then LV: Shrink filesystem first, then reduce LV—critical for data safety
•Activation is required: LVs must be activated before use; managed at boot via initramfs
•Type conversion is possible: lvconvert can add mirrors, create RAID, or change configurations online

What's Next:

Page Complete

3 / 5