Linux - LVM

Logical Volume Manager

Overview

Physical Volume

A physical volume is a block device that has to be marked as such to take part in Logical Volume Management. This is the first step in setting up a volume group which we use to create logical volumes which can be used as 'regular' disks.

The physical volume does not have to be a full disk, it can be a partition of a disk. In fact it can be any blockdevice.

# pvcreate <BLOCK_DEVICE>

# pvs

PV VG Fmt Attr PSize PFree

/dev/sda3 cl lvm2 a-- 18.41g 0

# pvdisplay

--- Physical volume ---

PV Name /dev/sda3

VG Name cl

PV Size 18.41 GiB / not usable 2.00 MiB

Allocatable yes (but full)

PE Size 4.00 MiB

Total PE 4713

Free PE 0

Allocated PE 4713

PV UUID zaP4cW-tjgV-Wggs-RHmg-RZlh-PifQ-J1ULI4

Discovering physical volumes

In order to discover block devices on your system you could use

# cat /proc/partitions

# fdisk -l

# lsblk # tree view

# lsblk -l # list view

# fdisk -l

Disk /dev/sda: 20 GiB, 21474836480 bytes, 41943040 sectors

Units: sectors of 1 * 512 = 512 bytes

Sector size (logical/physical): 512 bytes / 512 bytes

I/O size (minimum/optimal): 512 bytes / 512 bytes

Disklabel type: gpt

Disk identifier: 1729418E-F806-434E-94DD-F9FE8948E3B4

Device Start End Sectors Size Type

/dev/sda1 2048 1230847 1228800 600M EFI System

/dev/sda2 1230848 3327999 2097152 1G Linux filesystem

/dev/sda3 3328000 41940991 38612992 18.4G Linux LVM

Disk /dev/mapper/cl-root: 16.4 GiB, 17620271104 bytes, 34414592 sectors

Units: sectors of 1 * 512 = 512 bytes

Sector size (logical/physical): 512 bytes / 512 bytes

I/O size (minimum/optimal): 512 bytes / 512 bytes

Disk /dev/mapper/cl-swap: 2 GiB, 2147483648 bytes, 4194304 sectors

Units: sectors of 1 * 512 = 512 bytes

Sector size (logical/physical): 512 bytes / 512 bytes

I/O size (minimum/optimal): 512 bytes / 512 bytes

# cat /proc/partitions

major minor #blocks name

11 0 1048575 sr0

8 0 20971520 sda

8 1 614400 sda1

8 2 1048576 sda2

8 3 19306496 sda3

253 0 17207296 dm-0

253 1 2097152 dm-1

# lsblk

NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT

sda 8:0 0 20G 0 disk

├─sda1 8:1 0 600M 0 part /boot/efi

├─sda2 8:2 0 1G 0 part /boot

└─sda3 8:3 0 18.4G 0 part

├─cl-root 253:0 0 16.4G 0 lvm /

└─cl-swap 253:1 0 2G 0 lvm [SWAP]

sr0 11:0 1 1024M 0 rom

# lsblk -l

NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT

sda 8:0 0 20G 0 disk

sda1 8:1 0 600M 0 part /boot/efi

sda2 8:2 0 1G 0 part /boot

sda3 8:3 0 18.4G 0 part

sr0 11:0 1 1024M 0 rom

cl-root 253:0 0 16.4G 0 lvm /

cl-swap 253:1 0 2G 0 lvm [SWAP]

Volume Group (VG)

Now that we have physical volumes available we can create volume groups. We can think of a volume group as a pool of available storage space that we can use to create logical volumes (virtual disks).

# lsblk

# pvcreate /dev/sda /dev/sdb /dev/sdc

# vgcreate bigdisk /dev/sda /dev/sdb /dev/sdc

# vgs

# vgdisplay

Let's mark the block devices as physical volumes.

# pvcreate /dev/loop0 /dev/loop1 /dev/loop2

Physical volume "/dev/loop0" successfully created.

Physical volume "/dev/loop1" successfully created.

Physical volume "/dev/loop2" successfully created.

# pvs

PV VG Fmt Attr PSize PFree

/dev/loop0 pool lvm2 a-- 96.00m 96.00m

/dev/loop1 pool lvm2 a-- 96.00m 96.00m

/dev/loop2 pool lvm2 a-- 96.00m 96.00m

# pvdisplay

--- Physical volume ---

PV Name /dev/loop0

VG Name pool

PV Size 100.00 MiB / not usable 4.00 MiB

Allocatable yes

PE Size 4.00 MiB

Total PE 24

Free PE 24

Allocated PE 0

PV UUID t6JwEn-1YhG-MHBo-Atco-TNfF-bhPz-Sp4umx

--- Physical volume ---

PV Name /dev/loop1

VG Name pool

PV Size 100.00 MiB / not usable 4.00 MiB

Allocatable yes

PE Size 4.00 MiB

Total PE 24

Free PE 24

Allocated PE 0

PV UUID xXKauA-Ka73-mIt1-eQCP-vLym-BcqN-mufQBS

--- Physical volume ---

PV Name /dev/loop2

VG Name pool

PV Size 100.00 MiB / not usable 4.00 MiB

Allocatable yes

PE Size 4.00 MiB

Total PE 24

Free PE 24

Allocated PE 0

PV UUID fZcFEl-Jlrf-NEQS-JEwv-LE1e-4OaK-zx6Uis

Now that we have physical volumes we can create volume groups. Let's create a volume group named pool consisting of all three block devices we created previously.

# vgcreate pool /dev/loop0 /dev/loop1 /dev/loop2

Volume group "pool" successfully created

Logical Volume (LV)

Snapshots

Creating a snapshot

When creating a snapshot we actually create a new volume which gets marked as a snapshot volume and it has to reference an existing volume. You have to make sure there is enough space in your volume group to create the snapshot volume (just like a 'regular' logical volume).

# lvs

# lvcreate -s -n <NAME_OF_SNAPSHOT_VOLUME> -L <MAX_SIZE_OF_SNAPSHOT_VOLUME> <LVM_DEVICE_TO_TAKE_SNAPSHOT_OF>

# lvcreate -s -n centos8_snapshot -L 1G /dev/vms/centos8

Using default stripesize 64.00 KiB.

Logical volume "centos8_snapshot" created.

# lvs

LV VG Attr LSize Pool Origin Data% Meta% Move Log Cpy%Sync Convert

centos8 vms owi-aos--- 20.00g

centos8_snapshot vms swi-a-s--- 1.00g centos8 0.00

Resizing Logical Volumes

lvextend
lvreduce
lvresize

Extending Logical Volumes

# lvextend -L+30M -r /dev/pool/data

Rounding size to boundary between physical extents: 32.00 MiB.

Phase 1 - find and verify superblock...

Phase 2 - using internal log

- zero log...

- scan filesystem freespace and inode maps...

- found root inode chunk

Phase 3 - for each AG...

- scan (but don't clear) agi unlinked lists...

- process known inodes and perform inode discovery...

- agno = 0

- agno = 1

- agno = 2

- agno = 3

- process newly discovered inodes...

Phase 4 - check for duplicate blocks...

- setting up duplicate extent list...

- check for inodes claiming duplicate blocks...

- agno = 0

- agno = 1

- agno = 2

- agno = 3

No modify flag set, skipping phase 5

Phase 6 - check inode connectivity...

- traversing filesystem ...

- traversal finished ...

- moving disconnected inodes to lost+found ...

Phase 7 - verify link counts...

No modify flag set, skipping filesystem flush and exiting.

Size of logical volume pool/data changed from 152.00 MiB (38 extents) to 184.00 MiB (46 extents).

Logical volume pool/data successfully resized.

meta-data=/dev/mapper/pool-data isize=512 agcount=4, agsize=9728 blks

= sectsz=512 attr=2, projid32bit=1

= crc=1 finobt=1, sparse=1, rmapbt=0

= reflink=1

data = bsize=4096 blocks=38912, imaxpct=25

= sunit=0 swidth=0 blks

naming =version 2 bsize=4096 ascii-ci=0, ftype=1

log =internal log bsize=4096 blocks=1368, version=2

= sectsz=512 sunit=0 blks, lazy-count=1

realtime =none extsz=4096 blocks=0, rtextents=0

data blocks changed from 38912 to 47104