Add storage with LVM | Opensource.com

Add storage with LVM

LVM enables extreme flexibility in how you configure your storage.

Storage units side by side
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Scott Meyers. Modified by Opensource.com. CC BY-SA 2.0.

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Logical Volume Manager (LVM) allows for a layer of abstraction between the operating system and the hardware. Normally, your OS looks for disks (/dev/sda/dev/sdb, and so on) and partitions within those disks (/dev/sda1/dev/sdb1, and so on).

In LVM, a virtual layer is created between the operating system and the disks. Instead of one drive holding some number of partitions, LVM creates a unified storage pool (called a Volume Group) that spans any number of physical drives (called Physical Volumes). Using the storage available in a Volume Group, LVM provides what appear to be disks and partitions to your OS.

And the operating system is completely unaware that it's being "tricked."

lvm.png

Drive space

Opensource.com, CC BY-SA 4.0

Because the LVM creates volume groups and logical volumes virtually, it makes it easy to resize or move them, or create new volumes, even while the system is running. Additionally, LVM provides features that are not present otherwise, like creating live snapshots of logical volumes, without unmounting the disk first.

A volume group in an LVM is a named virtual container that groups together the underlying physical disks. It acts as a pool from which logical volumes of different sizes can be created. Logical volumes contain the actual file system and can span multiple disks, and don't need to be physically contiguous.

Features

  • Partition names normally have system designations like /dev/sda1. LVM volumes have normal human-understandable names, like home or media.
  • The total size of partitions is limited by the size of the underlying physical disk. In LVM, volumes can span multiple disks, and are only limited by the total size of all physical disks in the LVM.
  • Partitions can normally only be resized, moved, or deleted when the disk is not in use and is unmounted. LVM volumes can be manipulated while the system is running.
  • Partitions can only be expanded by allocating them free space adjacent to the partition. LVM volumes can take free space from anywhere.
  • Expanding a partition involves moving the data around to make free space, which is time-consuming and could lead to data loss during a power outage. LVM volumes can take free space from anywhere in the volume group, even on another disk.
  • Because it’s so easy to create volumes in an LVM, it encourages creating different volumes, like creating separate volumes to test features or to try different operating systems. With partitions, this process would be time-consuming and error-prone.
  • Snapshots can only be created in an LVM. It allows you to create a point-in-time image of the current logical volume, even while the system is running. This is great for backups.

Test setup

As a demonstration, assume your system has the following drive configuration:

NAME    MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
xvda    202:0    0   8G  0 disk
`-xvda1 202:1    0   8G  0 part /
xvdb    202:16   0   1G  0 disk
xvdc    202:32   0   1G  0 disk
xvdd    202:48   0   2G  0 disk
xvde    202:64   0   5G  0 disk
xvdf    202:80   0   8G  0 disk

Step 1. Initialize disks to use with LVM

Run pvcreate /dev/xvdb /dev/xvdc /dev/xvdd /dev/xvde /dev/xvdf. The output should be:

Physical volume "/dev/xvdb" successfully created
Physical volume "/dev/xvdc" successfully created
Physical volume "/dev/xvdd" successfully created
Physical volume "/dev/xvde" successfully created
Physical volume "/dev/xvdf" successfully created

See the result using pvs or pvdisplay:

"/dev/xvde" is a new physical volume of "5.00 GiB"
--- NEW Physical volume ---
PV Name               /dev/xvde
VG Name
PV Size               5.00 GiB
Allocatable           NO
PE Size               0
Total PE              0
Free PE               0
Allocated PE          0
PV UUID               728JtI-ffZD-h2dZ-JKnV-8IOf-YKdS-8srJtn

"/dev/xvdb" is a new physical volume of "1.00 GiB"
--- NEW Physical volume ---
PV Name               /dev/xvdb
VG Name
PV Size               1.00 GiB
Allocatable           NO
PE Size               0
Total PE              0
Free PE               0
Allocated PE          0
PV UUID               zk1phS-7uXc-PjBP-5Pv9-dtAV-zKe6-8OCRkZ

"/dev/xvdd" is a new physical volume of "2.00 GiB"
--- NEW Physical volume ---
PV Name               /dev/xvdd
VG Name
PV Size               2.00 GiB
Allocatable           NO
PE Size               0
Total PE              0
Free PE               0
Allocated PE          0
PV UUID               R0I139-Ipca-KFra-2IZX-o9xJ-IW49-T22fPc

"/dev/xvdc" is a new physical volume of "1.00 GiB"
--- NEW Physical volume ---
PV Name               /dev/xvdc
VG Name
PV Size               1.00 GiB
Allocatable           NO
PE Size               0
Total PE              0
Free PE               0
Allocated PE          0
PV UUID               FDzcVS-sq22-2b13-cYRj-dXHf-QLjS-22Meae

"/dev/xvdf" is a new physical volume of "8.00 GiB"
--- NEW Physical volume ---
PV Name               /dev/xvdf
VG Name
PV Size               8.00 GiB
Allocatable           NO
PE Size               0
Total PE              0
Free PE               0
Allocated PE          0
PV UUID               TRVSH9-Bo5D-JHHb-g0NX-8IoS-GG6T-YV4d0p

Step 2. Create the volume group

Run vgcreate myvg /dev/xvdb /dev/xvdc /dev/xvdd /dev/xvde /dev/xvdf. See the results with vgs or vgdisplay:

--- Volume group ---
VG Name               myvg
System ID
Format                lvm2
Metadata Areas        5
Metadata Sequence No  1
VG Access             read/write
VG Status             resizable
MAX LV                0
Cur LV                0
Open LV               0
Max PV                0
Cur PV                5
Act PV                5
VG Size               16.98 GiB
PE Size               4.00 MiB
Total PE              4347
Alloc PE / Size       0 / 0
Free  PE / Size       4347 / 16.98 GiB
VG UUID               ewrrWp-Tonj-LeFa-4Ogi-BIJJ-vztN-yrepkh

Step 3: Create logical volumes

Run the following commands:

lvcreate myvg --name media --size 4G
lvcreate myvg --name home --size 4G

Verify the results using lvs or lvdisplay:

--- Logical volume ---
LV Path                /dev/myvg/media
LV Name                media
VG Name                myvg
LV UUID                LOBga3-pUNX-ZnxM-GliZ-mABH-xsdF-3VBXFT
LV Write Access        read/write
LV Creation host, time ip-10-0-5-236, 2017-02-03 05:29:15 +0000
LV Status              available
# open                 0
LV Size                4.00 GiB
Current LE             1024
Segments               1
Allocation             inherit
Read ahead sectors     auto
- currently set to     256
Block device           252:0

--- Logical volume ---
LV Path                /dev/myvg/home
LV Name                home
VG Name                myvg
LV UUID                Hc06sl-vtss-DuS0-jfqj-oNce-qKf6-e5qHhK
LV Write Access        read/write
LV Creation host, time ip-10-0-5-236, 2017-02-03 05:29:40 +0000
LV Status              available
# open                 0
LV Size                4.00 GiB
Current LE             1024
Segments               1
Allocation             inherit
Read ahead sectors     auto
- currently set to     256
Block device           252:1

Step 4: Create the file system

Create the file system using:

mkfs.ext3 /dev/myvg/media
mkfs.ext3 /dev/myvg/home

Mount it:

mount /dev/myvg/media /media
mount /dev/myvg/home /home

See your full setup using lsblk:

NAME         MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
xvda         202:0    0   8G  0 disk
`-xvda1      202:1    0   8G  0 part /
xvdb         202:16   0   1G  0 disk
xvdc         202:32   0   1G  0 disk
xvdd         202:48   0   2G  0 disk
xvde         202:64   0   5G  0 disk
`-myvg-media 252:0    0   4G  0 lvm  /media
xvdf         202:80   0   8G  0 disk
`-myvg-home  252:1    0   4G  0 lvm  /home

Step 5: Extending the LVM

Add a new disk at /dev/xvdg. To extend the home volume, run the following commands:

pvcreate /dev/xvdg
vgextend myvg /dev/xvdg
lvextend -l 100%FREE /dev/myvg/home
resize2fs /dev/myvg/home

Run df -h and you should see your new size reflected.

And that's it!

LVM enables extreme flexibility in how you configure your storage. Try it out, and have fun with LVM!


This article was originally published on the author's personal blog and has been adapted with permission.

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About the author

https://ayushsharma.in
Ayush Sharma - I am a writer and AWS Solutions Architect. I work with startups and enterprises on Software Engineering, DevOps, SRE, and Cloud Architecture. I write about my experiences on https://ayushsharma.in.