LVM stands for Logical Volume Manager, it is a tool for file system and disk management on Linux. It can work in different ways like allocating whole devices, but the general usage is putting the LVM on a partition. One of the most common System [Engineer|Administrator] and DevOps interview topic is creating and resizing LVM volumes.
Using LVM gives us flexibility to extend, shrink or snapshot the filesystem. It is possible to mirror or stripe disks or partitions with it. With the snapshotting feature we can create filesystem snapshots. We can add new storage devices easily, and extend the volume of an existing setup. It is also possible to encrypt LVM volumes.
View my video about presenting a disk extension on a virtual Debian Linux!
Physical Volume
Physical Volumes are the storage devices or partitions attached to the system and handled by the LVM. For example the /dev/sdb device or its /dev/sdb1 is a partition.
Volume Group
A Volume Group consists of one or more Physical Volumes to allocate space from them. We can handle a Volume Group as a whole.
Logical Volume
The Logical Volumes are the virtual space allocations from Volume Groups. We can consider them as virtual partitions to create file system on them.
Basic operations with LVM
We can list our attached storage devices with the fdisk command. The following output is from a machine that has already installed LVM. It is the lvm2 package on Debian Linux.
root@debtop:~# fdisk -l
Disk /dev/sda: 30 GiB, 32212254720 bytes, 62914560 sectors
Disk model: VMware Virtual S
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: dos
Disk identifier: 0xdc94f4be
Device Boot Start End Sectors Size Id Type
/dev/sda1 * 2048 999423 997376 487M 83 Linux
/dev/sda2 1001470 62912511 61911042 29.5G 5 Extended
/dev/sda5 1001472 62912511 61911040 29.5G 8e Linux LVM
Disk /dev/mapper/debtop--vg-root: 10.14 GiB, 10888413184 bytes, 21266432 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/debtop--vg-swap_1: 976 MiB, 1023410176 bytes, 1998848 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/debtop--vg-home: 18.43 GiB, 19784531968 bytes, 38641664 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
From the output of the above command we can see that there is only one block device attached to the system. It is on /dev/sda and its size is exactly 30 GB.
There are three partitions on this device, sda[1,2,5]. The sda1 partition is flagged as boot partition, from here we can safely assume that it contains our /boot filesystem. Linux cannot boot from an LVM partition (the LVM is still not available at this time of the boot to read from). The sda2 partition is an extended type, it means it holds one or more further partitions, in this case it is the sda5. We see that this is an LVM partition, its size is 29.5 GB. Furthermore we can assume that the /dev/sda5 is a Physical Volume for LVM and it is added to the debtop-vg volume group.
List the Physical Volumes of the system (pvdisplay)
We can use the pvdisplay command to list the the Physical Volumes.
root@debtop:~# pvdisplay
--- Physical volume ---
PV Name /dev/sda5
VG Name debtop-vg
PV Size 29.52 GiB / not usable 2.00 MiB
Allocatable yes (but full)
PE Size 4.00 MiB
Total PE 7557
Free PE 0
Allocated PE 7557
PV UUID MTO8uI-gRWH-Kc2T-skB0-QR6j-W7mB-ad7xrwThe command output gives us an overall image about the attached volumes to the system. We assumed well that /dev/sda5 is a Physical Volume of the system containing a Volume Group called debtop-vg. We can see the maximum space we can allocate from this PV on the third line.
Investigate the Volume Groups (vgdisplay)
We can use the vgdisplay command to list the Volume Groups.
root@debtop:~# vgdisplay
--- Volume group ---
VG Name debtop-vg
System ID
Format lvm2
Metadata Areas 1
Metadata Sequence No 4
VG Access read/write
VG Status resizable
MAX LV 0
Cur LV 3
Open LV 3
Max PV 0
Cur PV 1
Act PV 1
VG Size <29.52 GiB
PE Size 4.00 MiB
Total PE 7557
Alloc PE / Size 7557 / <29.52 GiB
Free PE / Size 0 / 0
VG UUID 8X1Vdg-OEzV-ekRJ-4DRa-ycFJ-0wmp-soAUiaOur assumption regarding the debtop-vg Volume Group was correct. We can see from the output that currently this VG has three Logical Volumes (LV).
Logical Volumes of the system (lvdisplay)
The Logical Volumes can be displayed by the lvdisplay command.
root@debtop:~# lvdisplay
--- Logical volume ---
LV Path /dev/debtop-vg/root
LV Name root
VG Name debtop-vg
LV UUID zkXB5m-7i1r-BOLd-YFcS-q1Gi-qt3J-ffuz9K
LV Write Access read/write
LV Creation host, time debtop, 2022-12-19 07:51:04 +0100
LV Status available
# open 1
LV Size 10.14 GiB
Current LE 2596
Segments 1
Allocation inherit
Read ahead sectors auto
- currently set to 256
Block device 254:0
--- Logical volume ---
LV Path /dev/debtop-vg/swap_1
LV Name swap_1
VG Name debtop-vg
LV UUID iDl16X-TfL6-2m3V-iTLA-HX1d-OgHr-A2Q6BS
LV Write Access read/write
LV Creation host, time debtop, 2022-12-19 07:51:05 +0100
LV Status available
# open 2
LV Size 976.00 MiB
Current LE 244
Segments 1
Allocation inherit
Read ahead sectors auto
- currently set to 256
Block device 254:1
--- Logical volume ---
LV Path /dev/debtop-vg/home
LV Name home
VG Name debtop-vg
LV UUID f9s1fS-EffG-6g1e-pfhf-P67g-ba0T-Nr4Pp5
LV Write Access read/write
LV Creation host, time debtop, 2022-12-19 07:51:05 +0100
LV Status available
# open 1
LV Size <18.43 GiB
Current LE 4717
Segments 1
Allocation inherit
Read ahead sectors auto
- currently set to 256
Block device 254:2Excuse me for the long text, but from the output we can see the three LVs of the system with their name, size and other data.
Hands-on lab practice
Let’s add a new disk to the system and extend our /home with it!
Preparing the disk partition
I added a new 10 GB storage device to the system. It must show up in the output of the fdisk command now.
root@debtop:~# fdisk -l
Disk /dev/sdb: 10 GiB, 10737418240 bytes, 20971520 sectors
Disk model: VMware Virtual S
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytesI truncated the output of the command, but we can see that the new storage device is on /dev/sdb so we can start preparing the partition on it.
We are going to use the entire size of the disk for LVM. Using the fdisk /dev/sdb command we can create a Linux LVM partition type. Using fdisk is out of the scope of this article, but the disk settings must look like the followings. It is worth to mention that other partitioning tools are also fine for creating this setup.
root@debtop:~# fdisk -l
Disk /dev/sdb: 10 GiB, 10737418240 bytes, 20971520 sectors
Disk model: VMware Virtual S
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: dos
Disk identifier: 0x32d3237f
Device Boot Start End Sectors Size Id Type
/dev/sdb1 2048 20971519 20969472 10G 8e Linux LVMAs we can see the partition is ready for LVM: /dev/sdb1 2048 20971519 20969472 10G 8e Linux LVM
Adding the storage as Physical Volume
We will add the partition as Physical Volume to the LVM with the pvceate command. Note that pvcreate can work on both partition and device. We use a partition here.
root@debtop:~# pvcreate /dev/sdb1
Physical volume "/dev/sdb1" successfully created.Now it is part of our PVs.
root@debtop:~# pvdisplay
--- Physical volume ---
PV Name /dev/sda5
VG Name debtop-vg
PV Size 29.52 GiB / not usable 2.00 MiB
Allocatable yes (but full)
PE Size 4.00 MiB
Total PE 7557
Free PE 0
Allocated PE 7557
PV UUID MTO8uI-gRWH-Kc2T-skB0-QR6j-W7mB-ad7xrwAdding the PV to our Volume Group
Let’s extend our VG with this PV.
root@debtop:~# vgextend debtop-vg /dev/sdb1
Volume group "debtop-vg" successfully extended
root@debtop:~# vgs
VG #PV #LV #SN Attr VSize VFree
debtop-vg 2 3 0 wz--n- <39.52g <10.00g
We used the vgextend command here, added the /dev/sdb1 to the debtop-vg Volume Group. Note that I used the vgs command to list the VGs here.
With the vgdisplay command we can check the unallocated space in the VG.
root@debtop:~# vgdisplay
--- Volume group ---
VG Name debtop-vg
System ID
Format lvm2
Metadata Areas 2
Metadata Sequence No 5
VG Access read/write
VG Status resizable
MAX LV 0
Cur LV 3
Open LV 3
Max PV 0
Cur PV 2
Act PV 2
VG Size <39.52 GiB
PE Size 4.00 MiB
Total PE 10116
Alloc PE / Size 7557 / <29.52 GiB
Free PE / Size 2559 / <10.00 GiB
VG UUID 8X1Vdg-OEzV-ekRJ-4DRa-ycFJ-0wmp-soAUiaLet’s extend our home Logical Volume! We are going to extend the /home with the new space.
--- Logical volume ---
LV Path /dev/debtop-vg/home
LV Name home
VG Name debtop-vgExtending the LVM Logical Volume
Now we have the free space in our VG to extend our LV with it. Let’s use the lvextend command and add the maximum free space to the home filesystem. After reading the manual page carefully we can put together our command.
root@debtop:~# lvextend -l +100%FREE /dev/debtop-vg/home
Size of logical volume debtop-vg/home changed from <18.43 GiB (4717 extents) to 28.42 GiB (7276 extents).
Logical volume debtop-vg/home successfully resized.We can see from the output that the extension was successful. We can double-check the extension of the LV now.
root@debtop:~# lvs
LV VG Attr LSize Pool Origin Data% Meta% Move Log Cpy%Sync Convert
home debtop-vg -wi-ao---- 28.42g
root debtop-vg -wi-ao---- 10.14g
swap_1 debtop-vg -wi-ao---- 976.00mIt’s time to handle the file system extension.
Extending the ext4 filesystem on the LV
We have stretched the free space under the filesystem, but still a step is missing. We have to stretch the filesystem too. The method and command depend on the filesystem type.
On the Debtop machine the /home is on ext4, thus I will use resize2fs to do this operation.
root@debtop:~# resize2fs /dev/debtop-vg/home
resize2fs 1.46.2 (28-Feb-2021)
Filesystem at /dev/debtop-vg/home is mounted on /home; on-line resizing required
old_desc_blocks = 3, new_desc_blocks = 4
The filesystem on /dev/debtop-vg/home is now 7450624 (4k) blocks long.According to the command output our operation was successful and we extended the filesystem with the maximum available free space.
Let’s double check it with the df command!
root@debtop:~# df -h
Filesystem Size Used Avail Use% Mounted on
udev 1.9G 0 1.9G 0% /dev
tmpfs 390M 1.2M 389M 1% /run
/dev/mapper/debtop--vg-root 9.9G 4.1G 5.4G 44% /
tmpfs 2.0G 0 2.0G 0% /dev/shm
tmpfs 5.0M 4.0K 5.0M 1% /run/lock
/dev/sdb1 470M 99M 347M 23% /boot
/dev/mapper/debtop--vg-home 28G 63M 27G 1% /home
tmpfs 390M 860K 389M 1% /run/user/1000Our /home got the +10 Gigabytes free space now. We have managed to add a new disk and extend a Logical Volume with it.
Summary
With this longer article we have just scratched the surface of the LVM. There are more ways to use this versatile system and we just dipped our pinky in it.
We started the whole work with adding a disk to the system where we already had an LVM configuration. With the fdisk tool we created a new partition on the fresh disk marked as Linux LVM.
Then we added the partition as a Physical Volume to our LVM, extended our already present Volume Group with it.
At this point all was set to extend the Logical Volume with the free space on the system.
After extending the LV we stretched the ext4 filesystem too to use all the new free space. The filesystem extension can help online, when it is mounted. Please note that shrinking a filesystem cannot happen online, and it is a different process.
Please use this write up on your own risk!
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