logical volume

Introduction to LVM

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Most people still use standard partitioning schemes on their Linux desktops and servers. But as with most things in Linux, you have choices.

An alternative to traditional partitioning is to use Logical Volume Manager, or LVM. This blog post explains what LVM does, and how to use it.

Key Advantages

  1. You can create single volumes from multiple physical hard disks/volumes.
  2. In the case of virtualisation, you can easily add additional space to a virtual machine without a reboot, and online resize your LVM/partition.
  3. Consistent backups can be taken whilst the volume is online by using LVM snapshots.
  4. LVMs can be created to include raid functionality, including Raid 1, 5 and 6.
  5. Entire LVs can be striped across multiple physical volumes, much like Raid 0.

Objectives

In order to learn what can be achieved with LVM, I’m going to assume you’re running a Debian- based distribution (e.g. Ubuntu, Debian); and we’re going to carry out the following:

  1. Create physical volumes with LVM
  2. Create volume groups with LVM
  3. Create logical volumes with LVM
  4. Perform an online resize of a logical volume

Create a physical volume with LVM

You can see if there are any existing volumes in place by running pvdisplay at the command line. If you get a command not found error, then you’ll need to install lvm with apt-get install lvm2.

Now we can move on to create a physical volume. You can find if any are spare by running fdisk -l and taking note of any physical devices without any partitions present. In my case, I have /dev/sdb and /dev/sdc free to use. As such, I will go ahead and run:

root@debian-test:~# pvcreate /dev/sdb /dev/sdc
  Physical volume "/dev/sdb" successfully created
  Physical volume "/dev/sdc" successfully created
root@debian-test:~#

Now, I can see the existing physical volumes along with my newly created ones.

---Physical volume---
PV Name		/dev/sda5
VG Name 		debian-test-vg
PV Size		15.76 GiB / not usable 2.00 MiB
Allocatable 		yes (but full) 
PE Size		4.00 MiB
Total PE 		4034
Free PE 		0
Allocated PE		4034
PV UUID		DruG7X-n0H3-he8J-4KOb-sWyL-KBUz-1iT2ZD

“/dev/sdb” is a new physical volume of “16.00 GiB” 
---NEW Physical volume---
PV Name		/dev/sdb
VG Name 		
PV Size		16.00 GiB
Allocatable 		No
PE Size		0
Total PE 		0
Free PE 		0
Allocated PE		0
PV UUID		e3ZXGv-fa3c-1CBs-61Ue-FrLM-cbcM-PRucwn

“/dev/sdc” is a new physical volume of “16.00 GiB” 
---NEW Physical Volume---
PV Name		/dev/sdb
VG Name 		
PV Size		16.00 GiB
Allocatable 		No
PE Size		0
Total PE 		0
Free PE 		0
Allocated PE		0
PV UUID		YLPy6Q-CQJL-zPut-rTUt-1ro7-n2n1-Ksk0z2

Create volume groups with LVM

Now we’re ready to create a volume group with our new physical volumes. I’ll choose to create a volume group called ‘test’ on the new physical volumes we’ve added.

root@debian-test:~# vgcreate test /dev/sdb /dev/sdc
  Volume group "test" successfully created
root@debian-test:~#
We can list these by running vgdisplay

:

root@debian-test:~# vgdisplay
--- Volume group ---
VG Name		test
System ID
Formate		1vm2
Metadata Areas	2
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			2
Act PV			2
VG Size		31.99 GiB
PE Size		4.00 MiB
Total PE		8190
Alloc  PE/Size		0 / 0 
Free  PE/Size		8190 / 31.99 GiB
VG UUID		zcmtQ3-L7yB-qksc-s3d7-8cLM-am0F-5JbJ1d

Create logical volumes with LVM

For simplicity’s sake, I’m going to create a logical volume utilising 100% of the free space on the volume group by running the following:

root@debian-test:~# lvcreate -l 100%FREE -n test-lv test
  Logical volume "test-lv" created
root@debian-test:~#

The above command creates a logical volume named ‘test-lv’ on the volume group ‘test’.

We can see the result of our work by running lvdisplay :

root@debian-test:~# 1vdisplay
--- Logical volume ---
LV Path		/dev/test/test-lv
LV Name 		test-lv
VG Name		test 
LV UUID		tLOOIR-V0jb-de7s-Z4s9-LDpe-c105-ru8oQS
LV Write Access	read/write
LV Creation host, time debian-test,       2017-05-15 22:28:15 +0100
LV Status		available
# open			0
LV Size		31.99 GiB
Current LE 		8190
Segments 		2
Allocation		inherit
Read ahead sectors	auto 
-Current set to 	256
Block device 		254:2

At this point, we’ll want to create a filesystem on our logical volume to be able to utilise it within Linux. Simply run: mkfs.ext4 /dev/test/test-lv—From here we can simply mount the filesystem in a directory as one normally would by running mount /dev/test/test-lv /mnt/

We can see that the partition is mounted successfully by running df -h /mnt/test/ :

root@debian-test:~# df -h /mnt/test/
Filesystem			Size Used Avail Use% Mounted on 
/dev/mapper/test-test--1v	32G	48M	30G	1% /mnt/test
root@debian-test:~#

Perform an online resize of a logical volume

So, you’re all up and running with LVM and loving it…But Suzie from management says that a new project is coming your way that requires a whopping 60GB of space on the same server, and you can’t have any downtime. What do you do?!?! :O

This scenario is simple. If you’re running a physical server with hot swappable storage, you can simply add another disk whilst the server is running. Alternatively, as is often the case these days, you may be running a virtual machine where you can add a hard drive to the virtual machine without too much hassle. Once you’ve done this, search for it using fdisk -l the same way as before. Once you’ve found the new disk, simply run pvcreate as we did previously. In my case, it’s pvcreate /dev/sdd .

root@debian-test:~# pvcreate /dev/sdd
  Physical volume "/dev/sdd" successfully created
root@debian-test:~#

Now let’s extend our volume group by running vgextend test /dev/sdd where the name of the volume group we created earlier and /dev/sdd is the new physical volume we added:

root@debian-test:~# vgextend test /dev/sdd
  Volume group "test" successfully extended
root@debian-test:~#

We can see that we have grown our logical volume and its new size by running vgdisplay test :

root@debian-test:~# vgdisplay test
--- Volume group ---
VG Name		test
System ID
Formate		1vm2
Metadata Areas	3
Metadata Sequence No 3
VG Access 		read/write
VG Status		resizable
MAX LV 		0
Cur LV			1
Open LV		1
Max PV		0
Cur PV			3
Act PV			3
VG Size		91.99 GiB
PE Size		4.00 MiB
Total PE		23549
Alloc  PE/Size		8190 / 31.99 GiB 
Free  PE/Size		15359 / 60.00 GiB
VG UUID		zcmtQ3-L7yB-qksc-s3d7-8cLM-am0F-5JbJ1d

At this point we can extend our logical volume by running lvextend -l 100%FREE -n /dev/test/test-lv :

root@debian-test:~# lvextend -l 100%FREE -n /dev/test/test-lv
  Size of logical volume test/test-lv changed from 31.99 GiB (8190 extents) to 60.00 GiB (15359 extents).
  Logical volume test-lv successfully resized
root@debian-test:~#

To resize the filesystem resident on the logical volume, simply run resize2fs /dev/test/test-lv and it will perform an online resize!

root@debian-test:~# df -h /mnt/test/
Filesystem			size Used Avail Use% Mounted on
/dev/mapper/test-test--lv	59G	52M	57G	1%	/mnt/test
root@debian-test:~#

Conclusion

There is a lot more you can do with LVM, but so far in my career, the functionality shown above has proven to be most useful. Further links below are included to assist you in your LVM journey!

Resources

  1. Further reading on LVM: https://en.wikipedia.org/wiki/Logical_Volume_Manager_(Linux)
  2. Old (but still relevant) examples of LVM usage: http://tldp.org/HOWTO/LVM-HOWTO/
  3. An overview of LVM on Debian’s website: https://wiki.debian.org/LVM

Keith Rogers is an IT professional with over 10 years’ experience in modern development practices. Currently he works for a broadcasting organization in the DevOps space with a focus on automation.


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