In the last article, we have seen how to expand and shrink volume groups and logical volumes. In this article, we will focus on how to protect the data by creating LVM snapshots.
Table of Contents
What is LVM Snapshots?
There are two options to safeguard your data from disk failures or other issues. You can take full or incremental backups and store N number of copies. Alternatively with LVM, you can create a snapshot volume that will take snapshots when changes are made to the source volume.
LVM snapshots use a copy-on-write mechanism to take snapshots. Initially, when you create a snapshot volume it will hold some metadata about the source logical volume and its block details. When you make any changes in the source volume, LVM will monitor the changes and take a snapshot of the modified blocks. Here LVM just stores the changes blocked to the snapshot volume.
I have attached two new disks to my Ubuntu machine which will be used for demonstration purposes. If you wish to know how the following command works in detail, then check our introduction to the LVM guide given below.
$ lsblk /dev/sd[cd] NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT sdc 8:32 0 10G 0 disk sdd 8:48 0 15G 0 disk
Both the disks are initialized as the physical volume.
$ sudo pvcreate /dev/sd[cd] Physical volume "/dev/sdc" successfully created. Physical volume "/dev/sdd" successfully created.
A volume group named "ostechnix_lab" is created from both the initialized physical volume.
$ sudo vgcreate ostechnix_lab /dev/sd[cd] Volume group "ostechnix_lab" successfully created
A Logical volume named "lab" is created from the "ostechnix_lab" volume group. The logical volume size is 5GB.
$ sudo lvcreate -L 5GB -n lab ostechnix_lab Logical volume "lab" created.
Volume is formatted with the
ext4 file system and mounted on
$ sudo mkfs.ext4 /dev/ostechnix_lab/lab $ mkdir /opt/lvm_lab $ sudo mount /dev/ostechnix_lab/lab /opt/lvm_lab/
After creating the logical volume, currently there are nearly 20GB of free space.
$ sudo vgdisplay --- Volume group --- VG Name ostechnix_lab System ID Format lvm2 Metadata Areas 2 Metadata Sequence No 2 VG Access read/write VG Status resizable MAX LV 0 Cur LV 1 Open LV 1 Max PV 0 Cur PV 2 Act PV 2 VG Size 24.99 GiB PE Size 4.00 MiB Total PE 6398 Alloc PE / Size 1280 / 5.00 GiB Free PE / Size 5118 / 19.99 GiB VG UUID LCGSEd-WTnT-lUEN-N0a8-QWhI-TqDi-lUZExd
Creating Snapshot Volume
I am going to create a snapshot volume of 5GB. Run the following command to create a snapshot volume:
$ sudo lvcreate -L +5G --snapshot --name snap /dev/mapper/ostechnix_lab-lab Logical volume "snap" created.
Let me walk through what each flag in the above command does.
-L +5G=> The space value should be passed to
-Lflag. Here 5GB is allocated.
--snapshot=> This flag creates the volume as a snapshot volume. You can also use
--name=> Name given to the snapshot volume. In my case "snap" is the name of my snapshot volume.
/dev/mapper/ostechnix_lab-lab=> This is the source volume location.
Run the "
lvs" command to see the snapshot volume. Take a look at the "origin" section from the below output. It points to the source volume "lab" and the data% is "0.01". snapshots will be taken if any changes to the blocks in that volume.
$ sudo lvs LV VG Attr LSize Pool Origin Data% Meta% Move Log Cpy%Sync Convert lab ostechnix_lab owi-aos--- 5.00g snap ostechnix_lab swi-a-s--- 5.00g lab 0.01
You can also mount the snapshot volume and see what files and directories are present.
$ mkdir /mnt/LVM_Snapshot/
$ mount /dev/mapper/ostechnix_lab-snap /mnt/LVM_Snapshot/
How To Restore LVM Snapshot
Run the following command to create some random data in the source volume of 1GB in size.
$ dd if=/dev/zero of=/data/dummy_file2 bs=1G count=1 oflag=dsync
Binary file is created and the snapshot volume space is around 20% which you can see under the "Data%" section.
$ ls -lh /opt/lvm_lab/ostechnix_sample.txt -rw-r--r-- 1 root root 1.0G Feb 19 15:46 /opt/lvm_lab/ostechnix_sample.txt
$ sudo lvs LV VG Attr LSize Pool Origin Data% Meta% Move Log Cpy%Sync Convert lab ostechnix_lab owi-aos--- 5.00g snap ostechnix_lab swi-a-s--- 5.00g lab 20.09
To restore the LVM snapshot follow the below procedure. An important point to note is once the snapshot is restored the logical volume will be automatically removed.
STEP 1: Unmount the file system where the source logical volume is mounted.
$ sudo umount /opt/lvm_lab
STEP 2: Run the
lvconvert command to initiate the restore process.
$ sudo lvconvert --merge /dev/ostechnix_lab/snap sudo lvconvert --merge /dev/ostechnix_lab/snap Merging of volume ostechnix_lab/snap started. ostechnix_lab/lab: Merged: 80.04% ostechnix_lab/lab: Merged: 80.42% ostechnix_lab/lab: Merged: 81.00% ostechnix_lab/lab: Merged: 81.43% ostechnix_lab/lab: Merged: 89.06% ostechnix_lab/lab: Merged: 98.04% ostechnix_lab/lab: Merged: 100.00%
You can run the
lvs command and under the Attr section it will display "O" which means snapshot restore is in progress.
STEP 3: Once the merge process is complete the snapshot volume will be automatically removed. You can verify it by running the
$ sudo lvs LV VG Attr LSize Pool Origin Data% Meta% Move Log Cpy%Sync Convert lab ostechnix_lab -wi-ao---- 5.00g
STEP 4: Mount the logical volume and check if the data is restored.
$ sudo mount /dev/ostechnix_lab/lab /opt/lvm_lab
LVM Snapshot Volume Sizing and Corruption
It is always recommended to allocate an equal amount of space to the snapshot volume considering the source logical volume will use its full space. In some cases without any forecasting, upfront snapshot volume will be allocated with less space than the logical volume. In such cases, if the source logical volume is fully utilized and snapshot volume will not have any space beyond the allocated space and will get corrupted.
If the snapshot volume is corrupted, it is of no use and we have to remove it and create a new snapshot volume.
To overcome this issue, you have three options.
- Allocate the size of the snapshot volume and source volume equally.
- Manually extend the snapshot volume size.
- Automatically extend the snapshot volume size. This is the recommended method.
To extend the size of the logical volume manually run the following command. Here 3GB of size is added to the snapshot volume.
$ sudo lvextend -L +3G /dev/ostechnix_lab/snap
To extend the size automatically you have to edit the file "
/etc/lvm/lvm.conf". You have to modify two important parameters.
autoextend_threshold parameter will automatically extend the volume when a certain percentage of snapshot volume is filled. For example, if the threshold is set to 50% for 2GB volume, then after 1GB of space is filled the space will automatically be added.
In other words, if you have 2GB snapshot volume and the threshold is set to 50 which means if 50% of space is used which is 1GB, then auto extend should happen.
You have to set how much space should be added to the snapshot volume by setting the
In this article, we have seen what is LVM snapshots, how to create new snapshot volumes and restore snapshot volumes. Finally, we have seen how to extend the snapshot volume in both manual and automatic ways.