BTRFS is an open-source and modern filesystem implemented with advanced features. It works on copy on write (CoW) mechanism.
It is included in the mainline Linux kernel. Notable features of Btrfs are copy-on-write data and metadata, snapshots, Built-in RAID support, online resizing and defragmentation, transparent compression, efficient storage for small files, etc.
Snapshot is a given time copy of Btrfs subvolume. Snapshots are writeable by default. Btrfs allows creating readable snapshots which is helpful to take backup.
In this tutorial, we learn how to create Btrfs snapshots, Incremental backups, and restore it.
System information
I am using Fedora and now Fedora 33 has Btrfs by default. My system has 20GB boot disk and 10GB additional disk with Btrfs filesystem on it.
Here sda is boot disk while sdb is the additional disk we will use for btrfs snapshot.
Listing the disk information:
$ ls -l /dev | grep sd brw-rw----. 1 root disk 8, 0 Nov 21 08:42 sda brw-rw----. 1 root disk 8, 1 Nov 21 08:42 sda1 brw-rw----. 1 root disk 8, 2 Nov 21 08:42 sda2 brw-rw----. 1 root disk 8, 16 Nov 21 08:57 sdb brw-rw----. 1 root disk 8, 17 Nov 21 08:57 sdb1
To explain I have created LVM and mounted it at /mnt/linoxide_btrfs1 with btrfs filesystem.
Listing PV, VG, and LV created:
$ pvs && vgs && lvs PV VG Fmt Attr PSize PFree /dev/sdb1 linoxide_vg lvm2 a-- <10.00g <5.00g VG #PV #LV #SN Attr VSize VFree linoxide_vg 1 1 0 wz--n- <10.00g <5.00g LV VG Attr LSize Pool Origin Data% Meta% Move Log Cpy%Sync Convert linoxide_lv1 linoxide_vg -wi-a----- 5.00g
The output of mount information:
$ df -h Filesystem Size Used Avail Use% Mounted on devtmpfs 286M 0 286M 0% /dev tmpfs 294M 0 294M 0% /dev/shm tmpfs 294M 4.5M 289M 2% /run tmpfs 294M 0 294M 0% /sys/fs/cgroup /dev/sda2 20G 2.3G 18G 12% / /dev/sda1 200M 12M 189M 6% /boot/efi tmpfs 59M 0 59M 0% /run/user/1000 tmpfs 59M 0 59M 0% /run/user/0 /dev/mapper/linoxide_vg-linoxide_lv1 5.0G 17M 4.5G 1% /mnt/linoxide_btrfs1
Btrfs Snapshot
In order to create btrfs snapshot, first create a subvolume.
Step 1. Create subvolume
To create a subvolume named ‘/mnt/linoxide_btrfs1/subvol1’, run:
$ sudo btrfs subvolume create /mnt/linoxide_btrfs1/subvol1
Step 2. Copy files to subvolume
Let’s copy some data files to subvolume. The following command copy some files from /etc/ to subvol1.
$ cd /mnt/linoxide_btrfs1/subvol1 $ cp /etc/[abc]* . $ ls -l -rw-r--r--. 1 root root 16 Nov 21 09:25 adjtime -rw-r--r--. 1 root root 1529 Nov 21 09:25 aliases -rw-r--r--. 1 root root 12288 Nov 21 09:25 aliases.db -rw-------. 1 root root 541 Nov 21 09:25 anacrontab -rw-r--r--. 1 root root 55 Nov 21 09:25 asound.conf -rw-r--r--. 1 root root 2853 Nov 21 09:25 bashrc -rw-r--r--. 1 root root 112 Nov 21 09:25 boto.cfg -rw-r--r--. 1 root root 37 Nov 21 09:25 centos-release -rw-r--r--. 1 root root 51 Nov 21 09:25 centos-release-upstream -rw-r--r--. 1 root root 1053 Nov 21 09:25 chrony.conf
Step 3. Create snapshot
Let’s create a snapshot of the subvolume1 (/mnt/linoxide_btrfs1/subvol1).
Note the -r flag for readonly. Make sure to create snapshot as read-only if like use send/receive commands (explained in the next section).
To create a readonly snapshot of ‘/mnt/linoxide_btrfs1/subvol1’ in ‘/mnt/linoxide_btrfs1/.snapshots/subvol1-day1’, run:
$ sudo btrfs subvolume snapshot -r /mnt/linoxide_btrfs1/subvol1 /mnt/linoxide_btrfs1/.snapshots/subvol1-day1
Check the contents of snapshot:
$ cd .snapshots/ $ ls -l drwxr-xr-x. 1 root root 446 Nov 21 09:25 subvol1-day1 $ cd subvol1-day1/ $ ls -l -rw-r--r--. 1 root root 16 Nov 21 09:25 adjtime -rw-r--r--. 1 root root 1529 Nov 21 09:25 aliases -rw-r--r--. 1 root root 12288 Nov 21 09:25 aliases.db -rw-------. 1 root root 541 Nov 21 09:25 anacrontab -rw-r--r--. 1 root root 55 Nov 21 09:25 asound.conf -rw-r--r--. 1 root root 2853 Nov 21 09:25 bashrc -rw-r--r--. 1 root root 112 Nov 21 09:25 boto.cfg -rw-r--r--. 1 root root 37 Nov 21 09:25 centos-release -rw-r--r--. 1 root root 51 Nov 21 09:25 centos-release-upstream -rw-r--r--. 1 root root 1053 Nov 21 09:25 chrony.conf
Btrfs Snapshot using send
We have taken a snapshot on the same disk as subvolume which is not good for backup strategy, In case of a disaster like disk corruption, will lose the recovery optionl.
BTRFS has a solution for that. We can send snapshots to a remote system or external hard drive using the command btrfs send and btrfs receive. They are based on SSH. The remote system should be formatted as BTRFS as well.
Let me explain with example.
Create a new disk with btrfs files system mounted at /mnt/linoxide_btrfs2.
$ df -h Filesystem Size Used Avail Use% Mounted on devtmpfs 286M 0 286M 0% /dev tmpfs 294M 0 294M 0% /dev/shm tmpfs 294M 4.5M 289M 2% /run tmpfs 294M 0 294M 0% /sys/fs/cgroup /dev/sda2 20G 2.3G 18G 12% / /dev/sda1 200M 12M 189M 6% /boot/efi tmpfs 59M 0 59M 0% /run/user/1000 tmpfs 59M 0 59M 0% /run/user/0 /dev/mapper/linoxide_vg-linoxide_lv1 5.0G 17M 4.5G 1% /mnt/linoxide_btrfs1 /dev/mapper/linoxide_vg2-linoxide_lv2 5.0G 17M 4.5G 1% /mnt/linoxide_btrfs2
In this example, the destination Btrfs volume is the second btrfs disk mounted as /mnt/linoxide_btrfs2 .
The following command to send snapshot to remote destination:
$ sudo sudo btrfs send /mnt/linoxide_btrfs1/.snapshots/subvol1-day1 | sudo btrfs receive /mnt/linoxide_btrfs2 At subvol /mnt/linoxide_btrfs1/.snapshots/subvol1-day1 At subvol subvol1-day1
Verify the data files:
$ cd /mnt/linoxide_btrfs2/ $ ls -l drwxr-xr-x. 1 root root 446 Nov 21 09:25 subvol1-day1 $ cd subvol1-day1/ $ ls -l -rw-r--r--. 1 root root 16 Nov 21 09:25 adjtime -rw-r--r--. 1 root root 1529 Nov 21 09:25 aliases -rw-r--r--. 1 root root 12288 Nov 21 09:25 aliases.db -rw-------. 1 root root 541 Nov 21 09:25 anacrontab -rw-r--r--. 1 root root 55 Nov 21 09:25 asound.conf -rw-r--r--. 1 root root 2853 Nov 21 09:25 bashrc -rw-r--r--. 1 root root 112 Nov 21 09:25 boto.cfg -rw-r--r--. 1 root root 37 Nov 21 09:25 centos-release -rw-r--r--. 1 root root 51 Nov 21 09:25 centos-release-upstream -rw-r--r--. 1 root root 1053 Nov 21 09:25 chrony.conf
Now, You have a full backup which can be used as DR as well.
Incremental backup
Snapshot can also send backups in an incremental way. When some directory is renamed, Rsync deletes the directory and transfers all data again, while BTRFs just transfer some bytes of metadata.
Let’s take another snapshot.
$ sudo btrfs subvolume snapshot -r /mnt/linoxide_btrfs1/subvol1 /mnt/linoxide_btrfs1/.snapshots/subvol1-day2
Using -p flag you need to specify base snapshot and this snapshot has to exist in the source and destination. The following command will send the difference between two base snapshots to the destination.
$ sudo btrfs send -p /mnt/linoxide_btrfs1/.snapshots/subvol1-day1 /mnt/linoxide_btrfs1/.snapshots/subvol1-day2 | sudo btrfs receive /mnt/linoxide_btrfs2
How to restore Btrfs snapshot
In this section, I will explain how to restore data from snapshot taken earlier. Let’s take scenario where file is deleted by mistake. You can get it back last snapshot, or even recover an older version of the file from an older snapshots.
You can simply use the cp command to restore the deleted file:
$ sudo cp /mnt/linoxide_btrfs1/.snapshots/subvol1-day1/chrony.conf /mnt/linoxide_btrfs1/subvol1
Let’s assume that you deleted the entire subvolume, Even in that case, with the use of cp command, You can restore the entire content from the snapshot:
$ sudo cp -r /mnt/linoxide_btrfs1/.snapshots/subvol1-day1 /mnt/linoxide_btrfs1/subvol1
Or you can restore the subvolume by using the btrfs snapshot command:
$ sudo btrfs subvolume snapshot /mnt/linoxide_btrfs1/.snapshots/subvol1-day1 /mnt/linoxide_btrfs1/subvol1
Snapshot cleanup
When you are taking periodic incremental backup with btrfs snapshot, you can have lots of them piling up. This can create confusion and occupy a lots of space on disk. You can also purge old unwanted snapshot using snapshot delete. While purging keep in mind that in order to perform an incremental send, you need the last snapshot.
$ sudo btrfs subvolume delete /mnt/linoxide_btrfs1/.snapshots/subvol1-day1
Conclusion
Btrfs has matured at a fast pace. Today, it has a wide range of capabilities like compression, snapshots, data integrity and performance that make it to an production ready file system. I hope this article will be helpful in using snapshots of BTRFS. Let me know your views in comment section.
#Backup #Btrfs #Snapshots
Cool
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