Snapper is a command-line utility in Linux used to create and manage snapshots of LVM thin volumes. It can create, delete, and compare snapshots and revert changes done between snapshots. Snapper also allows for the easy creation and management of snapshots for Btrfs.
Installing snapper
Use the yum command to install the snapper software package.
# yum install snapper ... Transaction Summary ============================================================= Install 1 Package (+3 Dependent packages) Total download size: 499 k Installed size: 1.5 M Is this ok [y/d/N]: y ... Complete!
Use the rpm –ql command to view the files provided by the snapper package.
# rpm –ql snapper /etc/cron.daily/snapper /etc/cron.hourly/snapper /etc/dbus-1/system.d/org.opensus.Snapper.conf /etc/logrotate.d/snapper /usr/bin/snapper /usr/sbin/snapperd /usr/share/dbus-1/system-services/org.opensus.Snapper.service ...
– By default, snapper sets up a cron.hourly job to create snapshots in the .snapshots subdirectory of the volume and a cron.daily job to clean up old snapshots.
– You can edit the configuration file to disable or change this behavior.
1. Creating a Thinly Provisioned Logical Volume
Use the lvcreate command to create a thin provisioned volume named – thinvol1. Note that this volume needs to be mounted before we create the snapshot using the snapper command.
# vgs VG #PV #LV #SN Attr VSize VFree myvolg 1 0 0 wz--n- 1020.00m 1020.00m
Create a thin pool:
# lvcreate –v –L 100m –T myvolg/mythinpool ... Logical volume “mythinpool” created
Create thinly provisioned volume :
# lvcreate –v –V 200m –T myvolg/mythinpool –n thinvol1 ... Logical volume “thinvol1” created
Create a file system on the thin volume.
# mkfs.ext4 /dev/myvolg/thinvol1 ... Writing superblocks and filesystem accounting information: done
Mount the file system:
# mkdir /myvol1 # mount /dev/myvolg/thinvol1 /myvol1
2. Creating snapper configuration file
To create a snapshot using snapper, a configuration file is required for the LVM thin volume or Btrfs subvolume. The LVM and Btrfs volumes must also have a mounted file system. Use the create-config command to create the configuration file. The following example creates a configuration file named myvol_snap for an LVM ext4 file system mounted on /myvol1:
# snapper –c myvol1_snap create-config –f "lvm(ext4)" /myvol1
– This command adds an entry to /etc/sysconfig/snapper.
– This command creates the /etc/snapper/configs/myvol1_snap configuration file.
– This command creates a .snapshots directory in the /myvol1 directory.
# cat /etc/sysconfig/snapper ... SNAPPER_CONFIGS=”myvol1_snap”
# cat /etc/snapper/configs/myvol1_snap ... # subvolume to snapshot SUBVOLUME=”/myvol1” # filesystem type FSTYPE=”lvm(ext4)” ... # start comparing pre- and post-snapshot in background after... BACKGROUND_COMPARISON=”yes” # run daily number cleanup NUMBER_CLEANUP=”yes” ... # create hourly snapshots TIMELINE_CREATE=”yes” # cleanup hourly snapshots after some time TIMELINE_CLEANUP=”yes” ... # cleanup empty pre-post-pairs EMPTY_PRE_POST_CLEANUP=”yes” ...
Snapshots of the /myvol1 file system are stored in the .snapshots subdirectory.
# ls –la /myvol1 ... drwxr-x--- ... .snapshots ...
Types of snapshots
There are three types of snapshots that you can create by using snapper:
pre: Use to record the state of a volume before a modification. Pre snapshots should always have a corresponding post snapshot.
post: Use to record the state of a volume after a modification.
single: These snapshots have no special relationship to other snapshots.
3. Creating snapshot
Create some test file in the /myvol1 directory.
# echo "This is a test file" > /myvol1/test_file
Use the snapper create -t pre to create a pre snapshot of the volume defined by the myvol1_snap configuration file. Include the –p option to display the number of the snapshot being created. (In this example, the pre snapshot number is 1)
# snapper –c myvol1_snap create –t pre –p 1
The snapshots are stored by snapshot number in the .snapshots subdirectory of the volume. View the contents of the /myvol1/.snapshots directory.
# ls –l /myvol1/.snapshots ... drwxr-xr-x ... 1
View the contents of the /myvol1/.snapshots/1 directory.
# ls –l /myvol1/.snapshots/1 ... -rw------- ... info.xml drwxr-x--- ... snapshot
Now modify the content of /myvol1 directory by removing the test_file.
# rm /myvol1/test_file
Use the snapper create -t post to create a post snapshot of the volume defined by the myvol1_snap configuration file. Include the –pre-num 1 option to associate this post snapshot with the pre snapshot 1. Include the –p option to display the number of the snapshot being created.
# snapper –c myvol1_snap create –t post --pre-num 1 –p 2
You would see a new directory names 2 created in the .snapshots directory.
# ls –l /myvol1/.snapshots/2 ... -rw------- ... filelist-1.txt -rw------- ... info.xml drwxr-x--- ... snapshot
The filelist-1.txt contains the file that was deleted after the pre snapshot and before the post snapshot.
# cat /myvol1/.snapshots/2/filelist-1.txt -..... /test_file
Comparing pre and post snapshots
Use the snapper status command to display the files and directories that have been added, removed, or modified between pre snapshot 1 and post snapshot 2.
# snapper –c myvol1_snap status 1..2 -..... /test_file
Use the snapper diff command to display the differences between the contents of the files in pre snapshot 1 and post snapshot 2.
# snapper –c myvol1_snap diff 1..2 Binary files /myvol1/.snapshots/1/snapshot/test_file and /myvol1/.snapshots/2/snapshot/test_file differ
Use the snapper list command to list the snapshots that exist for volume defined by the myvol1_snap configuration file.
# snapper –c myvol1_snap list Type | # | Pre # | Date | User | Cleanup |Description| Userdata ------+---+-------+------+------+---------+------------+-------- single| 0 | | ... | root | | Current | pre | 1 | | ... | root | | | post | 2 | 1 | ... | root | | |
reverse the changes from post snapshot to pre shanshot
To undo the changes from post snapshot to pre snapshot i.e. to restore the test_file use the snapper undochange command.
# snapper –c myvol1_snap undochange 1..2 create:1 modify:0 delete:0
Verify
# ls /myvol1 lost+found test_file