Say that you are working with some data that is constantly changing, like some MySQL databases, and you are charged to take a valid backup without any data inconsistency or downtime. You may consider using mysqldump; however, this utility will lock tables during the duration of the backup and can cause down time (especially if using MyISAM table engine which performs Table level locking). Then, you think you may just copy or archive the MySQL table files (/var/lib/mysql/DB), however, this can cause data inconsistent as during the copy some of the data may of changed. Not to mention, if it is InnoDB table engine all table data is stored in the ibdata files.
At this point you may be scratching your head not knowing what to do. You could stand up another server and setup MySQL replication. Unfortunately, you do not have any available server’s or workstation to setup for this purpose. Fortunately, for you, this server will setup by default with LVM (Logical Volume Management) by your distribution. Using LVM, we are able to create a snapshot of an existing Logical Volume using free space on the Volume Group. Lets go ahead and see some of the syntax and examples below.
Below we have our Logical Volume mounted on /var/lib/mysql.
# df -h /var/lib/mysql/
Filesystem Size Used Avail Use% Mounted on
/dev/mapper/mysql-lvol0
591M 38M 524M 7% /var/lib/mysql
So lets go ahead and check out the information about this Logical Volume.
# lvdisplay /dev/mysql/lvol0
--- Logical volume ---
LV Name /dev/mysql/lvol0
VG Name mysql
LV UUID BW3zO3-KVvQ-ABvt-4ylN-bw8t-ivvj-Vsp48P
LV Write Access read/write
LV Status available
# open 1
LV Size 600.00 MB
Current LE 150
Segments 1
Allocation inherit
Read ahead sectors auto
- currently set to 256
Block device 253:2
Now lets check the Volume Group this Logical Volume is apart of and see if there is any free space.
# vgdisplay mysql
--- Volume group ---
VG Name mysql
System ID
Format lvm2
Metadata Areas 1
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 1
Act PV 1
VG Size 1020.00 MB
PE Size 4.00 MB
Total PE 255
Alloc PE / Size 150 / 600.00 MB
Free PE / Size 105 / 420.00 MB
VG UUID PtaSHo-cdQ4-ofy1-ug6O-7q3z-OLS6-4oAyRt
As we do have an available 420 MB or 105 PE (Physical extents) lets go ahead and create a snapshot, mount it and view its contents.
# lvcreate --size 400MB --snapshot --name snap /dev/mysql/lvol0
Logical volume "snap" created
# mount /dev/mysql/snap /mnt/
# ls -l /mnt
total 20568
-rw-rw---- 1 mysql mysql 10485760 Feb 24 09:19 ibdata1
-rw-rw---- 1 mysql mysql 5242880 Feb 24 09:25 ib_logfile0
-rw-rw---- 1 mysql mysql 5242880 Feb 24 09:18 ib_logfile1
drwx------ 2 mysql mysql 16384 Feb 24 09:14 lost+found
drwx------ 2 mysql mysql 4096 Feb 24 09:18 mysql
srwxrwxrwx 1 mysql mysql 0 Feb 24 09:25 mysql.sock
drwx------ 2 mysql mysql 4096 Feb 24 09:18 test
Now we have a snapshot of our MySQL data which will not change even though the live data mounted at /var/lib/mysql is contently changing. Using this snapshot we can safely copy the contents and if need be use for a restore in the future.
