A snapshot in computer systems is not very much different than the word when it refers to photography. Just as a snap is an actual copy of a thing or person, a snapshot indicates the exact state of a system at a specific point in time.
LVM (Logical Volume Manager) helps you to create a LVM snapshot- essentially then it’s a logical volume that allows you to temporarily preserve the original data of a dynamic logical volume. It’s usually taken when any critical modifications are going to be made to the Logical volume- the snapshot allows for a quick and fast way to create a data copy.
When a huge volume of online data has to be backed up it can create performance issues and if data backup is done offline, enterprises may face downtime. LVM snapshots give you an easy way to eliminate those pain points without impeding system operations. You neither have to worry about data being changed during the snapshot backup nor do you have to take the database volume offline.
A word of caution though- the size of the LVM snapshots matter a great deal. If the shot is too small it will fill up space and become invalid; on the other hand, as it’s difficult to decide how much space each snapshot will need, it’s difficult to assess how many snapshots will fit within a limited space. It’s imperative then to allow enough snapshot volume.
When snapshots are created, a new logical volume also happens and that is a replica of the original one. Initially, the snapshot volume doesn’t use any space, but as changes are made in the original volume, this reflects in the snapshot volume too. As you continue to make changes, the snapshot increasingly occupies more space, but if all the allotted space is used, then the snapshot becomes unusable.
In a nutshell, LVM snapshots are “specialized volumes thatcontain exact same copies of your data volume”. By ensuring an extra amount of space in the backup volume than your original data volume, the problems of unusability and invalidation can be safely circumvented.
However, the amount of volume depends on the size of the snapshot, so by keeping the snapshot more or less its original size, the possibility of overflow and invalidation is prevented.
A huge advantage of LVM snapshots is that the data remains unchanged even as the backup is going on. This is in contrast to backups of live volume, where data changes can cause inconsistency in backups. The difference lies in the fact that while taking a snapshot, LVM backups are smooth and consistent as no data is being written on it and no application or user is using it.
The biggestbenefit of LVM snapshots is that as they can be created in fractions of a second and they drastically reduce the amount of time that services are down. Once they are created they can be backed up and services can continue normal operation.
LVM snapshots can also be used for testing and reverting of updates. When the system is in a known good state, create a snapshot and later on you can make the changes or perform updates. Also, if there have been accidental file deletions after the creation of a snapshot, you can set your mind at rest as the snapshot will have all the original files.