About snapshot basics
The large active databases or file systems that must be available around-the-clock are difficult to back up without incurring a penalty. Often, the penalty takes one of the following forms:
To allow time for the backup, the entire database is taken offline or the file system is unmounted. The result is suspension of service and inconvenience to users.
The copy is made very quickly but produces an incomplete version of the data, since some transactions have failed to complete.
A solution to this problem is to create a snapshot of the data. A snapshot captures the data at a particular instant, without causing the client downtime. The resulting capture or snapshot can be backed up without affecting the performance or availability of the file system or database. Without a complete, up-to-date snapshot of the data, a correct backup cannot be made.
When a NetBackup media server or third-party copy device manages the backup, the data to back up must be contained in a snapshot. The backup agent can only access the data by means of the raw physical disk. Once the data is captured as a snapshot, the NetBackup client "maps" the logical representation of the data to its physical disk address. These disk addresses are sent to the backup agent over the LAN. The data is then read from the appropriate disk by the backup agent.
Two types of snapshots are available, both supported by NetBackup: copy-on-write and mirror (or clone). In mode, NetBackup automatically decides the best type of snapshot which the storage array supports.