snapshot
- Snapshot is the term used to describe snapshots. The essential feature of snapshots is the time at which they are triggered. Snapshots are used in many technical applications to capture and document a time-related state. The term snapshot is well known from photography. Other examples are the screenshot of a screen display, where the displayed screen content is saved, the frame grabbing of video sequences or the snapshot of a process with which the state of the process at a certain point in time can be reconstructed. Likewise the backup of data or the time-related storage of multithread activities of a server.
- The snapshot technique is of particular importance in the storage area. It is a copy of a data medium at a specific point in time. Data changes made after the snapshot are not taken into account. Applications and users can access the snapshot files via a special path. There are various snapshot procedures for storage snapshots. These include Copy-on-Write( CoW), Redirect-on-Write( RoW), Instant Copy and Incremental Snapshot. All these methods are suitable for fast data backup. Applications can be shut down briefly for this purpose. After the stop, the snapshot is taken and the application can be started up again. The situation is different for snapshots for tape drives on which several 100 GB of data are stored. In these cases you work with the mirrored data. You disconnect the mirroring and take a snapshot of the mirrored storage medium. Depending on the data volume and data transfer rate, this can take several hours. Afterwards, the mirrored storage is reconnected to the master storage and the changes are reloaded. Since usually only a small part of the data of a file system changes, a simpler procedure is based on freezing the file system at the time of the snapshot and entering the new data in copies of the data blocks. Thus, only the data blocks in which data changes are made are managed twice. Snapshot solutions can be hardware or software oriented. The implementation of a snapshot software can be found in Microsoft Storage Server 2003, which was developed especially for Network Attached Storage( NAS).