Snapshot (computer storage)

The word snapshot ( snapshot English ) has several meanings in Information Technology:

• 4.1 File Systems
• 4.2 Block - level
• 4.3 applications

Distributed Systems

In the area of ​​distributed systems is the term used for a recording of the global state at any given time. This can be achieved, for example, the snapshot algorithm.

Screenshot

The arbitrary content on a computer screen is conveyed via the " pressure ", also called " PrtSc ( Print Screen ) " in the buffer. This is a snapshot (English actually screenshot) from the screen content. After calling a word processing or image editing program, the screen contents can be inserted here from the cache.

Snapshot of the browser content

A snapshot of the browser content is a special form of screenshots or screenshots. This also is not directly visible, or only accessible by scrolling the screen content can be included in the snapshot.

Version Management

As a snapshot is called computer programs, a program version that is issued directly from the database versioning and not as an official version (in English "release" ). Most of these versions are generated from the current actual state of the version control of source code or from the working copy of the project of a developer.

Mass storage

When the hard disk, a snapshot is a special memory area that receives Younger or Older versions of changed data. It contains an entire copy of the data base, but is filled gradually with each change. One distinguishes the method redirect -on-write and copy- on-write. When redirect -on-write all changes will be redirected to the snapshot. With copy-on -write changes are held back until the original data is copied to the snapshot. Now if the snapshot is read, it is first checked whether the part is to be read there yet. If this is the case, this part is used, otherwise, is read from the original file.

In file systems

Snapshots can be implemented in the file system where it can be accessed by reading in a special directory on older versions of the directory tree, for example.

Examples are the WAFL file system, UFS in FreeBSD 5, the Btrfs file system, ZFS or NTFS with the Volume Shadow Copy Service in Windows.

At block level

Some disk arrays and logical volume manager can hold snapshots of data blocks to their virtual disks or logical volumes.

Applications

From snapshot backups can be made without having to rely on the consistency of the current data set. Some applications and file systems also offer the possibility to put the data used in a defined state and to delay new write requests to the underlying system has created a snapshot.

For example, a database may be stopped and a snapshot will be created. The database can then be restarted immediately and the snapshot is written, for example, elsewhere on magnetic tape. However, professional databases enable consistent backups without stopping and using snapshots.

Is the file system snapshot available, users can restore their files, for example, after an accidental overwriting itself.

However, it is always important that the time of the snapshot no longer possible write operation is active, in order to avoid inconsistent data in the snapshot. Snapshots can therefore be used only if the consistency of the current data set is guaranteed.