6/13/2023 0 Comments Drive snapshot![]() ![]() ![]() Some Unix systems have snapshot-capable logical volume managers. This method of pointer-based snapshots consumes less disk capacity than if the data set was repeatedly cloned. In some systems once the initial snapshot is taken of a data set, subsequent snapshots copy the changed data only, and use a system of pointers to reference the initial snapshot. In other words, the time and I/O needed to create the snapshot does not increase with the size of the data set by contrast, the time and I/O required for a direct backup is proportional to the size of the data set. Most snapshot implementations are efficient and can create snapshots in O(1). To avoid downtime, high-availability systems may instead perform the backup on a snapshot-a read-only copy of the data set frozen at a point in time-and allow applications to continue writing to their data. High-availability 24/7 systems, however, cannot bear service stoppages. ![]() This is tolerable for low-availability systems (on desktop computers and small workgroup servers, on which regular downtime is acceptable). One approach to safely backing up live data is to temporarily disable write access to data during the backup, either by stopping the accessing applications or by using the locking API provided by the operating system to enforce exclusive read access. Version skew may also cause corruption with files which change their size or contents underfoot while being read. For example, if a user moves a file into a directory that has already been backed up, then that file would be completely missing on the backup media, since the backup operation had already taken place before the addition of the file. This prevents the backup from being atomic and introduces a version skew that may result in data corruption. On multi-tasking or multi-user systems, there may be writes to that data while it is being backed up. The term was coined as an analogy to that in photography.Ī full backup of a large data set may take a long time to complete. In computer systems, a snapshot is the state of a system at a particular point in time. Example of snapshots of a Btrfs filesystem, managed with snapper ![]()
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