Creating and maintaining snapshots of file systems
A snapshot of an entire GPFS file system can be created to preserve the contents of the file system at a single point in time. Snapshots of the entire file system are also known as global snapshots. The storage overhead for maintaining a snapshot is keeping a copy of data blocks that would otherwise be changed or deleted after the time of the snapshot.
Snapshots of a file system are read-only; changes can only be made to the active (that is, normal, non-snapshot) files and directories.
The snapshot function allows a backup or mirror program to run concurrently with user updates and still obtain a consistent copy of the file system as of the time that the snapshot was created. Snapshots also provide an online backup capability that allows easy recovery from common problems such as accidental deletion of a file, and comparison with older versions of a file.
- Because snapshots are not copies of the entire file system, they should not be used as protection against media failures. For more information about protection against media failures, see Recoverability considerations.
- Fileset snapshots provide a method to create a snapshot of an independent fileset instead of the entire file system. For more information about fileset snapshots, see Fileset-level snapshots.
- A snapshot of a file creates a new file that captures the user data and user attributes from the original. The snapshot file is independent from the original file. For DMAPI managed file systems, the snapshot of a file is not automatically managed by DMAPI, regardless of the state of the original file. The DMAPI attributes from the original file are not inherited by the snapshot. For more information about DMAPI restrictions for GPFS, see DMAPI restrictions for GPFS.
- When snapshots are present, deleting files from the active file system does not always result in any space actually being freed up; rather, blocks may be pushed to the previous snapshot. In this situation, the way to free up space is to delete the oldest snapshot. Before creating new snapshots, it is good practice to ensure that the file system is not close to being full.
- The use of clones functionally provides writable snapshots. See Creating and managing file clones.