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A Storage Checkpoint is a disk and I/O efficient snapshot technology for creating a "clone" of a currently mounted file system (the primary file system). Like a snapshot file system, a Storage Checkpoint appears as an exact image of the snapped file system at the time the Storage Checkpoint was made. However, unlike a snapshot file system that uses separate disk space, all Storage Checkpoints share the same free space pool where the primary file system resides unless a Storage Checkpoint allocation policy is assigned. A Storage Checkpoint can be mounted as read-only or read-write, allowing access to the files as if it were a regular file system. A Storage Checkpoint is created using the dbed_ckptcreate command.
Initially, a Storage Checkpoint contains no data—it contains only the inode list and the block map of the primary fileset. This block map points to the actual data on the primary file system. Because only the inode list and block map are needed and no data is copied, creating a Storage Checkpoint takes only a few seconds and very little space.
A Storage Checkpoint initially satisfies read requests by finding the data on the primary file system, using its block map copy, and returning the data to the requesting process. When a write operation changes a data block n in the primary file system, the old data is first copied to the Storage Checkpoint, and then the primary file system is updated with the new data. The Storage Checkpoint maintains the exact view of the primary file system at the time the Storage Checkpoint was taken. Subsequent writes to block n on the primary file system do not result in additional copies to the Storage Checkpoint because the old data only needs to be saved once. As data blocks are changed on the primary file system, the Storage Checkpoint gradually fills with the original data copied from the primary file system, and less and less of the block map in the Storage Checkpoint points back to blocks on the primary file system.
You can set a quota to limit how much space a file system will give to all storage checkpoints, to prevent the checkpoints from consuming all free space. See the command dbed_ckptquota for more information.
Storage Rollback restores a database, a tablespace, or datafiles on the primary file systems to the point-in-time image created during a Storage Checkpoint. Storage Rollback is accomplished by copying the "before" images from the appropriate Storage Checkpoint back to the primary file system. As with Storage Checkpoints, Storage Rollback restores at the block level, rather than at the file level. Storage Rollback is executed using the dbed_ckptrollback command.
Note
Whenever you change the structure of the database (for example, by adding or deleting datafiles, converting PFILE to SPFILE, or converting SPFILE to PFILE), you must run dbed_update.
Mountable Storage Checkpoints can be used for a wide range of application solutions, including backup, investigations into data integrity, staging upgrades or database modifications, and data replication solutions.
If you mount a Storage Checkpoint as read-write, roll back to this Storage Checkpoint will not be permitted. This ensures that any Storage Checkpoint data that has been modified incorrectly cannot be a source of any database corruption. When a Storage Checkpoint is mounted as read-write, the dbed_ckptmount command creates a "shadow" Storage Checkpoint of and mounts this "shadow" Storage Checkpoint as read-write. This enables the database to still be rolled back to the original Storage Checkpoint.