Using Storage Checkpoints and Storage Rollback for Backup and Restore

VERITAS Storage Foundation for Oracle provides a Storage Checkpoint facility that is similar to the snapshot file system mechanism; however, a Storage Checkpoint persists after a system reboot. A Storage Checkpoint creates an exact image of a database instantly and provides a consistent image of the database from the point in time the Storage Checkpoint was created. The Storage Checkpoint image is managed and available through the VxDBA utility, the GUI, or the VERITAS Storage Foundation for Oracle command line interface (CLI). VERITAS NetBackup also makes use of Storage Checkpoints to provide a very efficient Oracle backup mechanism.

A direct application of the Storage Checkpoint facility is Storage Rollback. Because each Storage Checkpoint is a consistent, point-in-time image of a file system, Storage Rollback is the restore facility for these on-disk backups. Storage Rollback rolls back blocks contained in a Storage Checkpoint into the primary file system for restoring the database faster. For more information on Storage Checkpoints and Storage Rollback, see the VERITAS File System Administrator's Guide.

Understanding Storage Checkpoints and Storage Rollback

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 or the GUI.

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.

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 or the GUI.

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, the VxDBA utility and GUI will not allow you to roll back to this Storage Checkpoint. 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 allows the database to still be rolled back to the original Storage Checkpoint.