InfoScale™ 9.0 Storage and Availability Management for DB2 Databases - AIX, Linux

Storage Checkpoints and Rollback process

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.

Initially, a Storage Checkpoint contains no data. The Storage Checkpoint only contains 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 required 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 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.

Database Storage Rollback restores a database on the primary file systems to the point-in-time image created during a Storage Checkpoint.

Database Storage Rollback is accomplished by copying the "before" images from the appropriate Storage Checkpoint back to the primary file system. As with Storage Checkpoints, Database Storage Rollback restores at the block level, rather than at the file level. Database Storage Rollback is executed using the vxsfadm command with the -o restore operation.

For example:

$ $ /opt/VRTSdbed/bin/vxsfadm -s checkpoint /
 -a db2 -o restore --checkpoint_name checkpoint1

Mountable Storage Checkpoints can be used for a wide range of application solutions including the following:

If you mount a Storage Checkpoint as read-write, the command 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, then a "shadow" Storage Checkpoint of the original Storage Checkpoint gets created, and this "shadow" Storage Checkpoint is mounted as read-write. This allows the database to still be rolled back to the original Storage Checkpoint.