What is a Storage Checkpoint?

The VERITAS File System provides a unique Storage Checkpoint facility that quickly creates a persistent image of a file system at an exact point in time. Storage Checkpoints significantly reduce I/O overhead by identifying and maintaining only the file system blocks that have changed since the last Storage Checkpoint or backup via a copy-on-write technique (see How a Storage Checkpoint Works). Unlike a disk-based mirroring technology that requires a separate storage space, this VERITAS technology minimizes the use of disk space by creating a Storage Checkpoint within the same free space available to the file system.

Storage Checkpoints are data objects that are managed and controlled by the file system; as a result, Storage Checkpoints are persistent across system reboots and crashes. You can create, remove, and rename Storage Checkpoints because they are data objects with associated names (see Storage Checkpoint Administration). After you create a Storage Checkpoint of a mounted file system, you can also continue to create, remove, and update files on the file system without affecting the logical image of the Storage Checkpoint. This technology preserves not only the name space (directory hierarchy) of the file system, but also the user data as it existed at the moment the Storage Checkpoint was taken.

Storage Checkpoints differ from VERITAS File System snapshots in the following ways because they:

• Allow write operations to the Storage Checkpoint itself.
• Persist after a system reboot or failure.
• Share the same pool of free space as the file system.
• Maintain a relationship with other Storage Checkpoints by identifying changed file blocks since the last Storage Checkpoint.
• Have multiple, read-only Storage Checkpoints that reduce I/O operations and required storage space because the most recent Storage Checkpoint is the only one that accumulates updates from the primary file system.

Various backup and replication solutions can take advantage of Storage Checkpoints. The ability of Storage Checkpoints to track the file system blocks that have changed since the last Storage Checkpoint facilitates backup and replication applications that only need to retrieve the changed data. Storage Checkpoints significantly minimize data movement and may promote higher availability and data integrity by increasing the frequency of backup and replication solutions.

Storage Checkpoints can be taken in environments with a large number of files (for example, file servers with millions of files) with little adverse impact on performance. Because the file system does not remain frozen during Storage Checkpoint creation, applications can access the file system even while the Storage Checkpoint is taken. However, Storage Checkpoint creation may take several minutes to complete depending on the number of files in the file system.