How a Storage Checkpoint Works

The Storage Checkpoint facility freezes the mounted file system (known as the primary fileset), initializes the Storage Checkpoint, and thaws the file system. Specifically, the file system is first brought to a stable state where all of its data is written to disk, and the freezing process momentarily blocks all I/O operations to the file system. A Storage Checkpoint is then created without any actual data; the Storage Checkpoint instead points to the block map (described below) of the primary fileset. The thawing process that follows restarts I/O operations to the file system.

You can create a Storage Checkpoint on a single file system or a list of file systems. A Storage Checkpoint of multiple file systems simultaneously freezes the file systems, creates a Storage Checkpoint on all of the file systems, and thaws the file systems. As a result, the Storage Checkpoints for multiple file systems have the same creation timestamp. The Storage Checkpoint facility guarantees that multiple file system Storage Checkpoints are created on all or none of the specified file systems (unless there is a system crash while the operation is in progress).

As mentioned above, a Storage Checkpoint of the primary fileset initially contains a pointer to the file system block map rather than to any actual data. The block map points to the data on the primary fileset. The figure below shows the file system /database and its Storage Checkpoint. The Storage Checkpoint is logically identical to the primary fileset when the Storage Checkpoint is created, but it does not contain any actual data blocks.

Primary Fileset and Its Storage Checkpoint

Click the thumbnail above to view full-sized image.

In the figure below, each block of the file system is represented by a square. Similar to the previous figure, this figure shows a Storage Checkpoint containing pointers to the primary fileset at the time the Storage Checkpoint is taken.

Initializing a Storage Checkpoint

Click the thumbnail above to view full-sized image.

The Storage Checkpoint presents the exact image of the file system by finding the data from the primary fileset. As the primary fileset is updated, the original data is copied to the Storage Checkpoint before the new data is written. When a write operation changes a specific data block in the primary fileset, the old data is first read and copied to the Storage Checkpoint before the primary fileset is updated. Subsequent writes to the specified data block on the primary fileset do not result in additional updates to the Storage Checkpoint because the old data needs to be saved only once. As blocks in the primary fileset continue to change, the Storage Checkpoint accumulates the original data blocks.

In the following figure, the third block originally containing C is updated. Before the block is updated with new data, the original data is copied to the Storage Checkpoint. This is called the copy-on-write technique, which allows the Storage Checkpoint to preserve the image of the primary fileset when the Storage Checkpoint is taken.

Every update or write operation does not necessarily result in the process of copying data to the Storage Checkpoint. In this example, subsequent updates to this block, now containing C', are not copied to the Storage Checkpoint because the original image of the block containing C is already saved.

Updates to the Primary Fileset

Click the thumbnail above to view full-sized image.