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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 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.
Note
The calling application is responsible for cleaning up Storage Checkpoints after a system crash.
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. Primary fileset and its Storage Checkpoint 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
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In Initializing a Storage Checkpoint, a square represents each block of the file system. This figure shows a Storage Checkpoint containing pointers to the primary fileset at the time the Storage Checkpoint is taken, as in Primary fileset and its Storage Checkpoint,
Initializing a Storage Checkpoint
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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.