About quiescing the system

Before a useful snapshot can be created, the data to back up must be transactionally consistent or complete. A transaction is a single data action, such as updating a patient's record in a medical database, or creating a record for a new patient. Such a transaction is composed of multiple I/O requests (search, copy, send, write, and so forth). Until the transaction's I/O requests are complete, the data is inconsistent and may be unsuitable for backup.

Transactions affect all levels of the storage management stack (file system, volume manager, and so forth). A transaction generates further transactions as a request is handed off to the next level of the stack. For instance, in the file system, an I/O request from a database application constitutes a transaction and may be split into many disk references. All these disk references must be complete for the original request to be fulfilled. Thus, the creation of the snapshot must be coordinated with any application or process that can affect the transactional consistency of the data.

The means of coordination is called quiesce (literally, to make quiet or place in repose). Quiesce involves pausing the database application or process until the data is transactionally consistent. Applications and the storage management stack must all be quiesced before a useful snapshot can be made.