Seagate ST39175LW Guía para resolver problemas descargar pdf (Pagina 33)

Chapter 1, Introduction 19

Processing Before and After the Frozen Image

Transactions affect all levels of the storage management stack (file system, volume

manager, and so forth), generating further transactions as a request is handed off to the

next level of the stack. From the viewpoint of the file system, for instance, an I/O request

issued by a database application constitutes a transaction and may be split into many disk

references, all of which must be complete for the original request to be fulfilled. As a

result, the creation of the frozen image must be coordinated with any application or other

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). This

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 frozen image can be made.

Quiescing the Database Application

Most database applications are transactionally consistent only at particular points in time.

Sometimes, they are consistent only after they have been shut down. Since there is a

growing need for database applications to remain up and available constantly, many

applications are now designed to reach a point of transactional consistency at regular

intervals or in response to an external event. This process is called application quiesce,

described below and in the figure.

In database application quiesce, an external signal or message is sent to a receptive

database. In response, the database finishes the current transaction or group of

transactions, signaling the frozen image consumer when this is complete. (The database

then waits for a second signal indicating that normal operations can resume.) After the

database signals that it has reached a state of transactional consistency, the frozen image

consumer proceeds with the final steps of creating the frozen image.

Once the frozen image has been created, another signal is sent to the waiting database

telling it to resume processing. This is called unquiescing the application.

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Seagate ST39175LW Guía para resolver problemas Pagina 33