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Figure 3-7. Competing processors interfering with each other Right after Processor 1 tests the flag and finds it clear, Processor 2 does the same thing. Since both find the flag clear, they both set it and think they have exclusive access to the associated data structure. The problem is caused by the fact that between the test and set operations, the bus controller changed briefly, allowing an interleaved test operation by Processor 2. Multiprocessing buses provide so-called atomic test-and-set bus cycles to prevent this type of interference. When a processor assumes hardware control of the bus for such a cycle, the processor executes both the test and set operation in the same bus cycle. You can prevent interference between competing processors in other ways, but atomic test-and-set cycles are probably the simplest and safest. As long as all the processors in the system respect the software locks in shared memory, everyone gets along. You can use software flags to lock any portion of shared memory, from a single data structure to the entire memory.

The difference between consistent and inconsistent backups is simple A consistent backup doesn t need to go through a recovery process When a backup is used to recover a database or a part of a database (such as a tablespace or a datafile), first you need to restore the backup, and then you recover the database In the case of a consistent backup, you don t have to perform any recovery steps An inconsistent backup, on the other hand, always needs to undergo a recovery Oracle assigns every transaction a unique system change number (SCN) Each commit, for example, will advance the SCN forward Each time Oracle performs a checkpoint, all the changed data in the online datafiles is written to disk And each time there is a checkpoint, the thread checkpoint in the control file is updated by Oracle.

During this thread checkpoint, Oracle makes all the read/write datafiles and the control files consistent to the same SCN A consistent database means that the SCNs stored in all the datafile headers are identical and are also the same as the datafile header information held in the control files The important thing is that the same SCN number must appear in all the datafiles and the control file(s) The identical SCN means that the datafiles contain data taken from the same point in time Since the data is consistent, you don t need to perform any recovery steps after you restore (or copy back) a set of backup files.

When using shared memory to send notifications, an area of memory is set aside to handle the notification payload. The area must be associated with a lock, so the sender and receiver don t try to modify it at the same time. The area must accommodate at least one notification, but may be large enough to hold many in a buffer. Figure 3-8 shows the timing of the operations performed by the sender and receiver.

To make a consistent backup, either the database needs to be closed (with a normal SHUTDOWN or SHUTDOWN TRANSACTIONAL command, not a SHUTDOWN ABORT command) or it needs to be in a mount position after being started (again, after a clean shutdown) An inconsistent backup is a backup in which the files contain data from different points in time Most production systems can t be shut down for a consistent backup Instead, you need to operate those databases on a 24/7 basis You thus must back up the datafiles of these databases online; that is, while the database is open for transactions Since the datafiles are being modified by users while you are backing them up, you end up with an inconsistent backup Inconsistent backups don t mean there is anything wrong with your backups However, during a recovery process, it isn t sufficient to merely restore these backups.

In addition to restoring these backups, you must also supply all archived and online redo logs from the time of the backup to the time to which you want to recover the database Oracle will read these log files and apply all necessary changes to the restored backup files Since you can make an inconsistent backup of a database while it s open, most production databases use inconsistent backups as the foundation of their backup strategy..