ocp 053:
656.The BACKUP_TAPE_IO_SLAVES parameter is set to FALSE for the database instance. Which
statement is true while performing a tape backup in an RMAN session?
A. The tape I/O performed is asynchronous
B. The tape buffer is allocated from the System Global Area (SGA)
C. The tape buffer is allocated from the Program Global Area (PGA)
D. Oracle I/O uses an interrupt mechanism to determine when each I/O completes
Answer: C
答案解析：
参考：http://docs.oracle.com/cd/E11882_01/backup.112/e10642/rcmtunin.htm#BRADV90074
BACKUP_TAPE_IO_SLAVES specifies whether I/O server processes (also called slaves) are used by Recovery Manager to back up, copy, or restore data to tape. When the value is set to true, Oracle uses an I/O server process to write to or read from a tape device. When the value is set to false (the default), Oracle does not use I/O server process for backups. Instead, the shadow process engaged in the backup accesses the tape device.
Note:
You cannot perform duplexed backups unless you enable this parameter. Otherwise, Oracle returns an error. When this parameter is enabled, Recovery Manager will configure as many server processes as needed for the number of backup copies requested.RMAN allocates the tape buffers in the System Global Area (SGA) or the Program Global Area (PGA), depending on whether I/O slaves are used. If you set the initialization parameter BACKUP_TAPE_IO_SLAVES=true, then RMAN allocates tape buffers from the SGA. Tape devices can only be accessed by one process at a time, so RMAN starts as many slaves as necessary for the number of tape devices. If the LARGE_POOL_SIZE initialization parameter is also set, then RMAN allocates buffers from the large pool. If you set BACKUP_TAPE_IO_SLAVES=false, then RMAN allocates the buffers from the PGA.
If you use I/O slaves, then set the LARGE_POOL_SIZE initialization parameter to dedicate SGA memory to holding these large memory allocations. This parameter prevents RMAN I/O buffers from competing with the library cache for SGA memory. If I/O slaves for tape I/O were requested but there is not enough space in the SGA for them, slaves are not used, and a message appears in the alert log.
The parameter BACKUP_TAPE_IO_SLAVES specifies whether RMAN uses slave processes rather than the number of slave processes. Tape devices can only be accessed by one process at a time, and RMAN uses the number of slaves necessary for the number of tape devices.
BACKUP_TAPE_IO_SLAVES
specifies whether I/O server processes (also called slaves) are used by Recovery Manager to back up, copy, or restore data to tape.
TRUE
When the value is set to true, Oracle uses an I/O server process to write to or read from a tape device.then RMAN allocates tape buffers from the SGA.
False
Oracle does not use I/O server process for backups. Instead, the shadow process engaged in the backup accesses the tape device.then RMAN allocates the buffers from the PGA.
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原文：http://www.cnblogs.com/gispf/p/3792386.html