使用buffer memory 参数来调整rman的性能。
本文翻译自mos文章：RMAN Performance Tuning Using Buffer Memory Parameters (Doc ID 1072545.1)
rman 性能调整的目的是分辨一个特定的backup or restore job的瓶颈。
并使用使用rman命令、初始化参数 或者对physical media的调整来提高整体的性能.
由于数据库容量持续变大，在客户的环境中，几十到几百TB的数据库很常见，
server和存储系统也在resource， network，io方面增大容量以支持数据库backup 和restore windows
在此种背景下（against this backdrop），rman的性能调整变得比以往都重要。
本文详细描述了在backup和restore操作期间，rman性能调整中，与buffer memory 相关的guidelines
这些guidelines 以如下系统为目标：
the available I/O exceeds 100 MB/s, while memory, CPU, network consumption,
and tape drive throughput do not constrain overall RMAN performance.
在very high performance configurations中，这是尤为正确的，比如说Exadata。
在这些环境中，默认的rman buffer size 是不充分的（inadequate），需要加大。
When restoring from backup sets:
output buffer的 size为128KB/buffer
output buffer的 number为4 buffers
Tuning guideline:
第一：output buffer的number (数量) 可以使用 _backup_ksfq_bufcnt参数来增大。
第二：output buffer的size (大小) 可以使用 _backup_ksfq_bufsz参数来增大。
第三：Note that the default buffer sizes for restoring files are smaller than what are used when backing up those same files.
These buffers should be increased to match the corresponding backup if restore times are significantly slower than the corresponding backups.
When restoring from image copies:
默认的output buffer size 是1MB/buffer
默认的output buffer number 是4 buffers
Input and output buffers are allocated and tuned largely the same as in the preceding section,
with the exception of the following changes:
第一：When restoring from backup sets, the default output buffer size is increased from 128 KB to 1 MB.
第二：_db_file_direct_io_count is not used to adjust any RMAN buffer sizes. This means that RMAN buffer sizes can be adjusted without affecting any other database I/O.
第三：_backup_ksfq_bufcnt can be used to adjust the number of input and output buffers for all operations using DISK channels.
Oracle Database 11g Release 2
All RMAN I/O operations with ASM files select an optimal buffer size and count based on the allocation unit (AU) size and disk count of the ASM disk group.
Additional parameters are introduced to give more fine-grained control over RMAN buffer allocations. Although these parameters are supported for all RMAN operations, they are primarily intended for use in non-ASM systems, since RMAN operations in ASM will automatically
select an optimal buffer configuration.
第一：_backup_seq_bufsz/_backup_seq_bufcnt
o These parameters set the buffer size and count for both creating and restoring backup pieces using SBT channels with third-party media managers.
o Note that these parameters are not needed when using Oracle Secure Backup, as the optimal buffer size is automatically selected.
第二：_backup_disk_bufsz/_backup_disk_bufcnt
o These parameters set the buffer size and count for both creating and restoring backup pieces using DISK channels.
第三：_backup_file_bufsz/_backup_file_bufcnt
o These parameters set the input buffer size and count for all backup operations, and the output buffer size and count for all restore operations.
第四：
Note that _backup_ksfq_bufsz and _backup_ksfq_bufcnt are still supported in Oracle Database 11g Release 2 for compatibility reasons, but it is recommended that you tune the RMAN buffer sizes instead using the above parameters.
