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
Tuning guideline:
第一：output buffer size 可以使用_db_file_direct_io_count来增大。
Oracle Database 11g Release 11.1.0.7 + Patch 8369105 (also in Exadata V1 Bundle Patch 2)
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.
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.