Performance tuning parameters are a valuable tool for the recovery administrator to increase efficiency of backup and restore operations by avoiding throughput bottlenecks. The performance of Oracle backup and restore operations can be optimized through the appropriate configuration of RMAN parameter options provided in the Subclient Properties dialog box. A brief description of certain key performance tuning parameters and their recommended uses is provided below.
The Data Files per BFS parameter defines the number of datafiles to be bundled in each RMAN backupset, for datafile backups. It is often used in conjunction with the Max Open Files parameter to establish the proper RMAN multiplexing factor for disk buffer allocation. For example, assume that you are backing up six datafiles with one RMAN channel. If FILESPERSET=6 and MAXOPENFILES=1, then the channel includes 6 datafiles in a backupset but does not multiplex the files because RMAN is not reading from more than one file simultaneously. The channel reads one file at a time and writes to the backup piece. In this case, the level of multiplexing is 1 and would result in relatively slower backups because of the throughput bottleneck. Ideally, the MAXOPENFILES parameter should be set in such a way that the number of files read simultaneously is just enough to utilize the output device fully. In this example, if FILESPERSET=6 and MAXOPENFILES=3, the level of multiplexing is 2 and would result in a quicker more efficient backup, especially when the output device is tape, by allowing RMAN to provide the proper disk buffer allocation. However, keep in mind that multiplexing too many files can decrease restore performance depending on the hardware configuration.
Archive Files per BFS
The Archive Files per BFS parameter defines the number of archive files to be bundled in each RMAN backupset, for archive log backups. Its use is similar to the Data Files per BFS parameter discussed above.
Max Backupset Size (KB)
The Max Backupset Size parameter defines the maximum allowable size for an RMAN backupset. It can be used to adjust performance for either partial restore or whole database restores. The proper setting depends on whether faster whole database restores or partial restores are required. A smaller MAXBACKUPSETSIZE will result in faster partial restores, however, whole database restores will be slower. A larger MAXBACKUPSETSIZE will result in faster whole database restores, but may not be optimal for partial restores. It is generally recommended that you avoid entering too small a value for this setting, which should be at least 2000 KB. The exception is the default value of 0, which means unlimited.
Max Open Files
The Max Open Files parameter defines the maximum number of concurrent open datafiles that RMAN can read from simultaneously during a backup operation. A smaller MAXOPENFILES setting results in faster performance on most systems. However, it should be used in conjunction with the Data Files per BFS or Archive Files per BFS parameters to achieve the most efficient RMAN multiplexing level for optimizing disk buffer allocation. The goal is to set the number of files read simultaneously to fully utilize the output device. See Data Files per BFS above for a discussion of how these parameters work together. Keep in mind that the default value for this parameter is 8.
See Configure Backup Arguments for step-by-step instructions. See also publications from Oracle Corporation on the topic of RMAN performance tuning for a more complete understanding of this subject.
Disk Ratio
Disk ratio enables RMAN to read data files across disks and group them in a backup set. For example, consider data files distributed across 10 disks that supply data at 10 bytes/second and a tape drive that needs 40 bytes/second to keep streaming. In this case, you can set the disk ratio value to 4, which will direct RMAN to include data files from 4 disks in each backup set. Disk ratio groups the data files into backup sets and distributes the backup load across disks. Even though disk ratio facilitates backup performance, you should keep in mind that more the number of disks from which data files are grouped, slower will be the restore performance. Hence, make sure that a minimum possible value is set for disk ratio.