In Oracle Recovery Manager (RMAN) backup operations, one of the critical concepts that determines performance is “Multiplexing.” This refers to the technique of reading blocks from multiple data files in parallel and writing them interleaved into a single backup piece.
This article provides a professional perspective on the basic concepts of multiplexed backup sets, configuration methods, pros and cons, and recommended settings to maximize performance.
1. Mechanism of Multiplexed Backup Sets (Multiplexing)
RMAN multiplexing means that “a single channel reads multiple data files simultaneously and writes them to one backup piece.” This allows the system to maximize the throughput of the destination device (disk or tape) by reading from multiple files concurrently, even if the read speed of individual data files is slow.
Visualization of Multiplexing
Without multiplexing, RMAN finishes writing all of File A before moving to File B. With multiplexing, blocks from each file are arranged alternately.
- No Multiplexing (Normal):
[AAAAA][BBBBB][CCCCC] - Multiplexing Level 3:
[ABC][ABC][ABC][ABC]
By interleaving blocks in this manner, RMAN avoids I/O concentration on specific disks and reduces the overall backup time.
2. How to Configure Multiplexed Backup Sets
The level of multiplexing is determined by FILESPERSET (the number of files included in one backup set) or MAXOPENFILES (the number of files a single channel opens simultaneously).
2.1 Manual (Temporary) Configuration
Using SET BACKUP COPIES within a RUN block allows you to specify “duplexing (mirroring)” of the backup.
Note: The SET BACKUP COPIES command creates multiple identical files (Duplicated Backup). To adjust the block interleaving (multiplexing level), use FILESPERSET in conjunction.
# Example of creating 2 backup copies and applying multiplexing
RMAN> RUN {
ALLOCATE CHANNEL ch1 DEVICE TYPE DISK;
SET BACKUP COPIES 2;
BACKUP DATABASE FILESPERSET 4;
}
2.2 Changing Default (Persistent) Settings
You can use the CONFIGURE command to define multiple output destinations (multiplexed copies) for backups.
# Set output to two destinations (Duplexed backup configuration)
RMAN> CONFIGURE CHANNEL DEVICE TYPE DISK FORMAT '/backup1/%U', '/backup2/%U';
Verification of execution results:
RMAN> CONFIGURE CHANNEL DEVICE TYPE DISK FORMAT '/u01/app/oracle/backup1/%U', '/u01/app/oracle/backup2/%U';
old RMAN configuration parameters:
CONFIGURE CHANNEL DEVICE TYPE DISK FORMAT '/u01/app/oracle/backup1/%U', '/u01/app/oracle/backup2/%U';
new RMAN configuration parameters:
CONFIGURE CHANNEL DEVICE TYPE DISK FORMAT '/u01/app/oracle/backup1/%U', '/u01/app/oracle/backup2/%U';
new RMAN configuration parameters are successfully stored
released channel: ORA_DISK_1
released channel: ORA_DISK_2
released channel: ORA_DISK_3
3. Pros and Cons of Multiplexing
| Feature | Details |
| ✅ Pros | Faster Backups: Parallelizes reading from slow disks to maximize destination throughput. I/O Optimization: Distributes the load across specific data files to resolve overall system bottlenecks. Improved Fault Tolerance: When used with BACKUP COPIES, recovery is possible even if one backup is corrupted. |
| ❌ Cons | Increased Restore Time: Because blocks are interleaved, the entire backup piece must be scanned even when restoring a single file. Contention from Over-configuration: If the multiplexing level is too high, it may cause head seeking and degrade performance. |
4. Performance Test Results (Reference Values)
The following table shows the trend in backup time when changing the multiplexing level (the number of files read simultaneously).
| Multiplexing Level (FILESPERSET) | Backup Time (Seconds) | Data Size (GB) |
| None (1) | 120 | 50 |
| Low (2) | 90 | 50 |
| Medium (4) | 80 | 50 |
| High (8 or more) | 75 | 50 |
Analysis: Increasing the level reduces the time, but after a certain point (4–8 in this example), the reduction diminishes due to I/O contention.
5. Recommended Settings and Selection Criteria
The following are guidelines for the optimal multiplexing level (simultaneous open files) based on the environment:
- Disk Backup: 2 to 4. For modern fast storage, prioritize increasing parallel channels (
PARALLELISM) instead. - Tape (SBT) Backup: 4 to 8. Higher multiplexing is effective for maintaining the write speed (streaming) of the tape drive.
- Cloud Storage: 1 to 2. It is more efficient to increase the number of parallel channels to separate network sessions rather than increasing multiplexing.
Summary
- Multiplexing is a technique for reading multiple data files simultaneously to improve writing efficiency.
- It contributes to shortening backup time but has the side effect of slowing down restores if overused.
- It is particularly effective in SBT (Tape) environments. In disk environments, balance with PARALLELISM is important.
- Test in a real environment to determine whether the bottleneck is “read” or “write” before configuring.
This article is based on Oracle Database 19c (screens and default values may differ in other versions).
[reference]
Oracle Database Backup and Recovery Reference, 19c
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