“Parallel Backup” in Oracle Recovery Manager (RMAN) is a feature that dramatically shortens backup times by running multiple channels in parallel. In large-scale database operations, understanding this parallelization is essential to complete processing within a limited backup window.
This article focuses on two methods—manual channel allocation and automatic channel allocation—explaining their pros, cons, and specific configuration examples.
Conclusion: Which Allocation Method Should You Choose?
- To simplify routine backup operations: Automatic Channel Allocation (
CONFIGURE) is optimal. - For fine-grained control during temporary maintenance or specific environments: Use Manual Channel Allocation (
ALLOCATE).
1. Parallel Backup via Manual Channel Allocation
This method involves using the ALLOCATE CHANNEL command within a RUN block to define channels that are valid only for the duration of that execution.
Pros and Cons
| Pros | Cons |
| Parallelism can be dynamically changed for each backup target. | Scripts become longer, increasing the maintenance burden. |
| Specific disk drives or paths can be individually designated. | Channel definitions must be written every time the script runs. |
Execution Example
In manual allocation, it is recommended to explicitly release channels using RELEASE CHANNEL after use (though they are automatically released when the RUN block ends).
# Execute parallel backup by manually allocating 3 channels
RMAN> RUN {
ALLOCATE CHANNEL ch1 DEVICE TYPE DISK;
ALLOCATE CHANNEL ch2 DEVICE TYPE DISK;
ALLOCATE CHANNEL ch3 DEVICE TYPE DISK;
BACKUP DATABASE;
RELEASE CHANNEL ch1;
RELEASE CHANNEL ch2;
RELEASE CHANNEL ch3;
}
Point: Each ALLOCATE CHANNEL corresponds to one OS process (or thread) and creates backup pieces in parallel.
2. Parallel Backup via Automatic Channel Allocation
This method uses the CONFIGURE command to persistently record parallelism settings in the control file.
Pros and Cons
| Pros | Cons |
| Automatically applied to all subsequent commands after a one-time setup. | Difficult to flexibly change the degree of parallelism per session. |
| Backup scripts become extremely simple. | Requires pre-configuration (CONFIGURE) suited to the environment. |
Execution Example
By simply setting the degree of parallelism (PARALLELISM), RMAN automatically generates the required number of channels.
# Set parallelism to 3 (Persistent setting)
RMAN> CONFIGURE DEVICE TYPE DISK PARALLELISM 3;
# After setting, a 3-way parallel backup runs with just this:
RMAN> BACKUP DATABASE;
# Verify current settings
RMAN> SHOW ALL;
Point: In automatic allocation, channel names (such as ORA_DISK_1) are also assigned automatically.
3. Manual vs. Automatic Allocation: Comparison Summary
| Comparison Item | Manual (ALLOCATE) | Automatic (CONFIGURE) |
| Configuration Command | ALLOCATE CHANNEL | CONFIGURE ... PARALLELISM |
| Validity Period | Within the RUN block only | Persistent until changed |
| Flexibility | Very High (Free path designation, etc.) | Low (Uniform settings) |
| Recommended Scenario | Spot backups, special path configurations | Daily nightly backups, standard operations |
4. Points for Performance Tuning
Simply increasing parallelism may result in plateaued performance. Consider the following factors:
- Resource Considerations: Set the number of channels within a range that does not exceed the number of CPU cores or disk I/O bandwidth. Too many channels may cause “I/O wait,” potentially slowing down the process.
- Adjusting
FILESPERSET: Adjust the number of files included in one backup set to balance the load across each channel. MAXPIECESIZE: Limit the maximum size of a single backup piece to avoid file system limits and improve transfer efficiency.- Multi-section Backups: If you have massive data files, specifying
SECTION SIZEallows multiple channels to share the workload of backing up a single file.
Operational and Security Considerations
- Resource Contention: Increasing backup parallelism puts pressure on I/O and CPU for online operations. Consider executing during off-peak hours or controlling resources via Resource Manager.
- Reverting Settings: To cancel parallel settings (revert to default), execute
CONFIGURE DEVICE TYPE DISK CLEAR;. - Limitations: In Standard Edition 2 (SE2), RMAN parallelism (number of channels) is limited to a maximum of 1. Enterprise Edition is required to use multiple channels.
FAQ: Frequently Asked Questions
Q: If I set PARALLELISM to 10, will it be 10 times faster?
A: Theoretically, performance improves, but since storage write speeds and network bandwidth become bottlenecks, you must determine the optimal number based on actual measurements.
Q: What happens if both manual and automatic settings are configured?
A: If ALLOCATE CHANNEL is used within a RUN block, it takes precedence, and the automatic channels set via CONFIGURE will not be used.
Q: Is parallel backup effective for Tape (SBT) backups as well?
A: Yes, but be careful; allocating more channels than the number of physical tape drives will cause tape contention and drastically reduce efficiency.
Summary
- Use manual allocation when detailed control is required for specific tasks.
- Use automatic allocation to simplify daily operational management.
- Determining the degree of parallelism requires a balance with hardware resources (CPU/Disk).
- Keep in mind the parallelism limits in the SE2 license.
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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