Are you having trouble with I/O throughput on the network for transferring RMAN backups? How to adjust RMAN phases?
What is
the RATE parameter in Oracle RMAN?
RATE
Channel Parameter:
You can use
the RATE parameter to set an upper limit for bytes read so that RMAN
does not consume excessive disk bandwidth and degrade online performance.
Essentially, RATE serves as a backup throttle.
In
the ALLOCATE and CONFIGURE CHANNEL commands,
the RATE parameter specifies the bytes per second that are read on a
channel. For example, if you set RATE 1500K, and if each disk drive
delivers 3 megabytes per second, then the channel leaves some disk bandwidth
available to the online system.
RATE is a channel
parameter in RMAN (Recovery Manager) that limits the I/O throughput
of that channel — specifically, it sets a maximum data transfer rate for
backup, restore, or recovery operations.
It’s
expressed in bytes per second (B/s), kilobytes per second (K/s), megabytes
per second (M/s), or gigabytes per second (G/s).
Syntax
You use it
inside the ALLOCATE CHANNEL or CONFIGURE CHANNEL command.
RMAN>
ALLOCATE CHANNEL c1 DEVICE TYPE DISK RATE 50M;
This example
limits the channel c1 to a maximum of 50 MB/s of I/O throughput.
It’s not
for performance tuning, but for I/O throttling and workload control.
Common
use cases:
1.
Avoid I/O contention
o When backups run
on a production system during business hours, limiting the rate prevents RMAN
from consuming too much disk or network bandwidth.
o Example: Run
incremental backups during daytime with RATE 10M.
2.
Prevent network saturation
o For tape or
network backups, you can throttle backup traffic to avoid impacting other
network operations.
3.
Control load on standby or DR sites
o When restoring
or duplicating databases over network links, RATE ensures stable throughput and
predictable recovery time.
4.
Testing and Benchmarking
o DBAs can
simulate slower disk or network speeds to test restore performance, or tune
parallelism more safely.
How RATE
interacts with CHANNELS
- RATE applies per channel,
not globally.
- So, if you have multiple
channels, total throughput ≈ RATE × number of channels.
Example:
RUN {
ALLOCATE CHANNEL c1 DEVICE TYPE DISK RATE
100M;
ALLOCATE CHANNEL c2 DEVICE TYPE DISK RATE
100M;
BACKUP DATABASE;
}
→ Total
potential throughput ≈ 200 MB/s.
Notes:
|
Behavior |
Description |
|
Upper limit only |
RATE defines a maximum. RMAN may transfer less if
resources are unavailable. |
|
Does not affect
compression or encryption CPU time |
Only I/O bandwidth is throttled.
CPU use for compression/encryption is unaffected. |
|
Use with caution |
Over-throttling can unnecessarily increase backup or restore
time. |
|
In Data Guard
environments |
When doing DUPLICATE FROM ACTIVE
DATABASE, RATE can control how much redo or backup traffic flows across the
network. |
Example
use cases:
Example
1: Controlled Network Restore
RUN {
ALLOCATE CHANNEL c1 DEVICE TYPE sbt RATE 25M;
RESTORE DATABASE;
}
→ Useful for
restoring over slow network (25 MB/s cap).
Example
2: Configure Default Rate
CONFIGURE
CHANNEL DEVICE TYPE DISK RATE 75M;
→ All future
channels will inherit this limit automatically.
When not
to use it
- High-performance Exadata /
Flash-based systems where storage bandwidth is already managed and backups run during
low-load windows.
- When tuning — use parallelism, buffer size
(MAXPIECESIZE, FILES PER SET), or BACKUP AS COMPRESSED BACKUPSET instead.
Summary:
|
Parameter |
Purpose |
Typical Use |
|
RATE |
Limits I/O throughput per channel |
Control load, avoid I/O or network contention |
|
Default |
Unlimited |
RMAN uses full available bandwidth |
|
Syntax |
ALLOCATE CHANNEL … RATE 50M; |
Limits to 50 MB/s per channel |
Removing
the RATE Parameter from Channel Settings
The RATE parameter
on a channel is intended to reduce, rather than increase, backup throughput so
that more disk bandwidth is available for other database operations. If the
backup is not streaming to tape, then confirm that the RATE parameter
is not set.
To remove
the RATE parameter:
1.
Examine your backup script.
2.
Do one of the following:
·
If the backup is in a RUN command, then
remove the RATE parameter, if it is specified, from
the ALLOCATE command. Skip the remaining steps.
·
If the backup is not in a RUN command, then
start RMAN, connect to the target database, and proceed to the next step.
3.
Execute the SHOW ALL command to show the
currently configured settings.
4.
Remove the RATE parameter, if it is set,
from the CONFIGURE CHANNEL command.
*********
Tuning **********
Throughput Vs. Bandwidth
Distinction
between throughput and bandwidth is critical in
understanding and tuning RMAN backup performance. Let’s compare them in the
specific context of Oracle RMAN I/O operations (whether disk or tape, or
network-based like Data Domain or SBT channels).
Definition
and Core Difference
|
Concept |
Definition |
Focus |
Measurement |
|
Bandwidth |
The maximum capacity of a
communication or storage path (disk, network, or SAN) to transfer data per
unit time. |
Capability / limit of path |
MB/s or Gb/s |
|
Throughput |
The actual data rate achieved
during the RMAN backup. |
Real performance / achieved
transfer |
MB/s |
So,
Bandwidth = Potential,
Throughput =
Reality.
Example in RMAN Backup Context
Scenario:
- You back up an Oracle database
using RMAN to a NAS target.
- The network interface is 10
GbE (≈ 1,250 MB/s bandwidth).
- RMAN reports 400 MB/s
backup speed.
|
Metric |
Value |
Meaning |
|
Bandwidth |
1,250 MB/s |
Physical or network channel
capacity |
|
Throughput |
400 MB/s |
Actual RMAN data transfer rate
achieved |
|
Utilization |
32% |
Indicates potential bottlenecks
(CPU, disk I/O, RMAN compression, etc.) |
Factors That Limit Throughput (While
Bandwidth is High)
|
Layer |
Possible Limiter |
Example |
|
RMAN / CPU |
CPU compression overhead |
Using BACKUP AS COMPRESSED
BACKUPSET |
|
Disk I/O |
Slow source or destination disk |
Spinning disks or overloaded ASM
diskgroup |
|
Channel Parallelism |
Too few RMAN channels |
e.g. only 1 channel on a 16-core
system |
|
Network |
Latency or TCP window issues |
Especially over WAN |
|
Media Manager (SBT) |
Slow tape or DD Boost plugin limits |
Deduplication or encryption
overhead |
|
OS Buffering |
Insufficient I/O buffers |
Check LARGE_POOL_SIZE and db_writer_processes
impact |
Be careful that RMAN has 3 Phases and I/O
can be bottleneck on any phases:
1.
Read Phase
A channel
reads blocks from disk into input I/O buffers.
2.
Copy Phase
A channel
copies blocks from input buffers to output buffers and performs additional
processing on the blocks.
3.
Write Phase
A channel
writes the blocks from output buffers to storage media. The write phase can
take either of the following mutually exclusive forms, depending on the type of
backup media:
How RMAN
Reports Throughput
RMAN backup
logs show:
channel
ORA_DISK_1: backup set complete, elapsed time: 00:02:30
channel
ORA_DISK_1: throughput 200 MB/s
This
“throughput” is effective transfer speed across all I/O layers combined.
You can also
query:
SELECT
INPUT_BYTES_DISPLAY, OUTPUT_BYTES_DISPLAY, OUTPUT_BYTES_PER_SEC_DISPLAY FROM V$BACKUP_ASYNC_IO;
to analyze
detailed I/O throughput per file.
Tuning
Goal
- Bandwidth cannot be increased by RMAN
itself (it’s hardware/network capacity).
- Throughput can be improved by optimizing parallelism,
compression, and I/O path efficiency.
Quick
Tips to Increase RMAN Throughput:
|
Area |
Recommendation |
|
Channels |
Use multiple RMAN channels (ALLOCATE
CHANNEL or DEVICE TYPE DISK PARALLELISM N, controlling by RATE param) |
|
Compression |
Disable compression for performance
benchmarking |
|
Buffers |
Increase LARGE_POOL_SIZE or use BACKUP_DISK_IO_SLAVES,
use Async I/O |
|
ASM/Filesystem |
Separate read/write I/O paths |
|
Network |
Enable jumbo frames and optimize
TCP buffers |
|
Target Device |
Use DD Boost or fast disk staging |
Simple Analogy
|
Concept |
Analogy |
|
Bandwidth |
The width of a highway (how many
lanes) |
|
Throughput |
The number of cars actually passing
per second |
|
Bottleneck |
A traffic jam (one slow lane blocks
others) |
Summary
|
Metric |
Definition |
Controlled By |
RMAN Relevance |
|
Bandwidth |
Maximum data capacity of I/O path |
Hardware / Network |
Defines upper limit of
backup performance |
|
Throughput |
Actual achieved data rate |
RMAN + OS + I/O stack |
Real indicator of backup efficiency |
|
Goal |
Throughput ≈ Bandwidth |
Tune RMAN, parallelism, buffers |
Optimize backup performance |
********************Alireza Kamrani***********************
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