Databases Reference
In-Depth Information
Variables that you can control from RMAN are your incremental backup strategy and the level of multiplexing.
If your tape device is not streaming (continuously writing), then consider using an incremental backup strategy. Incremental
backups usually have fewer writes than full backups and therefore should lessen the bottleneck writing to tape.
The block change tracking feature enables RMAN to quickly identify which blocks have changed since the last
incremental backup. This feature can significantly improve the performance of incremental backups.
Also, consider altering the default multiplexing behavior of RMAN. Setting
filesperset
high and
maxopenfiles
low may increase the efficiency of writing to your tape device. You can also use the
diskratio
parameter to instruct
RMAN to balance the load if data files are distributed across several different disks. See Recipe 16-9 for details on how
to adjust the multiplexing of backup sets.
Maximizing Throughput to Backup Device
Problem
You suspect that your backup device is a bottleneck for backup operations. You want to adjust the throughput to the
backup device.
Solution
You can tune the throughput to backup devices by adjusting RMAN's level of multiplexing. RMAN multiplexing is
controlled by three parameters:
•
filesperset
•
maxopenfiles
•
diskratio
Using filesperset
Use the
fileperset
clause of the
backup
command to limit the number of data files in each backup set. For example,
if you wanted to limit the number of files being written to a backup set to only two files, you would use filesperset, as
shown here:
RMAN> backup database filesperset 2;
Using maxopenfiles
Use the
maxopenfiles
clause of the
configure channel
command or the
allocate channel
command to limit the
number of files that can be simultaneously open for reads during a backup. If you want to limit the number of files
being read by a channel to two files, use
maxopenfiles
as follows:
RMAN> configure channel 1 device type disk maxopenfiles 2;
To reset the channel
maxopenfiles
to the default setting, use the
clear
parameter as shown here:
RMAN> configure channel 1 device type disk clear;
