IBM Midrange System DS4000 Series Hardware Manual page 257

In most situations, recovery from an inconsistent redundancy group involves restoring the
volume from a backup source.
Note: In a FlashCopy or Enhanced Remote Mirror copy pair relationship, the pre-read
redundancy feature is only supported on the source logical drives.
When activating the pre-read redundancy check feature, the media scan feature no longer
automatically corrects discovered redundancy group inconsistencies.
Attention: Take care when enabling this feature, as it could have an impact on I/O
performance.
To enable this feature, perform these steps:
1. Select the logical drive where the pre-read redundancy check is to be enabled.
2. Select Logical Drive  Change  Pre-Read redundancy check, as shown in
Figure 4-104.
Figure 4-104 Pre-read redundancy check
Chapter 4. IBM System Storage DS planning and configuration
239