IBM Midrange System DS4000 Series Hardware Manual page 272

A media scan can be considered a surface scan of the hard drives, and a redundancy check
scans the blocks of a RAID 3, 5, or 6 logical drive and compares it against the redundancy
data. In the case of a RAID 1 logical drive, the redundancy scan compares blocks between
copies on mirrored drives.
Note: A media scan is only capable of resolving media errors and data or parity
mismatches. A media scan does not attempt to resolve any other sort of error occurring
during I/O operations.
We have seen no effect on I/O when we use a 30-day setting unless the processor is utilized
in excess of 95%. The length of time that it takes to scan the logical drives depends on the
capacity of all the logical drives on the system and the utilization of the controller.
Important: The media scan must be enabled for the entire storage subsystem and
enabled on each logical drive within the storage subsystem to protect the logical drive from
failure due to media errors. This is the default and recommended configuration.
Table 4-11 shows the errors and describes several of the actions that the DS5000 storage
subsystem takes as a result of a media scan and redundancy check operations.
Table 4-11 Media scan errors
Reported error
Unrecovered media
error
Recovered media
error
Redundancy
mismatches
Unfixable error
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IBM Midrange System Storage Hardware Guide
Description
The data cannot be read on its
first attempt, or on any
subsequent retries.
The drive cannot read the
requested data on its first
attempt, but succeeded on a
subsequent attempt.
Redundancy errors are found.
The data cannot be read, and
parity or redundancy
information cannot be used to
regenerate it.
Result
With redundancy check: Data is
reconstructed and scanned again.
Without redundancy check: No error
correction.
Data is written to drive and verified.
The first 10 redundancy mismatches found
on a logical drive are reported. Operating
system data checking operations should
be executed.
An error is reported.