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IBM Midrange System Storage Hardware Guide
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.
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.
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
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.
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.
Reported error
Description
Result
Unrecovered media
error
The data cannot be read on its
first attempt, or on any
subsequent retries.
With redundancy check: Data is
reconstructed and scanned again.
Without redundancy check: No error
correction.
Recovered media
error
The drive cannot read the
requested data on its first
attempt, but succeeded on a
subsequent attempt.
Data is written to drive and verified.
Redundancy
mismatches
Redundancy errors are found.
The first 10 redundancy mismatches found
on a logical drive are reported. Operating
system data checking operations should
be executed.
Unfixable error
The data cannot be read, and
parity or redundancy
information cannot be used to
regenerate it.
An error is reported.
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