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RAID Best Practice: Write Over a Media Error

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When a disk reports a media error on read it merely means that the read failed, it is hard to judge by that if the location is actually bad or if the media has deteriorated at this location. This is what the Hitachi Ultrastar 5K3000 spec had to say about this:

9.9.1 Auto Reassign Function

Non recovered read errors

When a read operation is failed after defined ERP is fully carried out, a hard error is reported to the host system. This location is registered internally as a candidate for the reallocation. When a registered location is specified as a target of a write operation, a sequence of media verification is performed automatically. When the result of this verification meets the criteria, this sector is reallocated.

Hence the right thing to do is to recover the data from the RAID parity and write it over, the disk is then tasked with writing the location and verifying that the write worked fine, if it did than we are good, if not, the disk will reallocate the data at that location and move it to one of the spare locations and it will be noted by an increase in the number of reallocations.

It would also be a good idea to use a WRITE VERIFY command to enforce and be sure of a proper write in this case as this is already a suspect location.