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==Overview==
Modern [[hard drive]]s feature an ability to recover from some read/write errors by internally remapping [[Disk sector|sector]]s and other forms of self test and recovery. The process for this can sometimes take several seconds or (under heavy usage) minutes, during which time the drive is unresponsive. [[RAID]] controllers are designed to recognize a drive which does not respond within a few seconds, and mark it as unreliable, indicating that it should be withdrawn from use and the [[RAID array|array]] rebuilt from [[parity data]]. This is a long process, degrades performance, and if a second drive should fail under the resulting additional workload, it can be catastrophic.
If the drive itself is inherently reliable but has some bad sectors, then TLER and similar features prevent a disk from being unnecessarily marked as 'failed' by limiting the time spent on correcting detected errors before advising the array controller of a failed operation. The array controller can then handle the data recovery for the limited amount involved, rather than marking the entire drive as faulty.
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