Time-Limited Error Recovery
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Time-Limited Error Recovery (TLER) is a name used by Western Digital for a hard drive feature that allows improved error handling in a RAID environment. In some cases, there is a conflict whether error handling should be undertaken by the hard drive or by the RAID controller, which leads to drives being marked as unusable and significant performance degradation, when this could otherwise have been avoided. A similar technology called Error Recovery Control (ERC) is used by competitor Seagate.
[edit] Overview
Modern hard drives feature an ability to recover from some read/write errors by internally remapping sectors 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recognize a drive which does not respond within a few seconds, and mark it as unreliable, indicating that it should be withdrawn from use and the array rebuilt from parity data. This is a long process, degrades performance, and if a second drive should fails 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.
[edit] TLER in a non-RAID environment
Effectively, TLER and similar features limit the performance of on-drive error handling, to allow RAID controllers to handle the error if problematic. In a non-RAID environment, such features are unhelpful, and manufacturers do not recommend their use.
TLER can be enabled or disabled on certain Western Digital drives, using the tool WDTLER on a DOS bootdisk. Western Digital states that this feature cannot be disabled. [1] However users and independed editors have have reported that this feature can be disabled. [2] [3] [4] [5] The tool allows this feature to be disabled by setting the values for read and write and 0 second. By default, Caviar Raid Edition comes with read disabled and write set to 7 seconds while both settings are disabled for Raptor hard disks.
