Time-Limited Error Recovery

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. Similar technologies are called Error Recovery Control (ERC), used by competitor Seagate, and Command Completion Time Limit (CCTL), used by Samsung and Hitachi.

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recognize a drive which does not respond within a few seconds, and mark it as unreliable, indicating that it should be withdrawn from use and the array rebuilt from parity data.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 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.

Desktop Computers and TLER Effect

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. [http://wdc.custhelp.com/cgi-bin/wdc.cfg/php/enduser/std_adp.php?p_faqid=1478 WD Customer Help FAQ 1478] However users and independent editors have have reported that this feature can be disabled. [http://techreport.com/reviews/2006q3/wd-500s/index.x?pg=1 TechReport.com] [http://www.silentpcreview.com/forums/viewtopic.php?t=38385&sid=16f0f601b1174a4ebf3fd6fd295b2351 SilentPCReview.com] [http://www.fatwallet.com/forums/messageview.php?catid=18&threadid=660910&start=0 FatWallet.com] [http://www.anandtech.com/storage/showdoc.aspx?i=2922 AnandTech.com] The tool allows this feature to be disabled by setting the values for read and write to 0 seconds.

By default, Western Digital [http://www.westerndigital.com/en/products/index.asp?Cat=2&Language=en Enterprise Drives] such as Raptor, Caviar RE2 and RE2-GP (RAID Edition) come with TLER Read "Disabled" (0 seconds) and TLER Write "Enabled" (7 seconds) while [http://www.westerndigital.com/en/products/index.asp?Cat=3&Language=en Desktop Drives] such as Caviar SE, SE16, and GP come with TLER Read and Write Disabled (0 seconds).

The ability to change the TLER option on Western Digital Desktop and Enterprise hard drives coincides with the current trends in desktop computing allowing the user to have the flexibility of buying a start-up computer with a single desktop class hard drive and then upgrading to two or more drives in the future. Most of the motherboard manufacturers these days use chipsets from [http://www.intel.com/products/desktop/chipsets/index.htm?iid=chipsets_body+dt_all Intel] and [http://www.nvidia.com/page/mobo.html nVidia] that include on-board RAID controllers as a standard option and anywhere from 4, 6, or up to 8 SATA ports allowing for easy upgrades to multiple hard drives and RAID capability without the requirement of purchasing specialized add-on cards. Additionally, having the choice of changing the TLER option gives the user the flexibility of purchasing the high performance drives such as the Western Digital Raptor that have TLER enabled and that are designed to run in a RAID array and using them as stand-alone single drives in their desktop computers and disabling the TLER option to enable better data recovery and lower chance of corruption and drive failures.

Stand-Alone vs RAID Hard Disk Usage Considerations

It is important to understand that TLER should be "Enabled" for a hard disk if it is being used in a RAID array to prevent the recovery time from a disk read or write error from taking too long and preventing the RAID controller from flagging the drive as failed and dropping the drive from the array. If a drive is dropped from an array due to it passing the timeout threshold of the RAID controller due to taking too much time performing error correction, the hard disk will need to be manually re-added to the array, requiring a re-build and re-synchronization of the hard disk with the rest of the disks in the array. In the remote possibility that two drives that do not have TLER enabled happen to encounter a disk error that takes too much time to recover, both of these drives might be flagged as failed and dropped from the RAID array, effectively breaking the array and requiring either a complete data restore from backup or manual intervention to force the array to re-recognize the drives as clean and online.

On the other hand it would be prudent to disable TLER for a RAID Edition hard drive being used as a stand-alone drive to allow it more time to recover from disk read or write errors, increasing the probability of data recovery.

Western Digital Time Limit Error Recovery Utility - WDTLER.EXE

The WDTLER utility allows for the enabling or disabling of the TLER parameter in the hard disk's firmware settings allowing the user to determine the best setting for his particular usage as either a stand-alone or RAID drive. This utility is written for the DOS operating system and you will require a DOS bootable disk with this utility on it to use it.

The WDTLER utility works on and makes changes to all the connected and compatible Western Digital hard drives to the computer. It is important to remember that any change will affect all the hard drives. If you only wish to change specific hard drives on your computer then you should disconnect the other hard drives before you use this utility, then reconnect them after you are finished.

The WDTLER utility comes with three batch files, TLERSCAN.BAT to get the current state of the TLER setting on all the hard drives, TLER-ON.BAT to Enable TLER, and TLER-OFF.BAT to Disable TLER. The included TLER-ON.BAT will set the Read & Write TLER time to 7 seconds. If you wish to use a custom timeout value, you can use the WDTLER.EXE utility directly with the -r# -w# parameters to specify how many seconds the Time Limit value should be.

Below is the WDTLER output for Western Digital Caviar SE16 320 GB and 500 GB hard disk for the default TLER configuration before and after TLER has been Enabled.

Before - TLER Read & Write: Disabled

WDTLER Version 1.03Copyright (C) 2004-2006 Western Digital CorporationWestern Digital Time Limit Error Recovery Utility

Model: WDC WD3200KS-00PFB0 Serial Number: WD-WCAPD1234567 Read TLER is disabled. Write TLER is disabled.

Model: WDC WD3200KS-00PFB0 Serial Number: WD-WCAPD1234567 Read TLER is disabled. Write TLER is disabled.

Model: WDC WD5000KS-00MNB0 Serial Number: WD-WMANU1234567 Read TLER is disabled. Write TLER is disabled.

Model: WDC WD5000KS-00MNB0 Serial Number: WD-WMANU1234567 Read TLER is disabled. Write TLER is disabled.

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After - TLER Read & Write: 7 seconds

WDTLER Version 1.03Copyright (C) 2004-2006 Western Digital CorporationWestern Digital Time Limit Error Recovery Utility

Model: WDC WD3200KS-00PFB0 Serial Number: WD-WCAPD1234567 Read TLER time is 7.000 seconds. Write TLER time is 7.000 seconds.

Model: WDC WD3200KS-00PFB0 Serial Number: WD-WCAPD1234567 Read TLER time is 7.000 seconds. Write TLER time is 7.000 seconds.

Model: WDC WD5000KS-00MNB0 Serial Number: WD-WMANU1234567 Read TLER time is 7.000 seconds. Write TLER time is 7.000 seconds.

Model: WDC WD5000KS-00MNB0 Serial Number: WD-WMANU1234567 Read TLER time is 7.000 seconds. Write TLER time is 7.000 seconds.

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External links

* [http://www.wdc.com/en/library/sata/2579-001098.pdf Time-Limited Error Recovery (TLER) Information Sheet]
* [http://www.samsung.com/global/business/hdd/learningresource/whitepapers/LearningResource_CCTL.html Samsung CCTL]