View source for Error recovery control

{{Refimprove|date=April 2010}}
In [[computing]], '''error recovery control''' ('''ERC''') ([[Western Digital]]: '''time-limited error recovery''' ('''TLER'''), [[Samsung]]/[[Hitachi GST|Hitachi]]: '''command completion time limit''' ('''CCTL''')) is a feature of [[hard disk]]s which allow a system administrator to configure the amount of time a drive's [[firmware]] is allowed to spend recovering from a read or write error. Limiting the recovery time allows for improved error handling in hardware or software [[RAID]] environments. In some cases, there is a conflict as to whether error handling should be undertaken by the hard drive or by the RAID implementation, which leads to drives being marked as unusable and significant performance degradation, when this could otherwise have been avoided.

==Overview==
Modern [[hard drive]]s feature an ability to recover from some read/write errors by internally remapping [[Disk sector|sectors]] and performing 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. Hardware RAID controllers and software RAID implementations are designed to recognise a drive which does not respond within a few seconds, and mark it as unreliable, indicating that it should be withdrawn from use and the array rebuilt from [[Parity bit#Parity block|parity data]]. This is a long process, degrades performance, and if more drives fail under the resulting additional workload, it may 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.

==Typical defaults==
Effectively, TLER and similar features limit the performance of on-drive error handling, to allow hardware RAID controllers and software RAID implementations to handle the error if problematic.

Generally, Western Digital [[Enterprise disk drive|enterprise drives]] such as [[Western Digital Raptor|Raptor]], Caviar RE2 and RE2-GP (RAID Edition) come with TLER Read "Enabled" (7 seconds) and TLER Write "Enabled" (7 seconds) while desktop drives such as Caviar SE, SE16, and GP come with TLER Read and Write Disabled (configured as 0 seconds, to disable).

== Standalone vs. RAID considerations ==
It is best for TLER to be "enabled" when in a RAID array to prevent the recovery time from a disk read or write error from exceeding the RAID implementation's timeout threshold. If a drive times out, the hard disk will need to be manually re-added to the array, requiring a re-build and re-synchronization of the hard disk. Enabling TLER seeks to prevent this by interrupting error correction before timeout, to report failures only for data segments. The result is increased reliability in a RAID array.

In a stand-alone configuration TLER should be disabled. As the drive is not redundant, reporting segments as failed will only increase manual intervention. Without a hardware RAID controller or a software RAID implementation to drop the disk, normal (no TLER) recovery ability is most stable.

In a software RAID configuration whether or not TLER is helpful is dependent on the operating system. For example, in FreeBSD the ATA/CAM stack controls the timeouts, and is set to progressively increase the timeouts as they occur. Thus, if a desktop disk without TLER starts delaying a response to a sector read, FreeBSD will retry the read with successively longer timeouts to prevent prematurely dropping the disk out of the array.

{| class="wikitable"
! Model
! TLER default ( read / write )
! Stand-alone recommendation
! RAID recommendation
|-
| '''Caviar, SE, SE16, GP, Raptor'''
| ''Disabled ( 0s / 0s )''
| ''Default''
| Enabled (if possible)
|-
| '''Caviar RE2, RE2-GP, Red'''
| ''Enabled ( 7s / 7s )''
| Disabled
| ''Default''
|}

=== ZFS ===
The [[ZFS|ZFS filesystem]] was designed to immediately write data to a sector that reports as bad or takes an excessively long time to read (such as non-TLER drives); this will usually force an immediate sector remap on a weak sector in most drives.{{Citation needed|date=March 2022}}

===RAID controllers===
Disconnect timeout values for different hardware [[Disk array controller|RAID controllers]] may vary between vendors; thus, TLER should trigger before the controller times out the drive. For example, 3ware 9650SE uses 20 seconds as the timeout,<ref>{{cite web|url=http://kb.lsi.com/KnowledgebaseArticle15639.aspx|archiveurl=https://web.archive.org/web/20120203053819/http://kb.lsi.com/KnowledgebaseArticle15639.aspx|title=User Guide for 9650SE 9690SA from 9.5.2 Complete Codeset|archivedate=3 February 2012|work=lsi.com|accessdate=10 June 2015}}</ref> while for the LSI Logic used in IBM x-series it is 10 seconds.<ref>Available in BIOS Raid Config Utility > Advanced Device Properties</ref>

Widely available [[Intel Rapid Storage Technology|Intel Matrix RAID / Intel Rapid Storage Technology]], embedded in [[Intel]] server motherboards and modern desktop motherboards, is a pseudo-hardware controller, not a true hardware RAID controller.

===Software RAID===
Linux [[mdadm]] simply holds and lets the drive complete its recovery – however, the default command timeout for the SCSI Disk layer (/sys/block/sd?/device/timeout) is 30 seconds,<ref>{{cite web|url=https://github.com/torvalds/linux/blob/master/drivers/scsi/sd.h#LC12|title=linux/sd.h at master · torvalds/linux · GitHub|work=GitHub}}</ref> after which it will attempt to reset the drive, and if that fails, put the drive offline.<ref>{{cite web|url=https://www.kernel.org/doc/html/v5.9/scsi/scsi_eh.html|title= Linux SCSI Subsystem: SCSI EH|work=kernel.org}}</ref>

== Changing ERC ==

===ATA-8 standard===
The 2006 ATA-8 standard defines a SCT {{tt|Error Recovery Control}} command.<ref>[https://www.singlix.org/trdos/8086/archive/specs/D1699r3e-ATA8_ACS.pdf ATA/ATAPI Command Set (ATA8-ACS) ]</ref> For hard drives that implement this interface, the {{Mono|smartctl}} utility (part of the [[smartmontools]] package) can be used to change the error-recovery timeout via {{code|-l scterc}}.<ref name=greg>{{Cite web |author=Richard Gregory |url=http://abatis.org.uk/projects/erc/ |title=Author's description of the original patch to smartctl that implemented that feature |access-date=2013-02-15 |archive-url=https://web.archive.org/web/20130910034510/http://cgi.csc.liv.ac.uk:80/~greg/projects/erc/ |archive-date=2013-09-10 |url-status=live }}</ref> In 2018, ACS-4 added a functionality for the setting to persist across reboot; it is now supported by smartctl.<ref>{{cite web |title=#1427 (Add support for SCT Error Recovery Timer features added in ACS-4) – smartmontools |url=https://www.smartmontools.org/ticket/1427 |website=www.smartmontools.org}}</ref>

Controlling the timeout behavior through the {{Mono|smartctl}} utility may not work on all hard disk drives because some manufacturers have changed their desktop drives not to include the support for the ERC parameter,<ref>{{cite web|url=http://www.spinics.net/lists/raid/msg38964.html|title=Re: md RAID with enterprise-class SATA or SAS drives|work=spinics.net}}</ref><ref>{{cite web|url=http://knowledge.seagate.com/articles/en_US/FAQ/203991en|title=Seagate FAQ: What is Error Recovery Control?|work=seagate.com}}</ref> purportedly to force sales of their more expensive RAID/enterprise models.{{Citation needed|date=April 2016}} Richard Gregory, who wrote the original ERC patch for smartctl, reports that Western Digital retracted ERC support by releasing a new model without notice.<ref name=greg/>

On Windows, the HDAT2 program is available in addition to smartctl (which is cross-platform).<ref name=greg/>

=== SCSI standard ===
SBC-4 describes a RECOVERY TIME LIMIT field in the Read-Write Error Recovery mode page used to define how the drive performs error recovery.<ref>{{cite web |title=INCITS 506-202x - Information technology - SCSI Block Commands - 4 (SBC-4) draft revision 22 |url=https://standards.incits.org/apps/group_public/download.php/124286/livelink |access-date=22 May 2023 |date=15 September 2020}}</ref> The sdparm program can change this setting with {{code|1=--set=RTL}}.<ref>{{man|8|sdparm|Linux}}</ref>

=== Vendor utilities ===
==== Western Digital ====
A {{Mono|WDTLER.EXE}} utility allows the enabling or disabling of the TLER parameter on Western Digital hard drives. This utility is written for [[DOS]]. The utility works on and makes changes to all compatible Western Digital hard disk drives connected to the computer. The change survives power-cycling. Western Digital used to mention the tool in an FAQ.<ref name=customer-service>{{cite web |title=TLER / CCTL / ERC thread |url=https://hardforum.com/threads/tler-cctl-erc-thread.1562128/ |website=[H]ard{{!}}Forum |date=16 November 2010}}</ref>

The utility comes with three batch files, {{Mono|TLERSCAN.BAT}} to get the current state of the TLER setting on all the hard drives, {{Mono|TLER-ON.BAT}} to enable TLER, and {{Mono|TLER-OFF.BAT}} to disable TLER. The included {{Mono|TLER-ON.BAT}} will set the Read & Write TLER time to seven seconds. It is possible to use the {{Mono|WDTLER.EXE}} utility directly with the <code>-r# -w#</code> parameters for a custom timeout.

Western Digital claims that using the {{Mono|WDTLER.EXE}} utility on newer drives can damage the firmware and make the disk unusable. The utility is no longer available from Western Digital, and new drives will not be able to have the TLER setting changed. RE disks are only suitable for RAID arrays and Caviar are only suitable for non-RAID use. The utility still works for older drives{{which|date=May 2023}}<!-- what is the cutoff? -->.

==== Hitachi ====
Hitachi customer service stated in 2009 that there is a Feature Tool for changing ERC (referred to as CCTL).<ref name=customer-service/>

==== Seagate ====
Seagate provides a {{Mono|openSeaChest}} utility to allow you to interrogate and change many firmware settings including TLER. If you cannot use <code>smartctl -l scterr,x,y</code> to set the TLER, the relevant command-line commands are <code>openSeaChest_Configure -d /dev/sg0 --sctReadTimer</code> and <code>openSeaChest_Configure -d /dev/sg0 --sctWriteTimer</code>.

==References==
{{Reflist}}

==External links==
* [https://raid.wiki.kernel.org/index.php/Timeout_Mismatch Linux Raid wiki: Timeout Mismatch]
* [http://wdc.custhelp.com/app/answers/detail/a_id/1397/p/227,283/session/L3RpbWUvMTMyMTQzOTc4NS9zaWQvdVhvYmpmSms%3D Western Digital FAQ answer ID 1397: Difference between Desktop edition and RAID (Enterprise) edition drives]{{Dead link|date=March 2024 |bot=InternetArchiveBot |fix-attempted=yes }}
* [http://www.wdc.com/wdproducts/library/other/2579-001098.pdf Time-Limited Error Recovery (TLER)  Information Sheet], Western Digital, January 2013
* [https://web.archive.org/web/20071103042201/http://www.samsung.com/global/business/hdd/learningresource/whitepapers/LearningResource_CCTL.html Samsung CCTL]

[[Category:Rotating disc computer storage media]]
[[Category:Hard disk computer storage]]