Editing RAID (section)

=== <span class="anchor" id="URE"></span><span class="anchor" id="UBE"></span><span class="anchor" id="LSE"></span>Unrecoverable read errors during rebuild ===
Unrecoverable read errors (URE) present as sector read failures, also known as latent sector errors (LSE). The associated media assessment measure, unrecoverable bit error (UBE) rate, is typically guaranteed to be less than one bit in 10<sup>15</sup>{{Disputed inline|Talk|date=October 2020}} for enterprise-class drives ([[SCSI]], [[Fibre Channel|FC]], [[Serial Attached SCSI|SAS]] or SATA), and less than one bit in 10<sup>14</sup>{{Disputed inline|Talk|date=October 2020}} for desktop-class drives (IDE/ATA/PATA or SATA). Increasing drive capacities and large RAID&nbsp;5 instances have led to the maximum error rates being insufficient to guarantee a successful recovery, due to the high likelihood of such an error occurring on one or more remaining drives during a RAID set rebuild.<ref name="Patterson_1994" />{{Obsolete source|reason=This source is 26 years old|date=October 2020}}<ref name="mojo2010">{{cite web|title=Does RAID 6 stop working in 2019?|url=http://storagemojo.com/2010/02/27/does-raid-6-stops-working-in-2019/|first=Robin|last=Harris|publisher=TechnoQWAN|work=StorageMojo.com|date=2010-02-27|access-date=2013-12-17}}</ref> When rebuilding, parity-based schemes such as RAID&nbsp;5 are particularly prone to the effects of UREs as they affect not only the sector where they occur, but also reconstructed blocks using that sector for parity computation.<ref>J.L. Hafner, V. Dheenadhayalan, K. Rao, and J.A. Tomlin. [https://www.usenix.org/legacy/event/fast05/tech/full_papers/hafner_matrix/hafner_matrix_html/matrix_hybrid_fast05.html "Matrix methods for lost data reconstruction in erasure codes. USENIX Conference on File and Storage Technologies], Dec. 13–16, 2005.</ref>

Double-protection parity-based schemes, such as RAID&nbsp;6, attempt to address this issue by providing redundancy that allows double-drive failures; as a downside, such schemes suffer from elevated write penalty—the number of times the storage medium must be accessed during a single write operation.<ref>{{Cite web|url=http://www.storagecraft.com/blog/raid-performance/|title=Understanding RAID Performance at Various Levels|last=Miller|first=Scott Alan|date=2016-01-05|website=Recovery Zone|publisher=StorageCraft|access-date=2016-07-22}}</ref> Schemes that duplicate (mirror) data in a drive-to-drive manner, such as RAID&nbsp;1 and RAID&nbsp;10, have a lower risk from UREs than those using parity computation or mirroring between striped sets.<ref name="UREs" /><ref>{{cite web
 |url = http://www.miracleas.com/BAARF/RAID5_versus_RAID10.txt
 |title = RAID&nbsp;5 versus RAID&nbsp;10 (or even RAID&nbsp;3, or RAID&nbsp;4)
 |date = March 2, 2011
 |access-date = October 30, 2014
 |first = Art S. |last=Kagel
 |website = miracleas.com
 |url-status = dead
 |archive-url = https://web.archive.org/web/20141103162704/http://www.miracleas.com/BAARF/RAID5_versus_RAID10.txt
 |archive-date = November 3, 2014
}}</ref> [[#SCRUBBING|Data scrubbing]], as a background process, can be used to detect and recover from UREs, effectively reducing the risk of them happening during RAID rebuilds and causing double-drive failures. The recovery of UREs involves remapping of affected underlying disk sectors, utilizing the drive's sector remapping pool; in case of UREs detected during background scrubbing, data redundancy provided by a fully operational RAID set allows the missing data to be reconstructed and rewritten to a remapped sector.<ref>{{cite book |first1=M. |last1=Baker |first2=M. |last2=Shah |first3=D.S.H. |last3=Rosenthal |first4=M. |last4=Roussopoulos |first5=P. |last5=Maniatis |first6=T. |last6=Giuli |first7=P |last7=Bungale  |title=Proceedings of the 1st ACM SIGOPS/EuroSys European Conference on Computer Systems 2006 |chapter=A fresh look at the reliability of long-term digital storage |date=April 2006|pages=221–234 |doi=10.1145/1217935.1217957 |isbn=1595933220 |s2cid=7655425 }}</ref><ref>{{Cite book |chapter-url=http://research.cs.wisc.edu/adsl/Publications/latent-sigmetrics07.pdf |first1=L.N. |last1=Bairavasundaram |first2=G.R. |last2=Goodson |first3=S. |last3=Pasupathy |first4=J. |last4=Schindler |title=Proceedings of the 2007 ACM SIGMETRICS international conference on Measurement and modeling of computer systems |chapter=An analysis of latent sector errors in disk drives |date=June 12–16, 2007|pages=289–300 |doi=10.1145/1254882.1254917 |isbn=9781595936394 |s2cid=14164251 }}</ref>