Raid 5 Rebuild Read Error
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is the best choice, ever [1]. There are cases where RAID0 is mathematically proven more reliable than RAID5 [2].
Unrecoverable Read Error Rate
RAID5 should never be used for anything where you value keeping your data. raid 5 ure calculator I am not exaggerating when I say that very often, your data is safer on a single hard drive than what happens if the array experiences a ure during the rebuild process? it is on a RAID5 array. Please let that sink in.The problem is that once a drive fails, during the rebuild, if any of the surviving drives experience an unrecoverable read error (URE),
Unrecoverable Read Error Ure
the entire array will fail. On consumer-grade SATA drives that have a URE rate of 1 in 10^14, that means if the data on the surviving drives totals 12TB, the probability of the array failing rebuild is close to 100%. Enterprise SAS drives are typically rated 1 URE in 10^15, so you improve your chances ten-fold. Still an avoidable risk.RAID6 suffers from the same fundamental flaw
Raid 10 Ure
as RAID5, but the probability of complete array failure is pushed back one level, making RAID6 with enterprise SAS drives possibly acceptable in some cases, for now (until hard drive capacities get larger).I no longer use parity RAID. Always RAID10 [3]. If a customer insists on RAID5, I tell them they can hire someone else, and I am prepared to walk away.I haven't even touched on the ridiculous cases where it takes RAID5 arrays weeks or months to rebuild, while an entire company limps inefficiently along. When productivity suffers company-wide, the decision makers wish they had paid the tiny price for a few extra disks to do RAID10.In the article, he has 12x 4TB drives. Once two drives failed, assuming he is using enterprise drives (Dell calls them "near-line SAS", just an enterprise SATA), there is a 33% chance the entire array fails if he tries to rebuild. If the drives are plain SATA, there is almost no chance the array completes a rebuild.[1] http://www.smbitjournal.com/2012/11/choosing-a-raid-level-by...[2] http://www.smbitjournal.com/2012/05/when-no-redundancy-is-mo...[3] http://www.smbitjournal.com/2012/11/one-big-raid-10-a-new-st... Twirrim 772 days ago Note that the 10^14 figure is only what the HDD mfgs publish, and it has been the same for something like a decade.
the unrecoverable data error in which he points out that while disk manufacturers quoted Bit Error Rates (BER) for hard disks are typically 10-14 or 10-15, SSD raid 6 ure BERs range from 10-16 for consumer drives to 10-18 for hardened enterprise drives.
Hard Drive Ure
Below the fold, a look at his analysis of the impact of this difference of up to 4 orders zfs ure of magnitude. When a disk in a RAID-5 array fails and is replaced, all the data on other drives in the array must be read to reconstruct the data from the failed drive. https://news.ycombinator.com/item?id=8306499 If an unrecoverable read error (URE) is encountered in this process, one or more data blocks will be lost. RAID-6 and up can survive increasing numbers of UREs. It has been obvious for some time that as hard disks got bigger without a corresponding decrease in BER that RAID technology had a problem, in that the probability of encountering a URE during reconstruction was going up, http://blog.dshr.org/2015/05/unrecoverable-read-errors.html and thus so was the probability of losing data when a drive failed.As Trevor writes: Putting this into rather brutal context, consider the data sheet for the 8TB Archive Drive from Seagate. This has an error rate of 10^14 bits. That is one URE every 12.5TB. That means Seagate will not guarantee that you can fully read the entire drive twice before encountering a URE. Let's say that I have a RAID 5 of four 5TB drives and one dies. There is 12TB worth of data to be read from the remaining three drives before the array can be rebuilt. Taking all of the URE math from the above links and dramatically simplifying it, my chances of reading all 12TB before hitting a URE are not very good. With 6TB drives I am beyond the math. In theory, I shouldn't be able to rebuild a failed RAID 5 array using 6TB drives that have a 10^14 BER. I will encounter a URE before the array is rebuilt and then I'd better hope the backups work. So RAID 5 for consumer hard drives is dead. Well, yes, but RAID-5, and RAID in general, is just o
here for a quick overview of the site Help Center Detailed answers to any questions you might have Meta Discuss the workings and policies of this site About http://superuser.com/questions/700177/why-ure-fails-raid-rebuild-and-renders-raid-5-unusable Us Learn more about Stack Overflow the company Business Learn more about hiring http://www.raid-failure.com/raid5-failure.aspx developers or posting ads with us Super User Questions Tags Users Badges Unanswered Ask Question _ Super User is a question and answer site for computer enthusiasts and power users. Join them; it only takes a minute: Sign up Here's how it works: Anybody can ask a question Anybody can answer The best read error answers are voted up and rise to the top Why URE fails raid rebuild and “renders RAID 5 unusable” [closed] up vote 1 down vote favorite I'm sorry but I just can't comprehend from a theoretical point of view. Why is it that running into a single URE, the raid controller decides everything else are ruined and just dies? Stupid. A 40 TB array is useless because unrecoverable read error 1mb is lost? Rebuild the whole damn thing, then just do a checksum check on all the files if the filesystem supports it. Even if not, it's just a case of being prompted with "file corrupted" when trying to open those files. This whole thing just screams stagnant hardware technology to me. Edit- It seems people just jump straight on band wagon of "you shouldn't rely on RAID for backup". Well, I'm not interested in that. Yes RAID is for availability, not durability. The fact remains, you still can salvage ~99% of the RAID if the rebuild just skips over the URE. raid share|improve this question edited Jan 13 '14 at 22:29 Kevin Panko 4,979113144 asked Jan 11 '14 at 13:00 Sleeper Smith 1126 closed as primarily opinion-based by Ƭᴇcʜιᴇ007, Heptite, Kevin Panko, Tog, Tanner Faulkner Jan 13 '14 at 16:02 Many good questions generate some degree of opinion based on expert experience, but answers to this question will tend to be almost entirely based on opinions, rather than facts, references, or specific expertise.If this question can be reworded to fit the rules in the help center, please edit the question. 4 Is there a question som
of disks in a RAID 5 array increases, the concern arises that a large array hasn't time to finish a rebuild before the second member disk fails. It is possible to calculate a probability of such an event if the disk reliability is given. The greater the array capacity and the less the disk reliability, the possibility of second disk failure is greater. Disk reliability is provided in a vendor specification where its value is usually between one error per 1014 and one error per 1016 bits read. Although such an approach of calculation arouses reasonable criticism, we have still created the calculator which works using this formula: P=(1-10-14)8*109*N*S where N is the number of disks in RAID, and S is the size of a single member disk, in GB Enter parameters Number of disks Disk size, GB Error probability, per bits read 1 per 10^14 bits 1 per 10^15 bits 1 per 10^16 bits 1 per 10^17 bits Result Please enter the data and click "Calculate". Copyright © 2011 - 2016 www.ReclaiMe.com