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What I don't understand about the URE is from [2]. If you have a 12TB raid 5 array and you need to rebuild. If 10^14 approaches an URE at around 12TB of data

by coned88 12y ago

What I don't understand about the URE is from [2]. If you have a 12TB raid 5 array and you need to rebuild. If 10^14 approaches an URE at around 12TB of data as the article says. What causes it to hit 12TB? Each disk has 10^14 which is 100TB. If you had 12TB from 4x3TB disks it should have alot to go through.

halfcat 12y ago
10^14 is bits. When you divide 10^14 by 8 you get 12.5 trillion bytes, or 12.5 TB. If you have 4x 4TB drives in RAID5, and one fails, then in order to rebuild with a replacement drive, you have to read all data from all surviving drives (3 x 4TB = 12TB). Here is an example from a manufacturer: http://www.seagate.com/files/www-content/product-content/constellation-fam/constellation-es/constellation-es-3/en-gb/docs/constellation-es-3-data-sheet-ds1769-1-1210gb.pdf http://www.seagate.com/files/www-content/product-content/con... They call it "Non-recoverable Read Errors per Bits Read" and then list "1 sector per 10^15". So for every 10^15 bits read they expect 1 sector to be unreadable.
- coned88 12y ago
  OK i see now but if it needs to read 12TB from all drives together then that's still far from 12.5TB per drive which is the limit. That's where I am confused.
  - halfcat 12y ago
    Say we have a department of a company with 36 employees, and one pair of dice. We decide that if any person out of the entire department rolls a 12, then everyone in the department will be fired. The chance of rolling a 12 is 1/36. It doesn't matter if one person keeps rolling the dice, or if they take turns, the chances of everyone being fired are close to 100%. The same is true for a disk array. Each read operation is an independent event (for the purpose of doing this math). The chance of one URE happening is 1/(10^14) for every bit read. It doesn't matter which disk it happens on. When it happens, the entire array is failed. Also 12.5 TB is not a hard limit, just an average. The URE could happen on the very first read operation, or you might read 100 TB without a URE.

by coned88 12y ago

halfcat 12y ago

10^14 is bits. When you divide 10^14 by 8 you get 12.5 trillion bytes, or 12.5 TB. If you have 4x 4TB drives in RAID5, and one fails, then in order to rebuild with a replacement drive, you have to read all data from all surviving drives (3 x 4TB = 12TB). Here is an example from a manufacturer: http://www.seagate.com/files/www-content/product-content/constellation-fam/constellation-es/constellation-es-3/en-gb/docs/constellation-es-3-data-sheet-ds1769-1-1210gb.pdf http://www.seagate.com/files/www-content/product-content/con... They call it "Non-recoverable Read Errors per Bits Read" and then list "1 sector per 10^15". So for every 10^15 bits read they expect 1 sector to be unreadable.

coned88 12y ago
OK i see now but if it needs to read 12TB from all drives together then that's still far from 12.5TB per drive which is the limit. That's where I am confused.
- halfcat 12y ago
  Say we have a department of a company with 36 employees, and one pair of dice. We decide that if any person out of the entire department rolls a 12, then everyone in the department will be fired. The chance of rolling a 12 is 1/36. It doesn't matter if one person keeps rolling the dice, or if they take turns, the chances of everyone being fired are close to 100%. The same is true for a disk array. Each read operation is an independent event (for the purpose of doing this math). The chance of one URE happening is 1/(10^14) for every bit read. It doesn't matter which disk it happens on. When it happens, the entire array is failed. Also 12.5 TB is not a hard limit, just an average. The URE could happen on the very first read operation, or you might read 100 TB without a URE.