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#1
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Raid5 vs Raid6 under degraded mode
Hey,
Assume a system with 9 SATA 1TB disks, and 1 hot spare. They are targeted to config as Raid5 or Raid6. If one of the disk failed, which system will perform better? Raid5 and Raid6? By how many %? Thanks. |
#2
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Raid5 vs Raid6 under degraded mode
On May 29, 3:46*am, howa wrote:
Assume a system with 9 SATA 1TB disks, and 1 hot spare. They are targeted to config as Raid5 or Raid6. If one of the disk failed, which system will perform better? Raid5 and Raid6? By how many %? It's not a performance question - it's an availability question. If you configure those volumes in a raid5, you will almost be guaranteed of losing your entire array if a drive fails. The math does not work in your favor - the odds are extremely high that under high I/O load - during your raid5 rebuild, you will have an unrecoverable error on one of the remaining members. At that time, your raid5 array will fail. Don't think it won't happen to you - I've seen double-disk failures happen on raid5 sets too often for it to be funny. Go with raid6. If you want performance, create 4 mirror sets instead of raid6. |
#3
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Raid5 vs Raid6 under degraded mode
Ed Wilts wrote:
On May 29, 3:46 am, howa wrote: Assume a system with 9 SATA 1TB disks, and 1 hot spare. They are targeted to config as Raid5 or Raid6. If one of the disk failed, which system will perform better? Raid5 and Raid6? By how many %? It's not a performance question - it's an availability question. No, it's not. You just didn't choose to answer the question that was asked. The answer to that question is that the RAID-5 and RAID-6 options should perform about the same after the disk failure since both will likely use the same algorithm to reconstruct data when it's needed and also to rebuild the failed disk's data on the hot spare (though the latter is a bit more complex for RAID-6 and thus might consume marginally more resources). If you configure those volumes in a raid5, you will almost be guaranteed of losing your entire array if a drive fails. Not unless the array is brain-damaged. The math does not work in your favor - the odds are extremely high that under high I/O load - during your raid5 rebuild, you will have an unrecoverable error on one of the remaining members. That is correct. At that time, your raid5 array will fail. That is not correct unless the array is (as I said above) brain-damaged. Rather, the stripe(s) containing an unreadable sector will be unable to reconstruct the sector from the failed disk and thus at most two sectors in the stripe (only one if the other was the parity sector) will be lost. A single disk doesn't take all its marbles and go home when a sector becomes unreadable, it just soldiers on without it (reporting the error, of course, and usually revectoring the damaged sector to a healthy one for future use). Why should an array act differently? It should report the sector(s) as lost, create a healthy stripe out of the valid remainder of the damaged stripe, and revector subsequent requests transparently to that new location. Don't think it won't happen to you - I've seen double-disk failures happen on raid5 sets too often for it to be funny. Two whole-disk failures very close together in a RAID-5 set of this size are certainly possible but of very low probability. The most likely cause would be some external environmental catastrophe of a nature which could quite possibly affect more than two disks anyway (in which case RAID-6 wouldn't have bought you anything). The next most likely cause would be some common flaw in the batch of disks being used, but even then the second disk would have to fail so soon that the hot spare would not have been rebuilt yet - a very unlikely occurrence. So if the original poster could indeed tolerate very minor data loss in the array should a disk fail (the probability of which is significant over a period of years) RAID-5 could be a very reasonable option. The likelihood that such very minor data loss would happen to occur in a critical area of the file system such that effectively far more data would become inaccessible is far smaller than the likelihood that some external problem would trash his/her data. - bill |
#4
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Raid5 vs Raid6 under degraded mode
Bill Todd wrote:
[SNIP] Two whole-disk failures very close together in a RAID-5 set of this size are certainly possible but of very low probability. The most likely cause would be some external environmental catastrophe of a nature which could quite possibly affect more than two disks anyway (in which case RAID-6 wouldn't have bought you anything). The next most likely cause would be some common flaw in the batch of disks being used, but even then the second disk would have to fail so soon that the hot spare would not have been rebuilt yet - a very unlikely occurrence. An example - disks in a RAID set being on a single controller which is starting to fail. Been there, done that, replaced controller and restored from backup. But this sort of problem does not often show up with enterprise-level hardware, it is more seen with consumer-grade equipment that is being pushed harder than it was costed/designed to do. And, with the increase of RAID being targeted as a consumer-level data storage solution, this will increase. (Not disputing any of the points you made, just expanding on this one.) Cheers, Gary B-) |
#5
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Raid5 vs Raid6 under degraded mode
On May 29, 3:35 pm, Ed Wilts wrote:
On May 29, 3:46 am, howa wrote: Assume a system with 9 SATA 1TB disks, and 1 hot spare. They are targeted to config as Raid5 or Raid6. If one of the disk failed, which system will perform better? Raid5 and Raid6? By how many %? It's not a performance question - it's an availability question. If you configure those volumes in a raid5, you will almost be guaranteed of losing your entire array if a drive fails. The math does not work in your favor - the odds are extremely high that under high I/O load - during your raid5 rebuild, you will have an unrecoverable error on one of the remaining members. At that time, your raid5 array will fail. Don't think it won't happen to you - I've seen double-disk failures happen on raid5 sets too often for it to be funny. Go with raid6. If you want performance, create 4 mirror sets instead of raid6. Guess you need to buy better disk arrays.... The disk arrays we use are constantly doing scrubs, preemptive replacements, etc. I have not had a disk array choke on a RAID-5 in YEARS... hmmm... about the same time we STOPPED using low end, bargain priced arrays... |
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