4 ms·

>Basically, if you want something striping, and care at all about data integrity, go with RAID10. Only use RAID5/6 if you don't care about data loss (whether du

by pedrocr 11y ago

>Basically, if you want something striping, and care at all about data integrity, go with RAID10. Only use RAID5/6 if you don't care about data loss (whether due to a comprehensive backup policy or a comprehensive redundancy policy on a machine-level), though in that case you might as well just cut to the chase and use RAID0. Your points are valid but this doesn't follow. RAID10 is strictly worse than RAID6 for redundancy. In a four drive array any two drives can fail with RAID6 whereas with RAID10 2 drives may be enough to blow up half your data. RAID10 is actually a less redundancy but higher performance tradeoff compared to RAID6.

deleted 11y ago
[deleted]
yellowapple 11y ago
I should've clarified that RAID1 is the absolute best if data integrity is your sole concern. Regardless... > RAID10 is strictly worse than RAID6 for redundancy. Only if you use two drives per mirror; RAID10's worst-case matches RAID6's when using three disks per mirror. RAID6 also lacks a "best case" that's distinct from its worst case. You lose three disks, you're hosed, period. You lose three disks in a RAID10 (even one with two disks per mirror), it's much more probable that your data will be intact. You can mitigate this further by using a bigger array (RAID10 total failure probability is affected by array size, unlike RAID6) or by using different vendors for each disk in the mirrors (example: each mirror has one Intel and one Samsung SSD of the same capacity) - which, while having some performance and capacity implications, actually mitigates the "oh no I bought a bad batch of hard drives and now my RAID is kaput" failure case (unlike with RAID6, where mixing/matching vendors won't help you unless you mix/match them for every disk). There are also some recoverability benefits of RAID10 v. RAID6; notably, recovering from a fault in RAID6 requires recomputing parity, while no such thing is necessary for RAID10. This mitigates the "oh no my array took too long to fix itself and now my RAID is kaput" failure case.
- pedrocr 11y ago
  >Only if you use two drives per mirror; RAID10's worst-case matches RAID6's when using three disks per mirror. That's not a proper comparison as you are now using more disks in the RAID10 array than the RAID6 to get the same space. If you're willing to have 6 disks to get the space of 2 you could use an array with 3 parity disks (what ZFS calls Z3) and now you can fail any 3 disks just like RAID10 and you've only used 5 disks instead of 6. If you have the option for 4 parity disks (I don't think ZFS provides a Z4 level) you could fail any 4 drives of 6 and get the space of 2, so 1 better than RAID10. N-Parity setups are strictly more redundant than RAID10 for the same number of disks and required capacity. >actually mitigates the "oh no I bought a bad batch of hard drives and now my RAID is kaput" failure case (unlike with RAID6, where mixing/matching vendors won't help you unless you mix/match them for every disk). This makes no sense to me. I mix and match vendors in RAID6 arrays just fine and it lowers the correlation of failures between disks just as well. In fact parity RAID is again strictly better at this as well. Since the redundancy is across the whole array instead of split into clusters whenever you add more drives from different manufacturers you're adding that redundancy to the whole array. With RAID10 if you have 3 drives per mirror your maximum redundancy before losing data is 3 manufacturers. >There are also some recoverability benefits of RAID10 v. RAID6; notably, recovering from a fault in RAID6 requires recomputing parity, while no such thing is necessary for RAID10. This mitigates the "oh no my array took too long to fix itself and now my RAID is kaput" failure case. Yeah, performance is the real advantage of RAID10 as I mentioned and indeed that performance can be important during rebuilds so it has some durability implications.
  - yellowapple 11y ago
    > That's not a proper comparison as you are now using more disks in the RAID10 array than the RAID6 to get the same space. You already have to use more disks for a RAID10 than in a RAID6, so I don't see why this is that big of a deal. Anything involving mirroring will involve at least a 50% cut in capacity (or more, such as with a 3-way or even 4-way mirror). RAID1 and RAID10 don't prioritize capacity; they prioritize robustness. > N-Parity setups are strictly more redundant than RAID10 for the same number of disks and required capacity. But not more robust against failure, as I've already described (hopefully) rather clearly. If we want to play this game, I can keep adding more disks per mirror, and now my worst-case will match that of your n-parity array while having a vastly better best-case (and, additionally, dragging the probability of that worst-case happening closer and closer to zero). Remember: if you're a RAID1 or RAID10 user who cares about disk capacity, you're doing something wrong. > I mix and match vendors in RAID6 arrays just fine and it lowers the correlation of failures between disks just as well. Do you mix and match all disks, though? Because if you have at least three (in the case of RAID6) disks, no matter where they are in the array, that have the same failure curve (i.e. are the same model/manufacturer), then you're gonna have a bad time when the first one of them dies. That's my point. In order to get the same inherent benefit as RAID10 here, you'd have to have a different vendor/model for every single drive in your RAID6 array. > whenever you add more drives from different manufacturers you're adding that redundancy to the whole array. Only if they're all different. You'll eventually get to the point where there aren't enough manufacturers in the world (or you'll be compromising on manufacturer diversity - which is, granted, what most people do). > With RAID10 if you have 3 drives per mirror your maximum redundancy before losing data is 3 manufacturers. And that's fine, because if one of those manufacturers sold me a defective batch, the problem's isolated to that side of the mirror. That's my point; with RAID10, it's no longer a game of sheer numbers of different models and manufacturers, but instead the more manageable strategy of giving each side of the mirror a different bathtub curve to ride, thus mitigating the chance of multiple hard drive batches failing at the same time.

by pedrocr 11y ago

deleted 11y ago

[deleted]

yellowapple 11y ago

I should've clarified that RAID1 is the absolute best if data integrity is your sole concern. Regardless... > RAID10 is strictly worse than RAID6 for redundancy. Only if you use two drives per mirror; RAID10's worst-case matches RAID6's when using three disks per mirror. RAID6 also lacks a "best case" that's distinct from its worst case. You lose three disks, you're hosed, period. You lose three disks in a RAID10 (even one with two disks per mirror), it's much more probable that your data will be intact. You can mitigate this further by using a bigger array (RAID10 total failure probability is affected by array size, unlike RAID6) or by using different vendors for each disk in the mirrors (example: each mirror has one Intel and one Samsung SSD of the same capacity) - which, while having some performance and capacity implications, actually mitigates the "oh no I bought a bad batch of hard drives and now my RAID is kaput" failure case (unlike with RAID6, where mixing/matching vendors won't help you unless you mix/match them for every disk). There are also some recoverability benefits of RAID10 v. RAID6; notably, recovering from a fault in RAID6 requires recomputing parity, while no such thing is necessary for RAID10. This mitigates the "oh no my array took too long to fix itself and now my RAID is kaput" failure case.

pedrocr 11y ago
>Only if you use two drives per mirror; RAID10's worst-case matches RAID6's when using three disks per mirror. That's not a proper comparison as you are now using more disks in the RAID10 array than the RAID6 to get the same space. If you're willing to have 6 disks to get the space of 2 you could use an array with 3 parity disks (what ZFS calls Z3) and now you can fail any 3 disks just like RAID10 and you've only used 5 disks instead of 6. If you have the option for 4 parity disks (I don't think ZFS provides a Z4 level) you could fail any 4 drives of 6 and get the space of 2, so 1 better than RAID10. N-Parity setups are strictly more redundant than RAID10 for the same number of disks and required capacity. >actually mitigates the "oh no I bought a bad batch of hard drives and now my RAID is kaput" failure case (unlike with RAID6, where mixing/matching vendors won't help you unless you mix/match them for every disk). This makes no sense to me. I mix and match vendors in RAID6 arrays just fine and it lowers the correlation of failures between disks just as well. In fact parity RAID is again strictly better at this as well. Since the redundancy is across the whole array instead of split into clusters whenever you add more drives from different manufacturers you're adding that redundancy to the whole array. With RAID10 if you have 3 drives per mirror your maximum redundancy before losing data is 3 manufacturers. >There are also some recoverability benefits of RAID10 v. RAID6; notably, recovering from a fault in RAID6 requires recomputing parity, while no such thing is necessary for RAID10. This mitigates the "oh no my array took too long to fix itself and now my RAID is kaput" failure case. Yeah, performance is the real advantage of RAID10 as I mentioned and indeed that performance can be important during rebuilds so it has some durability implications.
- yellowapple 11y ago
  > That's not a proper comparison as you are now using more disks in the RAID10 array than the RAID6 to get the same space. You already have to use more disks for a RAID10 than in a RAID6, so I don't see why this is that big of a deal. Anything involving mirroring will involve at least a 50% cut in capacity (or more, such as with a 3-way or even 4-way mirror). RAID1 and RAID10 don't prioritize capacity; they prioritize robustness. > N-Parity setups are strictly more redundant than RAID10 for the same number of disks and required capacity. But not more robust against failure, as I've already described (hopefully) rather clearly. If we want to play this game, I can keep adding more disks per mirror, and now my worst-case will match that of your n-parity array while having a vastly better best-case (and, additionally, dragging the probability of that worst-case happening closer and closer to zero). Remember: if you're a RAID1 or RAID10 user who cares about disk capacity, you're doing something wrong. > I mix and match vendors in RAID6 arrays just fine and it lowers the correlation of failures between disks just as well. Do you mix and match all disks, though? Because if you have at least three (in the case of RAID6) disks, no matter where they are in the array, that have the same failure curve (i.e. are the same model/manufacturer), then you're gonna have a bad time when the first one of them dies. That's my point. In order to get the same inherent benefit as RAID10 here, you'd have to have a different vendor/model for every single drive in your RAID6 array. > whenever you add more drives from different manufacturers you're adding that redundancy to the whole array. Only if they're all different. You'll eventually get to the point where there aren't enough manufacturers in the world (or you'll be compromising on manufacturer diversity - which is, granted, what most people do). > With RAID10 if you have 3 drives per mirror your maximum redundancy before losing data is 3 manufacturers. And that's fine, because if one of those manufacturers sold me a defective batch, the problem's isolated to that side of the mirror. That's my point; with RAID10, it's no longer a game of sheer numbers of different models and manufacturers, but instead the more manageable strategy of giving each side of the mirror a different bathtub curve to ride, thus mitigating the chance of multiple hard drive batches failing at the same time.