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>But I strongly disagree with RAID being the problem. Rather the increasing of capacity with bandwidth not really improving is the issue. Of course this is a in

>But I strongly disagree with RAID being the problem. Rather the increasing of capacity with bandwidth not really improving is the issue. Of course this is a inherent limitation of disk drives. I mean, yes, you could have ssds in raid and not have to worry about the drive failing before it's rebuilt, but we're talking about spinning disks here. What's the current recommendations around raid capacity before you have to seriously start worrying about drive failure before it can be rebuilt? This is a genuine question I don't know.

sandworm101 6y ago
>> What's the current recommendations around raid capacity before you have to seriously start worrying about drive failure before it can be rebuilt? That depends on the size of your drives and the number in the array. If you are running a small NAS with only one parity drive, you don't want drives that take more than a day to onboard. That limits you to 4/6TB per drive. If you have two parity drives in the array you can probably risk the 8/10/12 TB size. If you are running commercial-scale arrays of dozen of drives, arrays that can handle multiple failures at once, then the sky is probably the limit. But the story gets more complicated. Are all of your drives the same age? Are all the drives of the same model/manufacturer? Such things increase the risk ofone failure evolving into a multi-drive failure during rebuild. If/when I build a new array I want drives of different ages. So If I had 10 drives, I would start the array on five or six, holding some for later so that the entire array isn't the same age.
effie 6y ago
Drive failure during a rebuild isn't a real concern unless 1) the rebuild takes time comparable to characteristic lifetime of the drive, which on average is >5years. Rebuild times are more in the realm of days, maybe coming to weeks with the proposed 100TB models. Still far from substantial probability of failure. 2) the drives are very old or have bad SMART data. Then the probability of drive failure and (array loss) shoots up. What people often talk about regarding big drives and reliability concerns is probability of rebuild failure for RAID5, RAID6, i.e. some bit is read or written wrong and the array becomes inconsistent. This is much more probable but isn't really a big problem in practice because this can be detected and rebuild can be repeated.
- effie 6y ago
  OK the above isn't exactly correct in the last sentence. If there is URE due to medium error then we get a bad stripe and repeating rebuild will most often achieve nothing to remove it (although see SpinRite which can sometimes recover data from bad sectors). However, even if the bad sector can't be read, the "RAID failure" is still just loss of single or few sectors, which usually isn't a big deal. The array filesystem data is then best copied to a new array on new disks and then the old array discarded.
hinkley 6y ago
As long as you haven't hit the protocol bottleneck, aerial density almost always increases bandwidth. More bits per track equals more bits per revolution. If RPMs are constant then bits per second goes up as do bits per seek operation. It's the seeks that are murder.
- wing-_-nuts 6y ago
  Please ELI5, I know what each of these words mean in isolation but I know nowhere near enough about storage to understand what this means as a sentence.
  - hinkley 6y ago
    Hard drive only have a handful of different rates of spinning (rpm). The diameter of the platters are also limited to a couple options. The number of platters varies a bit, but there are numbers that are too stupid-low and ones that are too high to fit into the case, so there are maybe 3-5 varieties? If you have two drives with the same number of platters, the same diameter, and the same rotation speed, the one with the highest storage can only do that by increasing storage density - making the bits smaller. Smaller bits means more tracks, but also more bits per track. So in a single rotation (1/7200 of a second, for instance), the read head sweeps across more data. If it reads it as fast as it sees it (which they do), then that means a higher transfer rate.

sandworm101 6y ago

>> What's the current recommendations around raid capacity before you have to seriously start worrying about drive failure before it can be rebuilt? That depends on the size of your drives and the number in the array. If you are running a small NAS with only one parity drive, you don't want drives that take more than a day to onboard. That limits you to 4/6TB per drive. If you have two parity drives in the array you can probably risk the 8/10/12 TB size. If you are running commercial-scale arrays of dozen of drives, arrays that can handle multiple failures at once, then the sky is probably the limit. But the story gets more complicated. Are all of your drives the same age? Are all the drives of the same model/manufacturer? Such things increase the risk ofone failure evolving into a multi-drive failure during rebuild. If/when I build a new array I want drives of different ages. So If I had 10 drives, I would start the array on five or six, holding some for later so that the entire array isn't the same age.

effie 6y ago

Drive failure during a rebuild isn't a real concern unless 1) the rebuild takes time comparable to characteristic lifetime of the drive, which on average is >5years. Rebuild times are more in the realm of days, maybe coming to weeks with the proposed 100TB models. Still far from substantial probability of failure. 2) the drives are very old or have bad SMART data. Then the probability of drive failure and (array loss) shoots up. What people often talk about regarding big drives and reliability concerns is probability of rebuild failure for RAID5, RAID6, i.e. some bit is read or written wrong and the array becomes inconsistent. This is much more probable but isn't really a big problem in practice because this can be detected and rebuild can be repeated.

effie 6y ago
OK the above isn't exactly correct in the last sentence. If there is URE due to medium error then we get a bad stripe and repeating rebuild will most often achieve nothing to remove it (although see SpinRite which can sometimes recover data from bad sectors). However, even if the bad sector can't be read, the "RAID failure" is still just loss of single or few sectors, which usually isn't a big deal. The array filesystem data is then best copied to a new array on new disks and then the old array discarded.

hinkley 6y ago

As long as you haven't hit the protocol bottleneck, aerial density almost always increases bandwidth. More bits per track equals more bits per revolution. If RPMs are constant then bits per second goes up as do bits per seek operation. It's the seeks that are murder.

wing-_-nuts 6y ago
Please ELI5, I know what each of these words mean in isolation but I know nowhere near enough about storage to understand what this means as a sentence.
- hinkley 6y ago
  Hard drive only have a handful of different rates of spinning (rpm). The diameter of the platters are also limited to a couple options. The number of platters varies a bit, but there are numbers that are too stupid-low and ones that are too high to fit into the case, so there are maybe 3-5 varieties? If you have two drives with the same number of platters, the same diameter, and the same rotation speed, the one with the highest storage can only do that by increasing storage density - making the bits smaller. Smaller bits means more tracks, but also more bits per track. So in a single rotation (1/7200 of a second, for instance), the read head sweeps across more data. If it reads it as fast as it sees it (which they do), then that means a higher transfer rate.