RAID 10, 20 Disks, How Many Hot Spares
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This question was asked and I can't get the details of it right now, but while there is no way to get the details I figured that I would get the ball rolling on behalf of the person that was asking it. So the little that I know about the scenario is that there is a single RAID array of 20 spinning disks in RAID 10 and the person asking wants to know how many hot spares would be recommended.
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Zero with a couple of cold spares is what I'd suggest
Thats a lot of disks in a single array
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Generally, I would say that no hot spares would be needed. Mirrored RAID is insanely reliable and rebuilds very quickly and does not experience the increases drive failure risks of parity RAID. Also the risks of RAID 10 do not grow exponentially like parity RAID but linearly. Each mirrored set is discrete. So with rare exception, I would stick with zero hot spares.
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In a study of 160,000 RAID 1 array years, we lost zero arrays. Using hot spares is a crutch for people on parity RAID where risks are much higher. Going to mirrored, one of the key benefits is a lack of needing hot spares.
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@MattSpeller said in RAID 10, 20 Disks, How Many Hot Spares:
Zero with a couple of cold spares is what I'd suggest
If you have cold spares and you have available drive bays, you want them to be hot spares.
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@scottalanmiller said in RAID 10, 20 Disks, How Many Hot Spares:
@MattSpeller said in RAID 10, 20 Disks, How Many Hot Spares:
Zero with a couple of cold spares is what I'd suggest
If you have cold spares and you have available drive bays, you want them to be hot spares.
Why is that?
Aren't they still running and thus adding to the MTBF number?
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@BRRABill said in RAID 10, 20 Disks, How Many Hot Spares:
@scottalanmiller said in RAID 10, 20 Disks, How Many Hot Spares:
@MattSpeller said in RAID 10, 20 Disks, How Many Hot Spares:
Zero with a couple of cold spares is what I'd suggest
If you have cold spares and you have available drive bays, you want them to be hot spares.
Why is that?
Aren't they still running and thus adding to the MTBF number?
MTBF is a total myth, it's essentially useless. And no, they are not running, they are just sitting there, so they are not spinning and wearing out. But even if they were, it doesn't work like that. Yes they would wear out eventually, but at a fraction of the speed as if they were in actual use. But they are not, just sitting there idle.
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@scottalanmiller said in RAID 10, 20 Disks, How Many Hot Spares:
@MattSpeller said in RAID 10, 20 Disks, How Many Hot Spares:
Zero with a couple of cold spares is what I'd suggest
If you have cold spares and you have available drive bays, you want them to be hot spares.
Why wouldn't you want them to be part of the array? WTF is the point of hot spares in an array that does not need to resilver?
aka 20 drive array, 4 free slots -> populate free slots, redo array to 24 drives
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@MattSpeller said in RAID 10, 20 Disks, How Many Hot Spares:
@scottalanmiller said in RAID 10, 20 Disks, How Many Hot Spares:
@MattSpeller said in RAID 10, 20 Disks, How Many Hot Spares:
Zero with a couple of cold spares is what I'd suggest
If you have cold spares and you have available drive bays, you want them to be hot spares.
Why wouldn't you want them to be part of the array? WTF is the point of hot spares in an array that does not need to resilver?
aka 20 drive array, 4 free slots -> populate free slots, redo array to 24 drives
Because the larger the array, the higher the risk. So adding them to the array works against the purpose of the spares. Having them be cold doesn't provide as much value as being hot. Having them be hot, once you've paid for the protection, provides the best value in terms of safety.
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@scottalanmiller said in RAID 10, 20 Disks, How Many Hot Spares:
@MattSpeller said in RAID 10, 20 Disks, How Many Hot Spares:
@scottalanmiller said in RAID 10, 20 Disks, How Many Hot Spares:
@MattSpeller said in RAID 10, 20 Disks, How Many Hot Spares:
Zero with a couple of cold spares is what I'd suggest
If you have cold spares and you have available drive bays, you want them to be hot spares.
Why wouldn't you want them to be part of the array? WTF is the point of hot spares in an array that does not need to resilver?
aka 20 drive array, 4 free slots -> populate free slots, redo array to 24 drives
Because the larger the array, the higher the risk. So adding them to the array works against the purpose of the spares. Having them be cold doesn't provide as much value as being hot. Having them be hot, once you've paid for the protection, provides the best value in terms of safety.
That's a slippery slope argument against having more IOPS/space. I disagree and would populate the array to as many drives as I could. Hot spares have little or no value in SMB space where it's on premises and you have easy access.
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@MattSpeller said in RAID 10, 20 Disks, How Many Hot Spares:
That's a slippery slope argument against having more IOPS/space.
Not in the least. The purpose of the spares was for protection against failure. To buy something for one purpose and then use it for a counter-purpose is crazy. That's like saying "well we bought these bullets to protect ourselves against invaders, but since we have them let's shoot ourself in the foot with them so that we get use out of them." The goal is one thing, then you didn't just use the drives for a different purpose, but one that goes directly against the goal.
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@scottalanmiller Then we disagree on how much protection vs space/IOPS is warranted on a theoretical array. I see no value in wasting slots you paid a F(@*# ton of money for.
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@MattSpeller said in RAID 10, 20 Disks, How Many Hot Spares:
I disagree and would populate the array to as many drives as I could. Hot spares have little or no value in SMB space where it's on premises and you have easy access.
Hot spares and the SMB have no relationship. SMBs should be on premises the least, not the most. Combining mistakes doesn't make sense. You are making several assumptions that are wrong...
- That more IOPS are important enough to increase risk over past the point of the array being spec'd out already.
- That additional capacity has benefits beyond the specification point.
- That hot spares have no value, they always have some value in a mirrored array.
- That SMBs will be on premises.
All of those are wrong or potentially wrong. What we know in the OP's case is that the array was spec'd, now they are looking to invest in additional protection. Your recommendation is not just to invest, but to invest against protection and re-spec the array based on no data of the needs of the array at all. What if it is already way more IOPS and capacity than needed and any additional is just waste, but their risk aversion is high and the server is hosted on an island with no easy access?
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@MattSpeller said in RAID 10, 20 Disks, How Many Hot Spares:
@scottalanmiller Then we disagree on how much protection vs space/IOPS is warranted on a theoretical array.
No, you are having a discussion about protection vs space/IOPS and I am not. It's that simple. You are making a point that doesn't related to the question. The question is about investing in protection. You believe that "more capacity" is always better, even if there is no use for it?
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@scottalanmiller Argue a use case then, don't dance around making me chase you.
My use case is on prem easy access. Define yours and maybe we can agree on something.
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@MattSpeller said in RAID 10, 20 Disks, How Many Hot Spares:
I see no value in wasting slots you paid a F(@*# ton of money for.
No one said to waste them. They are talking about investing in additional protection.
Putting drives into the array when the IOPS and capacity are not needed is 100% wasted. So you just defeated your own point, there.
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@MattSpeller said in RAID 10, 20 Disks, How Many Hot Spares:
@scottalanmiller Argue a use case then, don't dance around making me chase you.
DOn't need a use case, risk aversion is the key. IOPS and capacity are spec'd properly, no more needed. Risk of the array is a concern. Hot spares would lower the risk, enlarging the array would increase the risk. This isn't complex. There is a goal: reducing risk. Your proposal is to undermine the goal for what reason? What makes you believe that risk protection is always bad and that higher risk is always good? Where would you stop with that logic? Always buy the biggest, fastest drives in the biggest possible arrays?
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@MattSpeller said in RAID 10, 20 Disks, How Many Hot Spares:
My use case is on prem easy access. Define yours and maybe we can agree on something.
- No one even suggested that on prem was going on, that's a totally false assumption. So you can't make up a use case and then use it to make the "it's always this way."
- Just because on prem is easy doesn't make off hours easy.
- Just because on prem is easy doesn't mean that wasting money on cold spares makes sense when hot spares are more reliable and less effort.
- Just because on prem is easy doesn't mean that we should increase risk for no known reason when the goal was to reduce risk.
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@scottalanmiller said in RAID 10, 20 Disks, How Many Hot Spares:
@MattSpeller said in RAID 10, 20 Disks, How Many Hot Spares:
@scottalanmiller Argue a use case then, don't dance around making me chase you.
DOn't need a use case, risk aversion is the key.
Bullshit, this is how you gish gallop all over anyone who disagrees with you - moving the goal posts. It's irritating as fuck tbh.
IOPS and capacity are spec'd properly, no more needed.
Again, this is crap - I know you can do better.
Give a real world example.
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@MattSpeller said in RAID 10, 20 Disks, How Many Hot Spares:
@scottalanmiller said in RAID 10, 20 Disks, How Many Hot Spares:
@MattSpeller said in RAID 10, 20 Disks, How Many Hot Spares:
@scottalanmiller Argue a use case then, don't dance around making me chase you.
DOn't need a use case, risk aversion is the key.
Bullshit, this is how you gish gallop all over anyone who disagrees with you - moving the goal posts. It's irritating as fuck tbh.
Sorry, but thats exactly what didn't happen. The goal never moved, at all. The goal was to reduce risk, you have a personal agenda that risk should never be reduced only increased and you are saying anything, including now making a personal attack, to support it. But you are not at all looking at the needs of the OP, just interjecting some personal goal that doesn't align.
No moving goal posts, none. You made up a new goal that didn't exist.
