RAID 5 URE Clarity Question
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Okay, let's use 6x 2TB drives to avoid confusion, and so the RAID 5 array = 12TB to match your math of a 12 TB RAID 5 being near a 100% to experience a URE.
Let's say drive E needs to be rebuilt.
You said each drive has a URE of 10^14.
How much data that matters needs to be read from drive D in order to help rebuild drive E? I would think a maximum of 400GB needs to be read from drive D. The data that was on drive E, is spread throughout the other 5 drives. So there is ~400GB of data on drive D that needs to be read so it can help rebuild the data that was on drive E. And each other drive will do the same thing.
Being that drive Ds URE is 10^14, which you said comes out to equal about 12TB of reads, I would think that a chance of a URE happening on drive D would be 3%. So isn't drive D only needed for the 400GB it contains of drive E to help rebuild it? That's 3% of the drives URE rate.
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@tim_g said in RAID 5 URE Clarity Question:
How much data that matters needs to be read from drive D in order to help rebuild drive E?
That you are asking this means you don't understand the issue. URE is a rate of failure. That alone, I think, should explain everything.
Or to state in another way, 400% of D has to be read to rebuild E.
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@tim_g said in RAID 5 URE Clarity Question:
I would think a maximum of 400GB needs to be read from drive A.
No, 2TB from EACH drive. Every drive has to be read 100% to recreate the data with all parity is lost. With RAID 6 it is more complex to explain if only one drive has failed, but the 400% number remains the same, it is just split over five drives instead of four. But the URE rate never changes. But only when two drives are lost do you have URE exposure.
So the only scenario that matters is 400% of one drive for 8TB of risk domain.
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@tim_g said in RAID 5 URE Clarity Question:
So isn't drive D only needed for the 400GB it contains of drive E to help rebuild it?
No, D doesn't contain ANYTHING of drive E. That's likely the root of confusion. At no point in parity RAID does any drive contain the contents of any other drive. That's mirroring, and mirroring doesn't have this risk at all.
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I wanted to change drive D to drive A so they don't get confused... but changed it back.... for anyone else wondering.
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@scottalanmiller said in RAID 5 URE Clarity Question:
@tim_g said in RAID 5 URE Clarity Question:
So isn't drive D only needed for the 400GB it contains of drive E to help rebuild it?
No, D doesn't contain ANYTHING of drive E. That's likely the root of confusion. At no point in parity RAID does any drive contain the contents of any other drive. That's mirroring, and mirroring doesn't have this risk at all.
That's not how I mean it... it contains 400GB of parity data that is used to help reconstruct the data in drive E, doesn't it?
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But not matter what, there's a good 400GB of crap on drive D that is needed to help rebuild the data that was on drive E...
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@tim_g said in RAID 5 URE Clarity Question:
@scottalanmiller said in RAID 5 URE Clarity Question:
@tim_g said in RAID 5 URE Clarity Question:
So isn't drive D only needed for the 400GB it contains of drive E to help rebuild it?
No, D doesn't contain ANYTHING of drive E. That's likely the root of confusion. At no point in parity RAID does any drive contain the contents of any other drive. That's mirroring, and mirroring doesn't have this risk at all.
That's not how I mean it... it contains 400GB of parity data that is used to help reconstruct the data in drive E, doesn't it?
No, it contains 2TB of parity data, every block of which is necessary for reconstructing the lost drive(s).
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@scottalanmiller said in RAID 5 URE Clarity Question:
@tim_g said in RAID 5 URE Clarity Question:
@scottalanmiller said in RAID 5 URE Clarity Question:
@tim_g said in RAID 5 URE Clarity Question:
So isn't drive D only needed for the 400GB it contains of drive E to help rebuild it?
No, D doesn't contain ANYTHING of drive E. That's likely the root of confusion. At no point in parity RAID does any drive contain the contents of any other drive. That's mirroring, and mirroring doesn't have this risk at all.
That's not how I mean it... it contains 400GB of parity data that is used to help reconstruct the data in drive E, doesn't it?
No, it contains 2TB of parity data, every block of which is necessary for reconstructing the lost drive(s).
Oh I see... I had it wrong the whole time.
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@tim_g said in RAID 5 URE Clarity Question:
But not matter what, there's a good 400GB of crap on drive D that is needed to help rebuild the data that was on drive E...
No, parity RAID is like a single file, when it corrupts, it is lost. Doesn't matter how many good blocks there are.
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@scottalanmiller said in RAID 5 URE Clarity Question:
@tim_g said in RAID 5 URE Clarity Question:
But not matter what, there's a good 400GB of crap on drive D that is needed to help rebuild the data that was on drive E...
No, parity RAID is like a single file, when it corrupts, it is lost. Doesn't matter how many good blocks there are.
So then it means the entire 2TB of EVERY drive needs to be READ to reconstruct the 2TB that was on the bad drive.
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@tim_g said in RAID 5 URE Clarity Question:
@scottalanmiller said in RAID 5 URE Clarity Question:
@tim_g said in RAID 5 URE Clarity Question:
@scottalanmiller said in RAID 5 URE Clarity Question:
@tim_g said in RAID 5 URE Clarity Question:
So isn't drive D only needed for the 400GB it contains of drive E to help rebuild it?
No, D doesn't contain ANYTHING of drive E. That's likely the root of confusion. At no point in parity RAID does any drive contain the contents of any other drive. That's mirroring, and mirroring doesn't have this risk at all.
That's not how I mean it... it contains 400GB of parity data that is used to help reconstruct the data in drive E, doesn't it?
No, it contains 2TB of parity data, every block of which is necessary for reconstructing the lost drive(s).
Oh I see... I had it wrong the whole time.
I figured that out
So it is 2TB, from every working drive in the array (4), for 8TB total. Which gives us somewhere around a 60% chance of hitting a URE. That's because 12T is an average, not a guarantee. If it was exactly every 12TB, it would be 67% chance of loss. -
@tim_g said in RAID 5 URE Clarity Question:
@scottalanmiller said in RAID 5 URE Clarity Question:
@tim_g said in RAID 5 URE Clarity Question:
But not matter what, there's a good 400GB of crap on drive D that is needed to help rebuild the data that was on drive E...
No, parity RAID is like a single file, when it corrupts, it is lost. Doesn't matter how many good blocks there are.
So then it means the entire 2TB of EVERY drive needs to be READ to reconstruct the 2TB that was on the bad drive.
Correct
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@scottalanmiller said in RAID 5 URE Clarity Question:
So it is 2TB, from every working drive in the array (4), for 8TB total,
to avoid confusion, do you mean (5), for 10TB total? Because there's 6 total, one went bad, 5 working ones left?
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@scottalanmiller said in RAID 5 URE Clarity Question:
t you buy, because that's what sets the failure rate. Obviously it is physical drives that fail, so it is the quality of the drives you
you guys are bouncing between RAID 5 and 6 conversations..
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@tim_g said in RAID 5 URE Clarity Question:
@scottalanmiller said in RAID 5 URE Clarity Question:
So it is 2TB, from every working drive in the array (4),
to avoid confusion, do you mean (5), for 10TB total? Because there's 6 total, one went bad, 5 working ones left?
No, because URE risk only matters when two drives are lost in RAID 6. If you had five drives, you have no URE risk.
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@scottalanmiller said in RAID 5 URE Clarity Question:
@tim_g said in RAID 5 URE Clarity Question:
@scottalanmiller said in RAID 5 URE Clarity Question:
So it is 2TB, from every working drive in the array (4),
to avoid confusion, do you mean (5), for 10TB total? Because there's 6 total, one went bad, 5 working ones left?
No, because URE risk only matters when two drives are lost in RAID 6. If you had five drives, you have no URE risk.
I'm talking about a 6x 2TB drives in a RAID 5. One of those drives goes bad, so you hot-swap it out with a good one and the rebuilding starts. At this point, URE matters because if a 2nd drive dies before the rebuild is complete, game over.
I'm not asking or saying anything at all about RAID 6.
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@tim_g said in RAID 5 URE Clarity Question:
@scottalanmiller said in RAID 5 URE Clarity Question:
@tim_g said in RAID 5 URE Clarity Question:
@scottalanmiller said in RAID 5 URE Clarity Question:
So it is 2TB, from every working drive in the array (4),
to avoid confusion, do you mean (5), for 10TB total? Because there's 6 total, one went bad, 5 working ones left?
No, because URE risk only matters when two drives are lost in RAID 6. If you had five drives, you have no URE risk.
I'm talking about a 6x 2TB drives in a RAID 5. One of those drives goes bad, so you hot-swap it out with a good one and the rebuilding starts.
I'm not asking or saying anything at all about RAID 6.
Whoops.
In that case you need 500% of a single drive. So the failure domain is 10TB, not 8TB. Sorry, got confused. You need the full capacity of all five remaining drives to restore the one that has been lost.
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@scottalanmiller said in RAID 5 URE Clarity Question:
@tim_g said in RAID 5 URE Clarity Question:
@scottalanmiller said in RAID 5 URE Clarity Question:
@tim_g said in RAID 5 URE Clarity Question:
@scottalanmiller said in RAID 5 URE Clarity Question:
So it is 2TB, from every working drive in the array (4),
to avoid confusion, do you mean (5), for 10TB total? Because there's 6 total, one went bad, 5 working ones left?
No, because URE risk only matters when two drives are lost in RAID 6. If you had five drives, you have no URE risk.
I'm talking about a 6x 2TB drives in a RAID 5. One of those drives goes bad, so you hot-swap it out with a good one and the rebuilding starts.
I'm not asking or saying anything at all about RAID 6.
Whoops.
In that case you need 500% of a single drive. So the failure domain is 10TB, not 8TB. Sorry, got confused. You need the full capacity of all five remaining drives to restore the one that has been lost.
Okay, that's what I thought and wanted to make sure or i'd be confused again.
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Sorry about the RAID 6 confusion. Everything referencing 8TB or 400% was me thinking this was six drives in RAID 6 and losing two, instead of six disks in RAID 5 losing one.
