Confused about removing drives


aqua

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I don't have an issue yet (knock on wood) but am planning out my server configuration and am trying to understand something: I purchased two new 8TB drives to set up the test system and have another 8TB, 5 TB, and 4TB in my current server that I will move over when I go live. I currently have about 10TB of content to be on the server. I was reading here about how to replace drives when they fail but if I were to put all the drives in the system and have way more space than is currently needed and a drive failed, I'd be inclined to just remove it and hope the data would get moved to other drives, but it seems like that may not be possible and that I'd be forced to replace the drive. In short, I'm having problems reconciling/understanding the language in that link and the warning language in the "new config" tool which says "DO NOT USE THIS UTILITY THINKING IT WILL REBUILD A FAILED DRIVE - it will have the opposite effect of making it impossible to rebuild an existing failed drive - you have been warned!"

 

Can someone explain/help? If my pool has lots of extra capacity can I just remove a failed drive without replacing it or will I be forced to replace it? If forced to replace it, how is this different than just removing a drive that hasn't failed yet? If I'm forced to replace, I'd assume it is just better to add drives when the capacity is needed rather than add ahead of time, correct?

 

When removing disks (that have not failed, I assume - there seems to be a distinction) the instructions say to make note of allocation and then to reset the array configuration and then to reasign the drives - it doesn't say explicitly but I assume this means to the same disk numbers that they were before, is that correct? Or are they just meaning to allocate back the same drives to the numbered disks and the parity ones to the parity disks, etc? If to the same slots as before removing a disk, I assume that means it is OK to leave certain numbers blank, such as having disk1, disk2, disk5, disk8 and nothing else assigned?

 

Thanks, appreciate any help.

Edited by aqua
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24 minutes ago, aqua said:

it is just better to add drives when the capacity is needed rather than add ahead of time, correct?

This, in a nutshell. Adding array assignments is easy, safe, and automatic, removal is not. Don't add more than you need.

 

Removing a disk from the array means recalculating parity based on the remaining disks. Data on the removed disk is gone with the disk.

 

Unraid never moves data from one array drive slot to another on its own, the only automatic moves are from the cache to the array and array to cache, depending on share setting. You can manually move data from one disk to another with various tools.

 

Slot assignments don't matter when recalculating parity, you can assign them to any slots. If you mess up and assign a data drive to a parity slot, it will be permanently erased, so don't do that.

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Maybe I will use the extra drive as a second parity drive then... Are there disadvantages to dual parity drives? Extra processing power? Easy to move a parity drive to the array if/when needed?

 

32 minutes ago, jonathanm said:

Slot assignments don't matter when recalculating parity, you can assign them to any slots. If you mess up and assign a data drive to a parity slot, it will be permanently erased, so don't do that.

Do slot assigments matter ever? When resetting the array configuration? This isn't super clear to me...

Edited by aqua
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Any modern processor can handle dual parity without issue.

 

It's not impossible to use parity2 as a replacement for a data slot, but it's not automatic.

 

Slot assignments are important if you have a drive assigned to parity2 and wish to maintain the same set of data drives, but the first parity drive slot is valid for any arrangement of the data slots as long as all the drives are included. Also, if you choose to restrict shares to a specific drive slot that can effect things if you reassign the drives differently.

 

If you are resetting the array with a different set of data drives, parity is invalid and must be recalculated anyway, so slots don't matter in that case.

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1 hour ago, jonathanm said:

If you are resetting the array with a different set of data drives, parity is invalid and must be recalculated anyway, so slots don't matter in that case.

Perhaps I'm still misunderstanding something, but if a drive fails and I have only 1 parity drive and I replace the failed drive, does order matter? Parity becomes invalid when? When I add or remove drives that haven't failed? I feel like I'm missing something fundamental here....

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Typically Your parity is always intact unless you click on the button in [tools] that says new config which invalidates your Parity and needs to be rebuilt. If your Parity drive dies you simply swap in a new one and it copies your Parity onto your new Parity drive.

 

As for order are you referring to which plug on your MotherBoard your using? If so unRAID doesn't care because it simply matches in its GUI.

 

Parity to Serial Number ***********

Disk1 to Serial Number ************

Disk2 to Serial Number ************

So on and so on. It doesn't care which SATA port your connected to.

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I'm meaning as to what disk1 goes to what serial number in terms of order mattering, not what sata port. The questions remain:

1. When does parity become invalid?

2. How is removing which hasn't failed voluntarily (and not replacing) different than removing a drive that has failed and not replacing it? It seems it is, but I don't understand why/how.

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6 hours ago, aqua said:

1. When does parity become invalid?

There are different conditions for the two parity slots, because the math is different between the two.

 

Parity 1 is valid as long as all the data drives are present and all writes have been accounted for.

 

Parity 2 adds the requirement that all the data drives are assigned to the original slots.

 

6 hours ago, aqua said:

2. How is removing which hasn't failed voluntarily (and not replacing) different than removing a drive that has failed and not replacing it? It seems it is, but I don't understand why/how.

It's not any different. In both cases parity is invalidated and must be rebuilt based on the new set of drives. Any data that was on the removed drives is lost.

 

If a drive fails, it will be emulated and the contents should be intact, the normal way forward is to replace the failed drive and allow the emulated content to be written to the new drive. If you set a new config while you have a drive being emulated, all that content will be lost, which is the reason for the warning.

 

There are no automatic provisions for removing a failed drive and saving the data, it must be replaced if you wish to keep the data in the same slot. You can manually copy the data off of the emulated failed drive slot and then set a new config, but that is on you to accomplish using whatever method you choose to copy the data. You can copy the emulated content to another array disk, but that would be slower than copying to an off array drive or rebuilding to a new drive.

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10 hours ago, trurl said:

It doesn't copy parity, it rebuilds it to the replacement. No doubt @kizer actually knows that.

 

 

Thanks @trurl

 

🤣 Well I had the right idea in mind when I was attempting to explain it. Lol

Sometimes I need to lay off the Phone when I'm cooking, cleaning and dodging the wives stares.

 

My simple thought process to all of this has been like this.

Drive1 + Drive2 = Parity

1 + 1 = 2 with all 3 drives present

1 + ?  = 2 missing disk 2

? + 1 = 2 missing disk 1

1 + 1 = ? missing parity

 

As long as you have no more than 1 failed drive you can rebuild your data ?. However In a Dual Parity setup you can rebuild two down drives.

 

If you remove a drive on purpose because your downsizing the amount of drives in your array you need to click the [New Config] button in tools.

If you need to rebuild your Data drive or your Parity drive because of a failure you simply swap out your failed drive and click [Rebuild]

 

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