Physically rearranging HDDs in array - is this okay? [SOLVED]


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Hi there - my setup has 7 x data, 1 x parity, 1 x cache disks. These disks are split between motherboard SATA and an LSI9201-8i card in no logical arrangement.

 

1. If I "tidied up" and physically rearranged the disks in the case, moving disks between the onboard SATA and the LSI card,  would that cause any problems for the array?

2. Would the arrangement of disks in the web GUI update to reflect the new physical arrangement of disks in the case?

 

Thanks!

Edited by kimguan
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1 hour ago, johnnie.black said:

It won't cause any problems but nothing will change on the GUI, except the disk identifiers.

 

Can I clarify:

 

If we start with this:

Physical disks:

(1) Data 2TB

(2) Data 4TB

(3) Parity

GUI (matches physical arrangement):

(1) Data 2TB

(2) Data 4TB

(3) Parity

 

And I physically rearrange the physical disks thusly:

(1) Parity

(2) Data 4TB

(3) Data 2TB

 

Will the GUI show:

Option 1 (matching original physical arrangement):

(1) Data 2TB

(2) Data 4TB

(3) Parity

Option 2 (matching new physical arrangement):

(1) Parity

(2) Data 4TB

(3) Data 2TB

Option 3:

Some other arrangment

 

If it is not Option 2, is there a way to change the GUI to match the new physical arrangement?

 

Thanks in advance for your help!

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3 hours ago, bonienl said:

In short, physically re-arranging the disks will not change the GUI layout and all remains as before.

 

Ok - that makes it clearer.

 

If the GUI does not automatically rearrange the order of the disks to reflect the physical location of the disks, can I do it manually without losing data or going through a parity rebuild? For example, if I physically swap the location of 2 data disks, can I stop the array, then unassign the 2 disks in the GUI and then reassign them to match their physical locations?

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1 hour ago, bonienl said:

If you want to GUI to follow the hardware layout, you will need to do a "New Config" and assign the disks as per your wishes.

 

 

So the procedure would be:

  1. shut down server;
  2. physically rearrange disks;
  3. start server;
  4. stop array;
  5. go to Tools > New Config;
  6. "Preserve current assignments": none; select "I want to do this"; click "Apply";
  7. Go to Main;
  8. Assign disks to slots; select "Parity is valid"; start array.

 

Is that correct?

Edited by kimguan
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20 minutes ago, kimguan said:

 

So the procedure would be:

  1. shut down server;
  2. physically rearrange disks;
  3. start server;
  4. stop array;
  5. go to Tools > New Config;
  6. "Preserve current assignments": none; select "I want to do this"; click "Apply";
  7. Go to Main;
  8. Assign disks to slots; select "Parity is valid"; start array.

 

Is that correct?

The usual advice at step 6 is preserve all. Then you can change them as you wish or not. If you preserve none then it won't remember where anything was. At the very least you should preserve parity since you can definitely screw up by assigning the wrong disk to parity.

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4 minutes ago, trurl said:

The usual advice at step 6 is preserve all. Then you can change them as you wish or not. If you preserve none then it won't remember where anything was. At the very least you should preserve parity since you can definitely screw up by assigning the wrong disk to parity.

 

Thanks that makes perfect sense. Thank you everyone for your help!

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Just a note for others finding this thread. Parity 1 is valid when disks are rearranged, if you have Parity 2 assigned, it will need to be rebuilt.

 

Whether or not you have only one parity drive is not relevant, only which slot it occupies. A single parity disk assigned to parity 2 would need to be rebuilt if you rearrange the slot assignments as discussed in this thread.

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4 hours ago, jonathanm said:

Whether or not you have only one parity drive is not relevant, only which slot it occupies. A single parity disk assigned to parity 2 would need to be rebuilt if you rearrange the slot assignments as discussed in this thread.

 

This is interesting!

 

This would imply that the parity algorithm is tied to the slot and not the number of disks. Slot 1 uses XOR and Slot 2 uses Reed-Solomon?

 

Going off on a tangent now: out of curiosity, why was the decision made to limit unRAID TO 2 parity drives? Reed-Solomon should be able to support an arbitrary number.

 

http://web.eecs.utk.edu/~plank/plank/papers/CS-96-332.pdf

Edited by kimguan
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On 4/30/2018 at 7:03 PM, kimguan said:

Going off on a tangent now: out of curiosity, why was the decision made to limit unRAID TO 2 parity drives? Reed-Solomon should be able to support an arbitrary number.

 

Just because something can be done, does not mean that it should be done.  You might want to look at this analysis of failure probability for a unRAID with no, single, and dual parity setups.

 

      https://lime-technology.com/forums/topic/50504-dual-or-single-parity-its-your-choice/

 

Note the observation that with dual parity, the likelihood of data loss from other physical threats (fire, wind, flooding, lightening theft, etc.) now exceeds that of NOT having a triple parity drive.  And that does not even address the issue of a sloppy, unobservant administrator.  IF your data is truly important to you, you need a well thought out backup plan that will truly protect that data which can simply not be replaced.  Without such a plan, not amount of multi-parity protection is going to provide any real security.  

 

Edited by Frank1940
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38 minutes ago, Frank1940 said:

And that does not even address the issue of a sloppy, unobservant administrator. 

 

You need to make sure you have Notifications setup to alert you by email or other agent. Frequently users will have an issue and not be aware of it until they get even more issues.

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