Parity Replacement Drive Question

[mikedpitt420]

I know this question has been asked and answered in several different forms, but I can find a definitive answer in my specific case. This is a friend's system, who has a 3TB drive has his parity drive, and it is a fairly new drive and working well, SMART tests are all fine. He came across a new free 4TB drive and wants to add it, thus making it the parity drive. Would the best practice be to:

1. Power down the system and physically connect the new 4TB drive

2. Power up, then stop the array

3. Un-assign the current parity drive completely

4. Assign the precleared 4TB drive as a parity drive, start the array, and allow it to calculate new parity

5. Immediately perform a parity check to ensure the new data has been written correctly and is readable

6. Assign the 3TB drive to an array slot, where I imagine it would be reformatted to XFS, as it was previously ReiserFS and then become usable array storage space.

 

My other question is : if the parity check fails, or something else goes wrong I should technically just be able to yank the new 4TB drive, and re-assign the 3TB back to parity, and pick up exactly where I left off?

Of course making backups of the config dir after stopping the array and having all other pertinent info at hand before attempting such a task.

 

Would this be the best practice for this particular procedure and (hopefully not) recovery?

[garycase]

Close.

 

I presume your goal is to retain the ability to use the old parity drive for a recovery "just in case" a data drive fails during the process of doing the new parity sync.  Assuming that's the case, your procedure will work fine.  You may need to Start & Stop the array between steps 3 & 4 so the parity drive is marked as "missing" [Not sure if that's needed with v6].

 

Assuming all goes well through step 5 (i.e. parity is recalculated and you've done a check to confirm it's good), you may want to then do a pre-clear on the old parity disk before assigning it to the array.    It's well tested already (through its usage as a parity drive) ... but a pre-clear will make adding it to the array MUCH quicker.  If it's not cleared, UnRAID will clear it before adding it ... a process that takes several hours, during which the array is not available.  If it's pre-cleared, it's added instantly and all you have to do is format it.

 

Note that the pre-clear script can be run with a  "-n" parameter (lower case is important), which will skip the pre- and post- read operations, simply clearing the drive and marking it as cleared ... this takes about 1/4th the time of a full pre-clear cycle.

 

 

There's another way to do this that also eliminates the clearing requirement for the old drive.  You can do a New Config at the beginning, assigning the new drive as parity and the old drive as a data drive.  Do NOT choose to format the new data drive (the old parity drive) ... just let UnRAID do the parity sync -- and then verify it with a check.    As long as that completes error-free, you can then simply let UnRAID format the "unformatted" data drive (the old parity drive).    Doing it this way does not require that the drive be cleared, since parity will be computed based on its current contents.    As long as you don't let UnRAID modify (i.e. format) the drive, it will still be possible to reconfigure the system for recovery if something was to go wrong before parity was recomputed.

 

 

 

[mikedpitt420]

Sweet. And doing it this latter way, will also allow me to still use this drive as a "backup parity disk" just in case still as it is not formatted until the very last step right?

 

Thanks very much sir. Do you think it's worth doing the preclear again? I don't mind doing it if that would be recommended. I just don't know if the stress would be worth it, since it's been working? Or would it? If so, I'd gladly preclear it before re-adding it as a data disk.

 

Thanks again sir.

[garycase]

Yes, that's why I said be sure you don't let UnRAID format the drive until you have a good new parity drive.

 

As for doing another pre-clear => certainly never hurts, but also IMHO not necessary.  I wouldn't bother if it was me.

 

 

[SSD]

There is a link in my sig to a best practice procedure for replacing parity.