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A share in only 1 drive, with 1 parity disk. If the 1 drive fails, can restore from parity?


jang430

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I've been using Unraid since the beginning with share spanning multiple drives.  Recently, since I'm running out of disk space on existing drives, and wanted my share to be limited across just 1 drive, I added a new drive.  I created a share just on this drive.  I have my parity.  If this drive fails, will I be able to restore from parity if I replaced this drive with a new one?  

 

It's been so long since I started using Unraid, I forgot about this already.

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15 minutes ago, jang430 said:

If this drive fails, will I be able to restore from parity if I replaced this drive with a new one?  

Yes, assuming all the rest of your drives can be read perfectly. Parity doesn't hold any data, it's the final piece of an equation involving all the drives. Any one drive is gone, the rest are required to fill out the rest of the equation.

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Just to confirm once again, the share involves just 1 drive (which in my evil scenario, will die), but all the other drives within the Unraid system (which aren't part of the share) will come to rescue and help build the share from that single drive that failed?

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2 minutes ago, jonathanm said:

Yes, assuming all the rest of your drives can be read perfectly. Parity doesn't hold any data, it's the final piece of an equation involving all the drives. Any one drive is gone, the rest are required to fill out the rest of the equation.

Didn't a simple "yes" suffice??

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5 hours ago, jang430 said:

Hehehe, kinda hard to believe, I guess that's what makes Unraid great! 

The point is that parity has no concept of shares or file systems - it works purely at the disk sector level.    When you rebuild a drive there is no understanding of the meaning of the contents being rebuilt - just that the sectors on it need to have the same contents as the drive previously had.

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The rest of the drives coming to the rescue in your "evil" scenario means that all other drives (which have none of your share data) must be fully functional until a rebuild is complete.  If you were to lose any other drive at the same time, all your share data are gone.

 

It is always true that two drive failures in unRAID mean the parity bits are invalid if using a single parity drive, (3 drive failures means same if using dual parity,)  so it is not to say confining a share to a single drive is bad or good, only to remind that any important data need to be backed up to be truly safe.

 

Side note: Dual parity does not necessarily mean you can lose 2 drives and still successfully recover all your data. See the following for reasoning:  https://lime-technology.com/forums/topic/58741-how-does-dual-parity-work-actually/?do=findComment&comment=576325

 

 

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The rest of the drives coming to the rescue in your "evil" scenario means that all other drives (which have none of your share data) must be fully functional until a rebuild is complete.  If you were to lose any other drive at the same time, all your share data are gone.
 
It is always true that two drive failures in unRAID mean the parity bits are invalid if using a single parity drive, (3 drive failures means same if using dual parity,)  so it is not to say confining a share to a single drive is bad or good, only to remind that any important data need to be backed up to be truly safe.
 
Side note: Dual parity does not necessarily mean you can lose 2 drives and still successfully recover all your data. See the following for reasoning:  https://lime-technology.com/forums/topic/58741-how-does-dual-parity-work-actually/?do=findComment&comment=576325
 
 


Thank you for the clarification. This helps.


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