How often do you run a parity check and do you think monthly is necessary?


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My Build in my profile which is using a 4TB Parity drive running all 5400 drives takes around 14hours. Starts at Midnight and I normally expect it to stop around 1pm/2pm. I use User Scripts and have my scripts setup only to run 2nd-31st of every month and Dockers they just do what they do whenever. 


Personally I like Monthly so I'm more aware of issues if any ever arises and I don't forget to run it since its setup to run automatically. 

Also I never schedule any Hardware changes or upgrades until after the end of the month since I know a parity check is going to be done. :D

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  • 2 years later...
5 minutes ago, razz said:

Is the parity drive udated everytime one puts new files on the unraid drives?

Yes. the parity drive(s) is updated anytime anything is written to any array data drive.  If the share to which the file is written is using the cache drive, the actual updating of the parity drive happens when the mover moves the file from cache to the array and not on the initial write which goes to the cache drive.


Parity is a real-time calculation that happens when any array write happens.


8 minutes ago, razz said:

Also what does it mean when a partiy check fails

This usually refers to an error being detected in the parity calculation.  For example, the parity drive says that parity calculation of the data at a particular location on all the data drives should result in a 0, yet it actually results in a 1.


You want your parity checks to result in 0 errors.  Anything other than 0 means there is a mismatch between the data on the disks and the parity calculation on the parity drive and something needs to be corrected.


Scheduled monthly, or however often you do them, parity checks are non-correcting by default.  In other words, they report errors but don't fix them.  Once you have looked into potential causes for the errors, you can choose to run a correcting parity check manually.

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21 hours ago, razz said:

Is the parity drive udated everytime one puts new files on the unraid drives?

The way this is worded is wrong. Unraid's parity doesn't deal with files in any way. It only recreates the whole drive, filesystem included. When you delete a file, an entry is made in the filesystem, which in turn updates parity. When a drive is formatted, parity changes to match. When you add a file, parity is changed. Every change to the drive is updated to parity, including if something corrupts a file, or the filesystem, causing all files to be unavailable. Parity will happily recreate the corruption on a new drive if you replace it, and the replacement drive will have the identical corruption.



Parity does not backup your files. Its only use is to rebuild a drive that has not accepted a write, for whatever reason, be it a hardware or controller failure. It cannot restore corrupted or deleted files. Only a second copy of files on another physical device is a backup.

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  • 1 year later...
On 10/18/2017 at 11:09 PM, SSD said:

I am doing parity checks every 2-3 months.


Always do one before and after any disk / hardware activity.


Between parity checks, I do md5 verification runs, which serve a similar purpose to parity check, but they check for file corruption instead of parity corruption.

@SSD How did you setup the md5 verification runs? I looked around but I could not find where to do that. Any help would be great.

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My server used to take about 8 hours with dual parity 4Tb drives. It's now about 12 hours for dual parity 6Tb drives. I'm assuming the amount of data disks has not much bearing on how long the process will take as the size of the parity disks is the main factor?

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