Re-Building larger drive. general interest question

[TheFloyd]

So decided to increase a 2TB drive in my array, with a 6TB drive and followed the normal process.  interestingly, this little drive had been evacuated for the most part and actually only had about 130GB of data on it.   The re-build process started and has generally averaged about 130MB/Sec in terms of re-build speed.   It got me thinking though...   I had no expectation that it would only rebuild the 130GB of data and call it done, as I assumed that fragments of information may be scattered in a non-contiguous way, all over the array.   however, after 6 hours, it has now rebuilt 2..4TB of the previous 2TB of HDD, onto the new 6TB drive.   if you look at the dashboard, you can see that all 130GB of data is now listed as "used", which would imply to me that the data on the disk has been re-constructed in full.   so my next thought was that this was some sort of initialization process for the remaining 4TB of the 6TB drive, as part of the expansion of the array.   but this is the bit i dont get...   on the main UI, i can see that it is full-time reading from all of the other drives in the array, and full-time writing to the new 6TB drive.   I get the writing bit, but what the heck is it reading from the parity and other three drives, when the array only ever knew this at a 2TB disk and it has already "written" 2.4TB to this new 6 TB drive ?

 

i would expect that like me, everyone will have theories about this operation, but i am hoping for general interests sake, that someone knows the exact reason and could enlighten me on this?    it still thinks it has 10.5 hours or rebuilding to do...   whaaaa?

[Encore]

 

i dont find the word in english, so i link this video  maybe after this all is clear

 

the parity needs to check /rebuild each sector of the whole disk as i know

[TheFloyd]

Hi Encore,   At first when i saw your response, i must admit that i thought it was a bit dismissive, because i asked a pretty specific question and wanted something a little better than to just be told to go and continue researching it for my self.   I have been watching spaceinvader one for years, and he is absolutely fantastic.

 

HOWEVER...   you were absolutely right.  I cant believe that this video was released two days ago, and i had not got around to watching it yet, when it absolutely explains what i wanted to know.   I completely understand what my server is doing now, as this larger volume is being integrated into the parity-protected array.

 

Thanks for flagging this video quicker than my youtube app did