Hey, 
I have 4 identical disks and 2 set for parity. 

 

I've been looking around and documenting myself about parity but there's something I wasn't able to find... maybe its too obvious but I need to get my mind around it. So the questions are:

1. If you do a parity check today and lose a drive a month later, then the drive restored will have today's data, so you can only "restore" to the last time you did the parity check, is this correct?

2. As I've seen, parity must be always build from start to beginning, with my setup this can take around 20 hours. Can't parity be updated on write? 

 

Thanks

Edited January 14, 20233 yr by Axmed
Readed more about it

sounds as if you have misunderstood how parity works on Unraid.
 

1.).  No.   Parity is updated in real-time so after a drive failure you are recovering to the the current data state.

 

2).  It is!    Updating the parity on all writes to the array is the main limiting factor on speeds writing to the array. The parity check process is just an (optional) house-keeping one to check that everything is as it should be.

 

53 minutes ago, Axmed said:

As I've seen, parity must be always build from start to beginning, with my setup this can take around 20 hours. Can't parity be updated on write? 

since parity always needs to be up to date, it has to be up-to-date from the start as well. otherwise it cannot be consistently updated. that is your 20 hour tax

See this for how parity works, https://wiki.unraid.net/Parity#How_parity_works

 

https://wiki.unraid.net/Manual/Overview#Parity-Protected_Array

1 hour ago, apandey said:

 

since parity always needs to be up to date, it has to be up-to-date from the start as well. otherwise it cannot be consistently updated. that is your 20 hour tax

See this for how parity works, https://wiki.unraid.net/Parity#How_parity_works

 

Oh ok, so the very first parity check is just to start the wheel. 
After that its live updated. So the following parity checks would in fact only identify the failures. 

Got it, many thanks