Is my parity OK?

[huladaddy]

I just recently built my unRaid machine. I started with one parity drive and two data drives. Because I was new to all of this, I wasn't really sure what I was doing, but proceeded methodically and slowly. I seem to recall only having to wait once for a very long time while unRaid did its thing. Looking back I assume it was when it was clearing. But how come I only remember one such long wait. Why not a second time while it cleared the second data drive? Did it clear both at the same time?

 

Finally, regardless of what happened originally, is my array healthy? I have since checked the parity, and it found no errors. Am I good to go?

[itimpi]

I just recently built my unRaid machine. I started with one parity drive and two data drives. Because I was new to all of this, I wasn't really sure what I was doing, but proceeded methodically and slowly. I seem to recall only having to wait once for a very long time while unRaid did its thing. Looking back I assume it was when it was clearing. But how come I only remember one such long wait. Why not a second time while it cleared the second data drive? Did it clear both at the same time?

 

Finally, regardless of what happened originally, is my array healthy? I have since checked the parity, and it found no errors. Am I good to go?

On an initial setup unRAID would not have to clear any drives.  A clear will only happen if adding a new data disk (that has not been precleared) to an existing parity protected array.  Perhaps it was the original build of parity that you were thinking of?  Or did you start the array and generate parity with only one data disk, and then add the second data disk after generating parity?

[huladaddy]

Ah, it must have been when it was building the parity. Thanks. I feel better now...

 

What does the clearing actually do then? My guess would be there are two benefits:

1. You get to exercise the drive by writing/reading to/from every sector on the drive.

2. By writing zeros, once done clearing, the parity doesn't need to be changed because of the zero-state of the drive.

 

Is that essentially it?

[itimpi]

Ah, it must have been when it was building the parity. Thanks. I feel better now...

 

What does the clearing actually do then? My guess would be there are two benefits:

1. You get to exercise the drive by writing/reading to/from every sector on the drive.

2. By writing zeros, once done clearing, the parity doesn't need to be changed because of the zero-state of the drive.

 

Is that essentially it?

the clearing process is to satisfy 2).  Note that no check is made during the unRAID clear process that the write of zeroes was done perfectly.  Also note that the array is not available for use while the clear happens.

 

The preclear plugin does more than this:

- allows the clear to be done in parallel with the array being available for use.

- checks that the writing of zeroes was done without error

- adds random head movements to better simulate real world use of the disk.

- gives a report on how successful it was and also if any critical SMART attributes changed.

- writes a signature to the drive that allow unRAID to recognise the drive has been cleared.

 

Adding a precleared disk only requires the array to be offline for a few seconds rather than the hours if you let unRAID carry out the clear process.  As it also provides a good confidence check on whether the drive is in a state to be used by unRAID so most experienced users run preclear against any new disk.

 

[huladaddy]

Thanks for the informative reply. I just read up on Preclear tonight as it happens and am in the process of prepping 3 drives... while my array is still up. Nice!