Parity check speed/behaviour question

[Kaldek]

Hi folks, I am hoping someone to clarify the behaviour of my parity check behaviour.  My setup is as follows:

• Parity: WD 8TB CMR
• Disks 1-3: WD 4TB CMR
  • Most shares.  All shares use Disks 1-3 only and Disk 4 is excluded for these shares.
• Disk 4: Seagate 8TB SMR
  • Video Share (only the Video share, no other shares.  Disks 1-3 are excluded for this share)

I split Disk 4 off for Videos because it's slower and I don't mind if this disk is the one which is mostly spun up (the other shares aren't used as much) because it reduces power consumption by only having one disk spinning.

 

Anyway when the parity check is running it starts out like the first image here where all disks are being checked:

 

Then it changes to the following behaviour for the last 40% or so where only the 8TB disk is being checked:

 

I'm sure this is normal, but why?

Edited February 1, 2021 by Kaldek

[Vr2Io]

Because 60% progress ( 60% of 8TB ) already over 4TB, so disk 1,2,3 wouldn't involved anymore.

[itimpi]

 

There are 2 things that affect speed:

1. the slowest disk in those currently being actively checked
2. disks slow down as they move from outer to inner tracks

It is therefore normal to see the speed gradually slowing due to #2 and then perhaps speed up again due to #1 as the check gets beyond the end of some of the smaller (typically slower) disks.

[Kaldek]

19 hours ago, Vr2Io said:

Because 60% progress ( 60% of 8TB ) already over 4TB, so disk 1,2,3 wouldn't involved anymore.

It took me a while to wrap my head around that but I think I get it.  What I think you're saying is:

• All disks are involved in parity calculations for the first 4TB
• Only the parity drive and 8TB drive are involved in parity calculations for the last 4TB.

Does that therefore mean that for the last 4TB of the 8TB drive, it's not technically parity but rather a bit mirror?

And that if I added another 8TB drive to my array it would switch from a bit mirror to actual parity?

 

I think I might be mentally barking up the wrong tree here

[Vr2Io]

8 minutes ago, Kaldek said:

• All disks are involved in parity calculations for the first 4TB
• Only the parity drive and 8TB drive are involved in parity calculations for the last 4TB.

Correct

 

9 minutes ago, Kaldek said:

rather a bit mirror?

No, it will be parity of last 4TB of disk4, if you add a 8TB disk5, then first 4TB was parity of disk 1-5 and last 4TB was disk 4,5.

[Kaldek]

20 minutes ago, Vr2Io said:

No, it will be parity of last 4TB of disk4, if you add a 8TB disk5, then first 4TB was parity of disk 1-5 and last 4TB was disk 4,5.

OK, yeah I think I'm just lacking in caffeine this morning.  It's no different in functionality to if I had a two drive array (one parity, one data).  It's still "parity" even though it might be conceptually similar to RAID1 (in that only two disks are taking part in the resiliency for this particular data).

 

I don't think I'd ever recommend that anyone visualised it that way.  It's just helped me wrap my head around what I saw.

 

 

Edited February 2, 2021 by Kaldek