[6.11.5] Quick question about dual parity

[ArdNsc]

I just want to double check if I am doing everything right.

 

What I want do achive: I have an array with single parity (10TB). I got a new drive (12TB) recently and I want to end up with an array with single parity (the new 12TB drive), containing the ex-parity drive (10TB) as an additional data disk.

 

I dont want to lose parity during any step of the process, so removing the parity device from the array and rebuilding it on the new drive was no option for me. Instead I planned the following steps:

 

1. Preclear the new 12TB drive (to test if it's good, succeeded)

2. Add the 12TB drive as Parity 2 and build parity on it (currently running, around 50% done)

3. Removing "Parity" 1 device (10TB drive)

4. Assign the old parity drive (10TB) as a new data drive, clear it.

5. Ending up with 10TB additional storage space in the array - and single parity.

 

Currently I am in the middle of step 2. I am getting some mixed feelings because the nomenclatura/terms/descriptions of the UI do not seem to be totally idiot proof/consistant; at least not to me: On the Main page under "Parity" it says "Data-Rebuild in progress... Completed: xx%". (I am not rebuilding any data? I am building up a second parity?) Under "Array" I have a green dot next to the "Parity" device and a yellow dot ("invalid") next to the "Parity 2" device. In The array Tab under "Array Devices" I have a green dot next to "Parity" and a yellow triangle next to "Parity 2" stating: "Parity is invalid". (All devices exept "Parity 2" are reading; "Parity 2" is writing. Seems right to me. The parity is considered "invalid" because it's still "building up"?) Under array operation I read: "Parity-Sync in progress." I am assuming that I am still having single parity at the moment and that I am currently building up dual parity. Right?

 

Also I want to make sure that it is okay to perform Step 3: When I remove Parity 1 and start the array, will "Parity 2" automatically become "Parity" (1) without the need to rebuild the parity again? This is what I am assuming.

 

And: When perfoming step 4 (adding new data drive) I also do not have to rebuild parity, it will remain valid. Right?

 

Tl;dr: Can I switch from dual parity to single parity just by removing the "Parity" device and keeping the "Parity 2" device? Will it become "Parity" (1) then?

 

Sorry for the redundant questions, I just don't want to mess things up.

 

[itimpi]

4 minutes ago, ArdNsc said:

Also I want to make sure that it is okay to perform Step 3: When I remove Parity 1 and start the array, will "Parity 2" automatically become "Parity" (1) without the need to rebuild the parity again? This is what I am assuming.

Not quite - parity2 will remain as parity2 and you will have no parity1.   The calculations for the two parity drives are completely different so they are not interchangeable.   It is perfectly valid to run without parity1 so in that sense parity2 remains valid.

 

regarding step 4 then yes parity remains valid.

 

[ArdNsc]

Okay. Thanks for clearing that up.

 

And in case I ever decide to switch to dual parity I just build up a (new) "Parity" (1) and keep the "Parity 2", right?

[itimpi]

17 minutes ago, ArdNsc said:

And in case I ever decide to switch to dual parity I just build up a (new) "Parity" (1) and keep the "Parity 2", right?

Yes

[JonathanM]

1 hour ago, ArdNsc said:

Can I switch from dual parity to single parity just by removing the "Parity" device and keeping the "Parity 2" device? Will it become "Parity" (1) then?

Just a quick note to finish out the differences, parity1 is mathematically simpler, and also doesn't require the disk slot numbers to be the same to stay valid. For example, with valid parity1, you can rearrange any data drives you wish to different slot numbers, like swapping disk1 and disk4, and as long as all the disks are present and none added, parity1 remains valid.

 

Parity2 on the other hand is a much nastier set of equations, and one of the factors of the parity2 math is the slot number of the drive. So all disks must remain in the same slots for it to remain valid, and it requires more CPU power to calculate. That also can cause a difference in write speed, depending on available CPU power. Recent CPU's you really can't even tell the difference, but on older CPU's with limited math power it can be quite noticeable.

[peterg23]

Quote

Parity2 on the other hand is a much nastier set of equations, and one of the factors of the parity2 math is the slot number of the drive. So all disks must remain in the same slots for it to remain valid, and it requires more CPU power to calculate. That also can cause a difference in write speed, depending on available CPU power. Recent CPU's you really can't even tell the difference, but on older CPU's with limited math power it can be quite noticeable.

 

✔️ Now that is something I did not know - thanks for the info, not that I need dual parity at the moment, but it's good to know when I do.

Edited January 16, 2023 by peterg23