Parity Disks

[SH4D0WFR0G6996]

Hello, I am going to be setting up a new unraid server here soon and racking my brain trying to think how many parity disks I really need.

I am going to be running 8 disks with 2 cache drives not sure exactly what my pools are going to look like yet but I was thinking of running 2 parity disks but I don't know if that is overkill what do you guys think should I have more space or more redundancy.

[trurl]

Do you mean 8 data disks, or 8 array disks total?

[trurl]

1 minute ago, trurl said:

8 array disks total

By which I mean data disks plus parity disks, cache is not part of the array

[SH4D0WFR0G6996]

8 disks total and yes I know cache doesn't count just felt I should add.

[trurl]

You could start with 6 data +  2 parity until you have more experience. Then later you could convert parity2 to data to get more capacity after you are confident you know how to work with your hardware and with Unraid.

 

If you have a lot more data disks then I would say you should have dual parity, but single parity is probably OK for as many disks as you are thinking about, as long as you are diligent (setup Notifications to alert you immediately by email or other agent as soon as a problem is detected) and careful (always test new disks before trusting them in your array, and always double check all connections when mucking about inside).

[trurl]

Just now, trurl said:

always double check all connections

Bad connections are probably the #1 reason people come to the forum asking for help when a disk gets disabled (kicked out of the array because out-of-sync).

 

[SH4D0WFR0G6996]

Ok sounds like a good setup I didn't know I could convert a parity disk to data but I guess that makes sense cause it would just move its parity calculations to the other disk.

Now that you say that if I add a Jbod or disk shelf in the future would I be able to move all the data from one disk to another and then make it a parity disk? ( I would like to have the disks physically beside each other for maintenance and ocd purposes lol).

[JonathanM]

1 hour ago, SH4D0WFR0G6996 said:

cause it would just move its parity calculations to the other disk.

Nope. Each parity disk is different, and uses the remainder of the data disks to rebuild 1 bad data disk. So, 2 parity disks allows rebuilding 2 different data disks.

 

It's complicated, but just keep in mind parity doesn't hold any of your data, it only completes the math using all the rest of the data drives needed to reconstruct a single failed drive. The math used on parity1 slot is different than the math used for parity2.

[SH4D0WFR0G6996]

Sorry I don't think I was entirely clear I didn't mean parity data I just meant normal data like some windows files for example would I be able to move those files off a disk completely within unraid, then turn the disk that had the windows files on it into a parity drive? its not super important if not im just curios.

[JonathanM]

Removing a parity disk and assigning it as a new data drive is easy. Doing the opposite is way more complicated.

 

Yes, you can remove files from a disk, and use it elsewhere or as a parity drive, but removing a data drive means recalculating parity from scratch, or going through an involved process to keep parity valid before removal.

 

Adding a data drive to Unraid is easy, removal is more complicated.

[SH4D0WFR0G6996]

Ah I see then my ocd will have to get over it cause id rather not rewrite parity when adding more disks.

Thank you.

[JonathanM]

49 minutes ago, SH4D0WFR0G6996 said:

id rather not rewrite parity when adding more disks.

When you add a data disk, it will be cleared before it is added to the parity array so it doesn't alter parity until you format it and start adding data.

So, no rewriting parity required when ADDING disks.

 

Removing a data disk, that's when parity will be redone from scratch.