How much disks or size the parity disks can protect?

[Ronan C]

Hello everyone!

 

I migrated to Unraid few months ago and i am very happy! works like a charm!

 

Yesterday i put a new disk as parity 2 causes when i setup the server i just put one disk for parity.

 

Now this is my config:

Parity 1 and 2 = 2X Seagate 4 TB 

3x Seagate 4 TB 

With a grand total of 12 TB of storage.

 

My question is, how much space are protected with 2 parity disks, its a little confuse i researched a lot, few people say with this setup i can lost one disk, others says 2, finally, how this works? for instance, the parity/protection is sticked by the size of the parity disk? for example, if i have one parity disk of 20 TB all my 12TB of storage are protected? or, if i have 2 parity disks of 6 TB all my 12TB of data are proected?

 

Thanks in advance!!!

 

[Squid]

It's not "space" that's protected by parity drives.  It's "disks" that are protected.

 

One parity disk can protect against a single data drive failure

Two parity disks can protect against two data drive failures.

 

If by some weird chance you have more drives fail than you have parity disks, then data loss will result.

 

However unlike a traditional RAID system with unraid you will only ever lose a portion of your data.  With a traditional RAID system you will have lost 100% of your data when you exceed your redundancy.

[itimpi]

The space that is protected is always the sum of the data drives regardless of how many data drives (a max of 28 with a Pro licence).    With a single parity drive you can handle 1 drive failing, whereas with 2 parity drives you can handle 2 drives failing simultaneously.

 

Note, however, that this is just a protection against drive failure (think of it as high availability).   It is not protection against other types of failure so you should still have a good backup strategy for any data that is important to you.

[Ronan C]

So, i think the best scenario for use unraid its only 4 disks, avoid increase the array with many disks... causes as many disks you have as many failures you can have.

 

For instance:

2x 12TB disks for parity

2x 12TB disks for files

 

I think this is the best way to keep the things under control, if you need more space, change the disks sizes one by one example from 12 TB to 20TB...

 

Thanks in advance

 

Ronan

[remotevisitor]

As has already been mentioned, even with such a setup you need to backup important/irreplaceable files to an external drive.

 

For instance in this setup if something causes file system corruption on a disk, it is quite possible (depending on what caused it) for this to be corruption to be reflected in an emulated drive (because parity was also updated with the change which caused the corruption).

 

The other common way of losing data is users accidentally deleting files/folders they didn’t mean to delete.   No number of parity disks is going to protect you from this.

 

If I had this system, rather than allocating a 2nd parity drive I would instead use that drive to maintain external backups of stuff I could not afford to lose.   But maybe you are already doing backups.

 

[Ronan C]

yes i did, large disks is very expensive in Brazil...

[MAM59]

1 hour ago, Ronan C said:

For instance:

2x 12TB disks for parity

2x 12TB disks for files

This is a clear overshooting the needs. With this setup you could simply use 2 raid-1 (mirror) pools. This makes no sense.

Even 2 parity drives are too much for less than (lets say) 8 data drives.

Remember, we speak of "simultanious" failures. How many drive failures did you have in the last few years?

Using 1 parity drive and 3 data drives is "safe" enough I think.

 

[Hoopster]

5 hours ago, MAM59 said:

Even 2 parity drives are too much for less than (lets say) 8 data drives.

@Ronan C I agree with this.  You really do not even need to consider dual parity until at least 8 data drives, unless of course you really want dual-parity protection with fewer drives.  I have 5 data drives with single parity and have never had an issue with a drive failing.  I did have one get disabled because of a write error but I was able to rebuild it onto itself.  It was a cabling problem.

 

Use one of your 12TB drive for parity and make the other three array data drives.

Edited December 2, 2022 by Hoopster

[Ronan C]

Thanks guys, i appreciate all opnions here!

 

But the point is: Last year i bought 3 seagate drives from a major merchant in Brazil, all of these 3 was bad, not working or with noise, i think the quality of drives can be a problem, so, for this i think 2 parity drives (tested ones) can save the day...

 

Big up

Ronan 

[JonathanM]

3 hours ago, Ronan C said:

Last year i bought 3 seagate drives from a major merchant in Brazil, all of these 3 was bad, not working or with noise, i think the quality of drives can be a problem

If they came in the same package it probably was used as a world cup ball substitute before it got to you.

 

Never buy more than 1 drive at a time if you can help it. The cost savings isn't worth the risk.