Correct quantity of Parity Drives to Use

[binar]

Fellow Forum Members,

My Unraid system is still in the planning phase. Can anyone please clarify the following for me. What is the correct amount of data parity hard drives one needs to use to protect against hard drive failures in a hypothetical Unraid system using five 14TB hard drives to store data in? Is using one additional hard drive adequate? Or does it require using  two additional hard drives? I'm asking because I don't know whether I need to buy 6 or 7 hard drives all together if my goal is to protect against drive failure in any of the 5 base hard drives making up my hypothetical Unraid system. 

 

Also, hypothetically if the 2019 Unraid configuration shown below with five base hard drives changes in 2022 to seven base hard drives, does the two parity hard drive count remain the same at two drives?  Or does one need to bump it up to three parity hard drives?   In other words, is the correct total drive count in the modified 2022 Unraid configuration need to be 9 drives or 10 drives or will 8 drives be adequate?

 

2019 Unraid Configuration:

Five 14TB base hard drives

Two parity hard drives to protect against drive failures

Total 14tb drives used = 7

 

2022 Modified Unraid Configuration:

Seven 14TB base hard drives

Two parity hard drives to protect against drive failures

Total 14tb drives used = 9

 

Mainly, I'm trying to learn how the parity drive count changes as time goes on and more hard drives are added to an Unraid configuration. 

 

Thanks in advance for any help.

[itimpi]

Each parity drive protects against the failure of any one data drive regardless of the number of data drives.   Whether you have single or dual parity is up to you.   There is no option for triple parity in the array - it is either none, single or dual.

[binar]

28 minutes ago, itimpi said:

Each parity drive protects against the failure of any one data drive regardless of the number of data drives.   Whether you have single or dual parity is up to you.   There is no option for triple parity in the array - it is either none, single or dual.

Thanks for your post Itimpi.  So in 2022 when I add two more 14tb hard drives will I have to erase the two parity drives and rebuild them so the Unraid system includes the addition of the two new 14TB hard drives with failure protection?  Or do I just leave the two parity hard drives alone and Unraid will automatically configure the two newly added 14TB hard drives so the data populated to them is also protected against drive failure? 

[itimpi]

1 minute ago, binar said:

Thanks for your post Itimpi.  So in 2022 when I add two more 14tb hard drives will I have to erase the two parity drives and rebuild them so the Unraid system includes the addition of the two new 14TB hard drives with failure protection?  Or do I just leave the two parity hard drives alone and Unraid will automatically configure the two newly added 14TB hard drives so the data populated to them is also protected against drive failure? 

This.

 

You can add drives of any size (as long as they are not larger than any of the parity drives) at any time you like and Unraid automatically adds them into the array protected by parity.

[binar]

Itimpi,

Thank you for your post.  I hope you  or anybody in this forum can clarify the following.  I am planning to buy three 10TB hard drives to start the data migration task. I am not 100% certain if that is going to be enough space. It may turn out  I need to buy 1 or 2 additional hard drives.

 

Long story short, is UnRaid flexible enough to allow me to setup the two Parity Backup drives  6 months after I complete the data migration task and sort out the actual drive quantities I need?  Also, I'm planning to buy two bigger sized 16TB hard drives to use as parity backup drives.  Therefore, will UnRaid operate  correctly if my configuration consists of a mishmash of hard drive capacities?  The list below is a possible outcome of my UnRaid hard drive inventory:

• Three 10TB hard drives
• One 12TB hard drive
• One 5TB hard drive
• Two 8TB hard drives
• Two 16TB Parity Backup hard drives I plan to buy sometime in 2020 when prices come down. (Will UnRaid allow me to setup these two Parity drives six months after I complete the data migration task?)

Lastly, is UnRaid going to like such a mishmash of drive capacities? Thank you very much in advance for any opinions.

 

[BRiT]

Unraid only has the limitations of the Parity Drive(s) must be as large or larger than any individual data drive.

 

Parity checks and parity rebuild speeds are limited by the slowest drive(s) involved.

 

Other thing to note is Parity is NOT Backup. Parity is protection from drive loss, but is not backups. If you delete a file on a data drive, the parity drive(s) will be updated in real time so you can not restore the file.

[binar]

Brit,

Thank you for your post. I learned something new. I was not aware that Parity checks and parity rebuild speeds are limited by the slowest drive involved. Therefore, it may not be a good idea I add an old 5TB hard drive to my configuration because that drive is old and very slow compared to the new drives I'm going to buy. In short, the last thing I want to do is sacrifice the write speed on my new drives all because I have included an old seven year old hard drive into the mishmash.

 

Parity drive providing protection against drive failure but not accidental data deletion is something I was already aware of. 

 

However, can anyone please clarify if setting up two Parity Drives can be done six months after the data migration task is done?  Or does UnRaid require the Parity Drives  be setup at the same time the migration task is being performed?  Thanks in advance for any help.

[BRiT]

Writes to the array are only limited by the slowest of the drives involved, with typical setting for write method of read/modify/write then only the single Data drive being written to and the Parity drives are involved.

 

If you have write method set as reconstruct writes, then all data drives are involved. This is also called Turbo Writes by some, since it can be much faster since it doesnt involve having to do a read-and-write in the data drive, just a write while the other drives do a read and the parity drives do a modify-write

 

Parity can be added later. Same with a second Parity drive.

 

The first time a Parity Drive is added, is when parity is calculated, that is the "plarity build" and it involves all drives. Then when you are validating the parity information it is called "parity check" and it involves all drives. If you ever have to rebuild a drive, that is "parity rebuild" or "drive rebuild" and it involves all other drives to recalculate what the replacement drive should contain.

[BRiT]

For more details on Parity methods, here's a good starting point to read:

 

 

[Goodboy428]

To kind of add to this conversation, does the parity drive combination need to be equal to the total storage of the array? I’m really familiar with regulars raid configurations and currently run a dell PowerEdge server in my house for use with plex, but am looking to swap over to unraid because of the flexibility of adding drives in the future. 
For configuration sake, I’m planning for an 8tb parity and 1x4tb, 3x2tb for storage which allows for 10tb (or thereabouts) storage capacity initially. Will the 8tb be sufficient parity? Or should it really be 10tb to equal storage capacity? I will replace 2tb drives with 4 or 8 Tb drives in the future to expand storage capacity. 

[BRiT]

Parity 1 >= max size of Data Drive(s)

Parity 2 >= max size of Data Drive(s)

 

Parity drive(s) must be greater than or equal to the max size of any data drive.

 

In @Goodboy428 situation, you can simply use a 4TB or larger for Parity since your max data drive is 4TB.

 

Edited December 18, 2019 by BRiT

[trurl]

14 minutes ago, Goodboy428 said:

To kind of add to this conversation, does the parity drive combination need to be equal to the total storage of the array?

No, each parity disk (1 or 2) must be at least as large as any single data disk. As mentioned parity is not a backup so why would you think it needed to be as large as the total capacity?

 

To add to what has already been said about parity not being a backup. Parity doesn't contain any of your data. Parity by itself cannot rebuild anything. Parity just contains parity bits that, together with the corresponding bits on ALL other disks, allows the data for a missing disk to be calculated.

 

Here is the wiki on parity:

 

https://wiki.unraid.net/UnRAID_6/Overview#Parity-Protected_Array

 

If you understand parity a lot of things about how Unraid works with the disks, and how you work with Unraid to manage the disks, makes a lot more sense.

[Goodboy428]

@trurl

No, each parity disk (1 or 2) must be at least as large as any single data disk. As mentioned parity is not a backup so why would you think it needed to be as large as the total capacity?

Yes, I understand its not a backup, I just wasn't sure if the Parity group needed to be equal to the total of the data array (and now that I type that out, it doesn't make sense in my head either)....  Thanks for link, it clarified/reminded me of how parity works.  I've forgotten that over the years...

 

I posted on another post about using rsync to back up data to en external drive for backup in case of loss.  I'm currently doing that with a batch file on a windows server.  Just running out of space there and evaluating other, growable, options instead of being locked down to a raid 5 array (which is what I have now).  

 

@BRiT I'm planning to put an 8tb in as parity so I can slowly swap the 2tb drives in the data array out with 8tb drives.