SOLVED 8x4TB RAIDZ2 = 20TB instead of 24TB

[f4242]

Hello,

I just created my first ZFS pool with FreeNAS. The pool contains 8 hard drives of 4TB in RAIDZ2. RAIDZ2 takes 2 parity disks, right? I assumed I would get 24TB usable [(8-2)*4TB], but FreeNAS UI tell me I only get 20TB. What I missed?

 

[sesipod]

I have 8x3TB Z1 and get 21.75 TB...
I'd like to know the answer to this again too but I think it's something to do with the the swap file per drive and extra space it reserves for operations.

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[f4242]

The UI use TiB, so this explain a part of the difference. 20 TiB = 22 TB. Still 2TB missing.

 

[sesipod]

Here is a good calculator that I use.

https://www.servethehome.com/raid-calculator/

Sent from my VS995 using Tapatalk

 

[sesipod]

The UI use TiB, so this explain a part of the difference. 20 TiB = 22 TB. Still 2TB missing.

I found this post on google. Kinda a step in the right direction. Lets see if someone else has a better idea ?

http://serverfault.com/questions/399137/raidz-in-freenas-eating-more-space-than-expected

 

[Vito Reiter]

Alright, well this is mostly expected when it comes to storage solutions. We've got to take into account a few things;

• The parity disks totaling 8TB
• True space that the disks can handle ~ < 4TB
• ZFS workspace

Your true RAW space is 32TB, minus the 8 is a true 24TB usable space. Now, if you're not new to the storage and hard drive world you'll probably realize that no drive on the planet has full usable space (1TB drives end up being like 931~GB usable) using that ratio you're going to be a little under 24TB. Then you have ZFS & Redundancy - ZFS stores data on the parity disks, but it also needs room to work with I/O's and space to know where all the data is. Since your data is spread out across 8 drives it needs room to say 'This data is here and here, but this other data is stored there and there' While a little complicated to understand, totaling 22TB is actually not too bad. That being said, although you don't have as much space as you want, your NAS most likely isn't broken, just functioning as normal.

Edit:

Here is a good calculator that I use.

https://www.servethehome.com/raid-calculator/

Sent from my VS995 using Tapatalk

Just saw that this was posted, pretty accurate as well and supports my claim. Anyway, hope our information helps you :)

Regards,
Vito Reiter

 

[melloa]

Or from @Bidule0hm: https://jsfiddle.net/Biduleohm/paq5u7z5/1/embedded/result/.

 

[f4242]

Thank you. This answer my question!