32TB Raw equals 20TB usable?

[lanlubber]

I have 8 4TB WD reds running in a RAIDZ2. After pool creation I only have 20TB usable? 12TB is a pretty big hit. Shouldn't it be closer to 8TB?

Thanks

Edit: So I found the calculator and apparently the numbers are correct, but damn that's a big hit. :(

 

[depasseg]

@Bidule0hm has a great ZFS space calculator. And look up the difference between TB and TiB.
https://forums.freenas.org/index.php?threads/zfs-raid-size-and-reliability-calculator.28191/
http://biduleohm.free.fr/zfsraidsarc/

 

[lanlubber]

Thanks... ouch :(

 

[HardChargin]

You're not alone, I recall coming to the same realization after investing in all those disks (8x4TB RAIDZ2). I had even done the calcs, but overhead somewhere in there put me 1-2 TB less than I had expected. Also, dont forget the 80% rule. Overall, Im still very happy with FreeNAS, FreeBSD, and ZFS managing my data. That and the amazing community/contributors here.

 

[depasseg]

A lot of that hit is due to the Marketing of "TB" 1 "TeraByte" (TB) is 0.9095" TibiBytes"(TiB) so when you buy a drive, you are getting 10% less than you think you bought. And then add in the metadata overhead and the suggestion to stay below 80% full for performance reasons and you are where you are.

 

[lanlubber]

I know I'll be happy with this over buying a NAS appliance for sure.

I thought about that after I posted that the advertised drive capacity is always off... oh well... I'm sure the lz4 compression will help.

 

[jgreco]

Thanks... ouch :(

If it makes you feel any better... when using three way mirrors (for redundancy) our 52TB raw disk space filer here makes a pool of 16TB, of which 8TB can be used due to the 50% iSCSI rule.

Count your blessings.

 

[lanlubber]

If it makes you feel any better... when using three way mirrors (for redundancy) our 52TB raw disk space filer here makes a pool of 16TB, of which 8TB can be used due to the 50% iSCSI rule.

Count your blessings. :)

Wow...