Why size available is small? 3TB x (3+1) RAIDZ

[moto]

Hi everyone,

I will use for the first time Freenas , but in trouble not know the correct information .

I used freenas 9.3. I've built a zpool(RAIDZ1) by 4drives (3 TB each).
I have created a disk pool and it shows 10.9TiB which is correct by my calcs.
But the RAIDZ dataset shows as 7.7TiB whereas I was expecting it to be closer to 8.2TiB.

Please tell me what reason is this difference(near 500GB) .
Also, what is a way to enable all of the area?

# zpool list
NAME SIZE ALLOC FREE EXPANDSZ FRAG CAP DEDUP HEALTH ALTROOT
zfs1 10.9T 5.80M 10.9T - 0% 0% 1.00x ONLINE /mnt

# df -h /mnt/zfs1
Filesystem Size Used Avail Capacity Mounted on
zfs1 7.7T 139k 7.7T 0% /mnt/zfs1

 

[Bidule0hm]

It's because of the overheads of ZFS and it's normal, you can't do anything to "correct it" :)

As an aside it's not recommended to use RAID-Z1 with drives of more than 3 TB (unless this is a backup server and you can tolerate a lower reliability), see the "don't use RAID-Z1" link in my signature to know why.

 

[cyberjock]

df and du are not appropriate tools for ZFS mounted partitions...

And if you understand RAIDZ1, you go from 12TB raw to 10.9T formatted, then lose approximately 1/4 because RAIDZ1 uses a disk for redundancy.

So nothing to see here. :P

 

[moto]

thanks for the reply.

I know that one drive used to parity by RAIDZ.
My calculation is as follows .

3TB disk = 3,000,000,000,000 bytes
3,000,000,000,000 bytes = 2.73 TiB
4 x 2.73TiB = 10.9TiB （It is disk pool size. It is correct.）

So , my expectation is
10.9TiB * (3/4) = 8.2TiB (My Expected dataset size)

But, dataset shows as 7.7TiB...

 

[DifferentStrokes]

You still have to factor in the overhead that @Bidule0hm was talking about.;)

 

[moto]

Thank you all!
It seems overhead is greater than I had thought .

 

[SweetAndLow]

If you are worried about 500GB you should also take into account that you shouldn't fill your pool past 80% full and never past 90%. So you get between 6.2TiB and 6.9TiB.