ZFS mirror capacity

[bobbus]

Hi,

I'm going for a new small NAS setup with 2 mirrored data disks of 1,5TB.
Looking at FreeNAS in a VM guest (to test with) I found that the capacity of a ZFS mirror is only half of what you would expect, meaning only 750GB instead of 1,5TB.

When you create a zpool from CLI in FreeNAS, with:
# zpool create -m /mnt/data data mirror ada1 ada2
then you do get 1,5TB of usable capacity on the mirror.

Where does FreeNAS use the extra (50%) capacity for? Snapshots? How do you bypass this behavior? Creating ZFS pools from CLI does not reflect in the GUI, so they're not usable...

Thanks for your help!
Bob

 

[matthewowen01]

the available space will be the size of your smallest drive in the mirror. 2, 1.5 TB drives will make a 1.5 TB mirror. a mirror with a 1 T drive and a 1.5 TB drive will make a 1 T mirror.

FreeNAS reserves 2 GB on each drive for a pagefile... did you create your VM with 4 GB disks perhaps?

 

[bobbus]

Thanks for your reply!
That makes sense, I tested with 4GB virtual disks indeed.
Thanks again!