can not add hard disk: out of space(500)

[aaronzhao]

HI totally newbie here. I have been trying to add a hard disk out of a ZFSPool to a vm which is running TrueNAS. The ZFS pool consists of 4 * 8T hard disks with RAID-Z1.
However, I ran into 'out of space' error when trying to create the hard drive like below

The capacity and available capacity both shown as 23.12TB,

• I don't understand why the actual capacity is way smaller? (only succeed when set the disk size around 13TB, anything above that will fail)
• is this the best way to add storage to TrueNAS? or for example, I should pass the hard disk to TrueNAS directly?

Thanks in advance.

 

[Dunuin]

There is padding overhead. Of your 32TB of raw capacity you lose 25% because of parity data. Of the remaining 24TB you lose 33% because of padding overhead (when using 4 disk raidz1 with ashift=12 and default 8K volblocksize) so only 16TB left. And a ZFS pool should always have 20% of free space left. So only 12.8TB of real usable capacity. And TrueNAS only supports ZFS. When running ZFS on top of ZFS you again need to keep 20% of the pool empty. So you lose again 20% of that, so in the end only 10.24TB effectivly usable to store files. And really terrible performance because of the multiplying overhead when running ZFS on top of ZFS.

Better to buy an LSI HBA, connect the 4 HDDs to it, use PCI passthrough to passthrough the whole HBA with all disks into that TrueNAS VM and then directly create the ZFS pool inside the VM. When just using datasets then you at least would get 19.2 TB of usable capacity with the same raidz1 and disks. If your hardware doesn't support PCI passthrough, have a look at disk passthrough as an alternative:https://pve.proxmox.com/wiki/Passthrough_Physical_Disk_to_Virtual_Machine_(VM)


See here to learn more about the padding overhead:
https://www.delphix.com/blog/delphi...or-how-i-learned-stop-worrying-and-love-raidz