Short story time: I’m in the process of backup/destroy/recreate my main storage pool. I’ve reached the point where I’m creating a 12 wide RaidZ2 vdev of 16TB drives (14.55 TiB). I’m also including a 3 way mirror svdev (2TB NVMe drives). When the pool is created, it is reporting a usable storage am…

I hate how ZFS gaslights you. How is one supposed to know how much capacity they truly have? 176TiB? 144TiB? 132TiB? My guess is that you have 144TiB usable capacity, but the TrueNAS GUI (and zfs command) wrongly report 132TiB, due to the calcuation being fixed on a 128K recordsize, even though yo…

[image] Krill: Does the GUI ever update to be accurate? I don’t remember where I read this, and I forget if it applies to TrueNAS specifically or ZFS in general, but the usable storage calculation for RAIDZ is always based on 128KB recordsize, no matter what you might actually be using. You’l…

So this would mean that the display widgets, and Truenas reporting, are always wrong in relation to RaidZ storage, because of record sizes, not considering svdev space, and raidz expansion miscalculations. Fantastic. Still doesn’t mean mirrors are better though, but the gap is closing.

[image] Krill: Still doesn’t mean mirrors are better though The philosopher of these forums, @etorix , would respectfully disagree with you.

Just to confirm, you do see a more accurate “available space” with zpool list mypool?

Output of sudo zpool list mypool: NAME SIZE ALLOC FREE CKPOINT EXPANDSZ FRAG CAP DEDUP HEALTH ALTROOT mypool 176T 4.05T 172T - - 0% 2% 1.00x ONLINE /mnt

Cheers, I agree with you in the main. Time to go down the rabbit hole and see if there are alternatives that make sense with record size etc but I suspect that it’s simply the case of “Don’t worry about it, just add more vdevs at 80% capacity”.

I found this , written by an engineer at iXsystems: For some reason, I remember reading about this from Matt Ahrens or on FreeBSD.

Storage tab - usable capacity query

TrueNAS General

winnielinnie September 11, 2025, 6:11pm 4

The philosopher of these forums, @etorix, would respectfully disagree with you.

Topic		Replies	Views
Pool replace/expand and Record Size Change with Decreased Capacity TrueNAS General ZFS	10	519	July 10, 2024
Upgrading VDEV with Larger Disks TrueNAS General SCALE , ZFS , RAID-Zx-expansion	12	1090	January 27, 2025
ZFS Pool Extend Weird Behavior TrueNAS General SCALE , ZFS	13	616	November 26, 2024
RAIDZ1 pool, 3x 20TB, only 18 TB available, wrong capacities TrueNAS General SCALE , ZFS , RAID-Zx-expansion	2	436	November 15, 2024
A funny thing happened when extending a pool TrueNAS General SCALE , ZFS , RAID-Zx-expansion	6	245	November 6, 2024

Storage tab - usable capacity query

Related topics