Usable spaces - asked a million times, but I' missing someting

[RichR]

FreeNAS 8.0.3, RAIDZ2
in experimenting, I have 2 volumes:
7 x 3TB drives - all 7 for data, no spare
8 x 3TB drives - all 8 for data, no spare

going by the "standard" to estimate usable space....
7(drives) -2(parity) * 3TB
15TB

8(drives) -2(parity) * 3TB
18TB

[root@pod2] ~# zfs list
NAME USED AVAIL REFER MOUNTPOINT
Store4 11.8M 12.5T 11.3M /mnt/Store4
Store5 11.9M 15.2T 11.3M /mnt/Store5


as you can see, I'm losing more than I expected. The volumes are empty but have about 12MB overhead (I guess that what it is). I'm losing approximately 2.5TB on Store4, and 2.8TB on Store5 -- nearly another entire drive.

Mathematically, after accounting for the 2 parity drives, usable is 83.33% (Store4), and 84.44% (Store5).

Is the loss of this much space (after parity drives) an expected behavior?

Thanks

 

[survive]

Hi RichR,

I'm not sure what the exact math is to figure out what you should expect your pool capacity to be but here are a few of the hidden drive space "taxes" you are possibly seeing:

The usable space of a 3TB drive (for a Seagate Barracudda XT) should be about 2794.52GB, so I think you are already down 2.2TB on your 11 data drives just from that. Then FreeBSD should be hiding\reserving another 10% of the total for itself for system "stuff" and to keep the user from running themselves out of space.

-Will