I have a FreeNAS server with 3 x 2 To disks in raidz1. I would expect to have about 4 To of space available. When I run zpool list I get:

[root@freenas] ~# zpool list
NAME          SIZE   USED  AVAIL    CAP  HEALTH  ALTROOT
main_volume  5.44T  3.95T  1.49T    72%  ONLINE  /mnt

I was expecting a size of 4 To.

Also, used space as reported by zpool list does not match what's reported by du:

[root@freenas] ~# du -sh /mnt/main_volume/
2.6T    /mnt/main_volume/

There are quite a few things that I dont yet completely understand about ZFS. But at the moment I am mostly worried that I misconfigured my system and that I dont have any storage redundancy. How can I make sure I did not do an horrible mistake ...

For the sake of completeness, here is the output of zpool status:

[root@freenas] ~# zpool status
  pool: main_volume
 state: ONLINE
 scrub: none requested
config:

    NAME                                            STATE     READ WRITE CKSUM
    main_volume                                     ONLINE       0     0     0
      raidz1                                        ONLINE       0     0     0
        gptid/d8584e45-5b8a-11d9-b9ea-5404a6630115  ONLINE       0     0     0
        gptid/d8f7df30-5b8a-11d9-b9ea-5404a6630115  ONLINE       0     0     0
        gptid/d9877cc3-5b8a-11d9-b9ea-5404a6630115  ONLINE       0     0     0

errors: No known data errors