Sstrhsrtj

This is a question regarding the available space as reported by zfs list that does not meet user expectations. I have looked at similar questions, but I still don't understand the zfs list output. Let me explain why:

I have a simple setup with just 4x WD30EFRX disks. Each disk is reported by Linux to have the following capacity:

5860533168 512-byte logical blocks: (3.00 TB/2.72 TiB)

That is, each drive has a capacity of 3000592982016 bytes. According to the math, the aggregated raw space for the four drives is 3000592982016*4 = 12002371928064 bytes (10.9 TiB), and zpool list seems to agree:

NAME   SIZE  ALLOC   FREE  EXPANDSZ   FRAG    CAP  DEDUP  HEALTH ALTROOT

nas   10.9T  1.15M  10.9T         -     0%     0%  1.00x ONLINE  -

So good so far.
Now, I put those 4 drives into a RAIDZ1, meaning 25% of the total capacity should be used for parity. I would expect zfs list to report something close to 2.72 TiB *3 = 8.16 TiB. Instead, I'm getting only 7.65 TiB.

zfs list nas

NAME   USED  AVAIL  REFER  MOUNTPOINT

nas    558K  7.65T   140K  /nas



zfs list nas -p

NAME    USED          AVAIL   REFER  MOUNTPOINT

nas   571392  8416155617280  142848  /nas

I also expect ZFS to use up some space for metadata and its own internal data structures and whatnot, but I'm missing over half a Terabyte of data:

Available space according to zfs list -p is

(8416155617280/1024^3) GiB = 7838 GiB

Available space after parity (3 drives of 5860533168 512-byte sectors) should be close to

5860533168*512/(1024^3)*3 GiB = 8383 GiB

8383 GiB - 7838 GiB = 545 GiB

Over half a terabyte, close to 5% of the aggregated raw disk space, is unaccounted for. Where did all that space go?

Note: I'm running Debian Jessie, ZFS 0.6.5.2-2. I did check for the quota, reservation, refquota and refreservation properties, which are all set to none. I just created this pool and there is no data in it, and of course no snapshots.
Here's my full zfs properties:

zfs get all nas                                                                            

NAME  PROPERTY              VALUE                  SOURCE

nas   type                  filesystem             -

nas   creation              Tue Feb  7 17:28 2017  -

nas   used                  558K                   -

nas   available             7.65T                  -

nas   referenced            140K                   -

nas   compressratio         1.00x                  -

nas   mounted               yes                    -

nas   quota                 none                   default

nas   reservation           none                   default

nas   recordsize            128K                   default

nas   mountpoint            /nas                   default

nas   sharenfs              off                    default

nas   checksum              on                     default

nas   compression           off                    default

nas   atime                 on                     default

nas   devices               on                     default

nas   exec                  on                     default

nas   setuid                on                     default

nas   readonly              off                    default

nas   zoned                 off                    default

nas   snapdir               hidden                 default

nas   aclinherit            restricted             default

nas   canmount              on                     default

nas   xattr                 on                     default

nas   copies                1                      default

nas   version               5                      -

nas   utf8only              off                    -

nas   normalization         none                   -

nas   casesensitivity       sensitive              -

nas   vscan                 off                    default

nas   nbmand                off                    default

nas   sharesmb              off                    default

nas   refquota              none                   default

nas   refreservation        none                   default

nas   primarycache          all                    default

nas   secondarycache        all                    default

nas   usedbysnapshots       0                      -

nas   usedbydataset         140K                   -

nas   usedbychildren        418K                   -

nas   usedbyrefreservation  0                      -

nas   logbias               latency                default

nas   dedup                 off                    default

nas   mlslabel              none                   default

nas   sync                  standard               default

nas   refcompressratio      1.00x                  -

nas   written               140K                   -

nas   logicalused           120K                   -

nas   logicalreferenced     40K                    -

nas   filesystem_limit      none                   default

nas   snapshot_limit        none                   default

nas   filesystem_count      none                   default

nas   snapshot_count        none                   default

nas   snapdev               hidden                 default

nas   acltype               off                    default

nas   context               none                   default

nas   fscontext             none                   default

nas   defcontext            none                   default

nas   rootcontext           none                   default

nas   relatime              on                     temporary

nas   redundant_metadata    all                    default

nas   overlay               off                    default

Thank you!