Hammer mirroring and disk space

[Eugene]

Hello All.

I'm trying to use HAMMER instead of RAID, as it was described by Siju 
George. The system slice layout is
# Device                Mountpoint      FStype  Options         Dump    
Pass#
/dev/da0s1a             /               ufs     rw              1       1
/dev/da0s2b             none            swap    sw              0       0
/dev/da0s2h             /HAMMER0        hammer  rw              2       2
/HAMMER0/pfs/var        /var            null    rw              2       2
/dev/da1s1a             /mirror         ufs     rw              1       1
/dev/da1s2b             none            swap    sw              0       0
/dev/da1s2h             /HAMMER1        hammer  rw              2       2
proc                    /proc           procfs  rw              0       0

The idea was to use slice da0s2 for HAMMER filesystem and to create a 
single master pfs on it, and use da1s2 for a mirror with HAMMER FS with 
a single slave pfs. A master pfs is mounted as /var, and a slave pfs is 
not mounted separately at all.
The question is an output of a df -h command:

Filesystem                Size   Used  Avail Capacity  Mounted on
/dev/da0s1a                77G    11G    59G    16%    /
HAMMER0                    66G   9.9G    56G    15%    /HAMMER0
/HAMMER0/pfs/@@-1:00001    66G   9.9G    56G    15%    /var
/dev/da1s1a                78G    12G    60G    16%    /mirror
HAMMER1                    66G    34G    32G    51%    /HAMMER1
procfs                    4.0K   4.0K     0B   100%    /proc
The hammer mirror-stream command is executed twice a day.

What can really use 35% (the percent grows up) on a mirroring (slave) 
filesystem and how to reduce this disk usage on a slave to return it to 
it's master's disk usage?

--
Sorry for my poor English.