8-STABLE and swap

[Robert Schulze]

We are running 8-STABLE (csupped at 20110504) on a NFS fileserver.
It has 32 GB RAM and uses ZFS:

home        ONLINE       0     0     0
	  raidz2    ONLINE       0     0     0
	    da1     ONLINE       0     0     0
	    da2     ONLINE       0     0     0
	    da3     ONLINE       0     0     0
	    da4     ONLINE       0     0     0
	    da5     ONLINE       0     0     0
	  raidz2    ONLINE       0     0     0
	    da6     ONLINE       0     0     0
	    da7     ONLINE       0     0     0
	    da8     ONLINE       0     0     0
	    da9     ONLINE       0     0     0
	    da10    ONLINE       0     0     0
	logs
	  mirror    ONLINE       0     0     0
	    da12    ONLINE       0     0     0
	    da13    ONLINE       0     0     0
	cache
	  ad4       ONLINE       0     0     0
	  ad8       ONLINE       0     0     0

Before upgrading from 8.0, the machine never used the whole system 
memory, it left about 10 GB free even after about 100 days uptime.
Now, it eats RAM insanely (wired is between 29 GB and 30 GB), which is 
quite good I think, but after about 3 days uptime, we now have 106 MB 
swapped out. Both L2ARC SSDs are ~74 GB in size, arc_summary prints the 
following values:

ARC Size:
	Current Size:			76.21%	23440.22M (arcsize)
	Target Size: (Adaptive)		76.52%	23535.40M (c)
	Min Size (Hard Limit):		12.50%	3844.77M (c_min)
	Max Size (High Water):		~8:1	30758.16M (c_max)

L2 ARC Size:
	Current Size: (Adaptive)		88466.19M
	Header Size:			0.29%	259.21M


The following sysctls were set:

security.bsd.see_other_uids=0
kern.maxvnodes=400000
kern.ipc.somaxconn=8192
kern.ipc.maxsockbuf=1024000
net.inet.udp.maxdgram=57344
vfs.ufs.dirhash_maxmem=25165824

My question now: why does the machine swap, is this normal behaviour?
Why is wired at about 30 GB if ARC=23 GB and L2ARC-header=259 MB?

with kind regards,
Robert Schulze