Unexpected zfs ARC behavior

[FF]

So on a somewhat loaded ZFS file server (all NFS) serving mostly VMs with
the load steadily increasing over the month (migrated from other
servers)... the ARC size has unexpectedly dropped (please see attached
graphic, if it makes through the mailing list server). The system peaks
around 2,000 NFS IOPS and hasn't exhibited any slow downs. The L2ARC has a
very low hit rate and prefetch has been turned off to increase the L2ARC
efficiency... but none of that should really matter as far as I can tell
since the L1ARC should try to use all the memory it can.

Some tuning of zfs sysctls (mostly write_boost and write_max) to increase
them, but this has since been backed off to default.

vmstat -m reports that 24G is dedicated to opensolaris:
    solaris 1113433 24565820K       - 6185559839
16,32,64,128,256,512,1024,2048,4096

And top reports memory free:

last pid: 32075;  load averages:  0.16,  0.13,
0.14                                    up 36+21:44:59  09:16:49
24 processes:  1 running, 23 sleeping
CPU:  0.0% user,  0.0% nice,  0.0% system,  0.6% interrupt, 99.4% idle
Mem: 25M Active, 1187M Inact, 26G Wired, 2048K Cache, 1536M Buf, 4160M Free
ARC: 14G Total, 1679M MFU, 12G MRU, 28M Anon, 156M Header, 21M Other

zfs-stats -a :

------------------------------------------------------------------------
ZFS Subsystem Report                            Sat Sep 27 09:17:07 2014
------------------------------------------------------------------------

System Information:

        Kernel Version:                         902001 (osreldate)
        Hardware Platform:                      amd64
        Processor Architecture:                 amd64

        ZFS Storage pool Version:               5000
        ZFS Filesystem Version:                 5

FreeBSD 9.2-RELEASE-p10 #0 r270148M: Mon Aug 18 23:14:36 EDT 2014 root
 9:17AM  up 36 days, 21:45, 2 users, load averages: 0.12, 0.12, 0.13

------------------------------------------------------------------------

System Memory:

        0.08%   26.22   MiB Active,     3.75%   1.16    GiB Inact
        82.36%  25.47   GiB Wired,      0.01%   2.00    MiB Cache
        13.80%  4.27    GiB Free,       0.00%   1.03    MiB Gap

        Real Installed:                         32.00   GiB
        Real Available:                 99.63%  31.88   GiB
        Real Managed:                   97.01%  30.93   GiB

        Logical Total:                          32.00   GiB
        Logical Used:                   83.03%  26.57   GiB
        Logical Free:                   16.97%  5.43    GiB

Kernel Memory:                                  23.54   GiB
        Data:                           99.90%  23.52   GiB
        Text:                           0.10%   23.13   MiB

Kernel Memory Map:                              29.76   GiB
        Size:                           76.40%  22.74   GiB
        Free:                           23.60%  7.02    GiB

------------------------------------------------------------------------

ARC Summary: (HEALTHY)
        Memory Throttle Count:                  0

ARC Misc:
        Deleted:                                90.09m
        Recycle Misses:                         2.44m
        Mutex Misses:                           794.67k
        Evict Skips:                            17.90m

ARC Size:                               44.78%  13.40   GiB
        Target Size: (Adaptive)         44.78%  13.40   GiB
        Min Size (Hard Limit):          12.50%  3.74    GiB
        Max Size (High Water):          8:1     29.93   GiB

ARC Size Breakdown:
        Recently Used Cache Size:       86.16%  11.55   GiB
        Frequently Used Cache Size:     13.84%  1.85    GiB

ARC Hash Breakdown:
        Elements Max:                           786.71k
        Elements Current:               87.05%  684.85k
        Collisions:                             153.35m
        Chain Max:                              16
        Chains:                                 194.92k

------------------------------------------------------------------------

ARC Efficiency:                                 506.24m
        Cache Hit Ratio:                87.56%  443.25m
        Cache Miss Ratio:               12.44%  62.99m
        Actual Hit Ratio:               80.06%  405.29m

        Data Demand Efficiency:         93.92%  372.74m
        Data Prefetch Efficiency:       49.49%  69.76m

        CACHE HITS BY CACHE LIST:
          Anonymously Used:             6.10%   27.05m
          Most Recently Used:           31.37%  139.05m
          Most Frequently Used:         60.07%  266.24m
          Most Recently Used Ghost:     0.80%   3.56m
          Most Frequently Used Ghost:   1.66%   7.35m

        CACHE HITS BY DATA TYPE:
          Demand Data:                  78.98%  350.09m
          Prefetch Data:                7.79%   34.53m
          Demand Metadata:              12.15%  53.86m
          Prefetch Metadata:            1.08%   4.77m

        CACHE MISSES BY DATA TYPE:
          Demand Data:                  35.95%  22.65m
          Prefetch Data:                55.93%  35.23m
          Demand Metadata:              4.49%   2.83m
          Prefetch Metadata:            3.63%   2.29m

------------------------------------------------------------------------
<cut>

Any suggestions? Is this expected or acceptable behavior?

Thanks in advance,

-- 
FF