ARC size limit

[George Kola]

Hi All,
       This is my first post to freebsd-stable fresh of Meet BSD California 2014.  We are switching our entire production to FreeBSD. Our storage servers have  256 GB of RAM , 4 TB of SSD and 40 TB of spinning disks. We are running ZFS root and the SSD is configured as L2ARC.  We are running FreeBSD 10.1 RC3
       I am finding that on all our machines, ARC is somehow limited to < 64 GB of memory and we have a huge inactive memory (180 G). The surprising thing is that ARC seems to have almost the same limit  (< 64 GB) on all of our storage boxes and ARC is not growing even though L2ARC hit shows that there is advantage in growing ARC.
       Any help/pointers is appreciated.
       What I am trying to do is to tune ZFS for our workload. We are hoping that we get a high hit rate. 
       Thanks to Justin Gibbs and Allan Jude for initial pointers and help. They suggested posting to the mailing list to get further help.

       I have pasted top output and zfs-stats output below and yes UMA is enabled.

Thanks,
George
       


top
last pid: 27458;  load averages:  3.30,  5.42,  5.34                                                                                                up 6+09:59:30  05:38:49
71 processes:  1 running, 70 sleeping
CPU:  4.2% user,  0.0% nice,  4.6% system,  0.2% interrupt, 90.9% idle
Mem: 11G Active, 181G Inact, 52G Wired, 1368M Cache, 4266M Free
ARC: 47G Total, 1555M MFU, 41G MRU, 35M Anon, 3984M Header, 709M Other
Swap: 64G Total, 2874M Used, 61G Free, 4% Inuse



sysctl vfs.zfs.zio.use_uma
vfs.zfs.zio.use_uma: 1




zfs-mon -a output

ZFS real-time cache activity monitor
Seconds elapsed:  62

Cache hits and misses:
                                  1s    10s    60s    tot
                     ARC hits:   124    126    103    101
                   ARC misses:    35     46     29     28
         ARC demand data hits:    55     90     61     61
       ARC demand data misses:    20     32     18     17
     ARC demand metadata hits:    69     36     42     40
   ARC demand metadata misses:     9     13     10      9
       ARC prefetch data hits:     0      0      0      0
     ARC prefetch data misses:     6      1      1      1
   ARC prefetch metadata hits:     0      0      0      0
 ARC prefetch metadata misses:     0      0      0      0
                   L2ARC hits:    16     28     14     14
                 L2ARC misses:    19     18     15     14
                  ZFETCH hits:   592   2842   2098   2047
                ZFETCH misses:   308   1326    507    494

Cache efficiency percentage:
                          10s    60s    tot
                  ARC:  73.26  78.03  78.29
      ARC demand data:  73.77  77.22  78.21
  ARC demand metadata:  73.47  80.77  81.63
    ARC prefetch data:   0.00   0.00   0.00
ARC prefetch metadata:   0.00   0.00   0.00
                L2ARC:  60.87  48.28  50.00
               ZFETCH:  68.19  80.54  80.56




zfs-stats -a output

ZFS real-time cache activity monitor
Seconds elapsed:  62

Cache hits and misses:
                                  1s    10s    60s    tot
                     ARC hits:   124    126    103    101
                   ARC misses:    35     46     29     28
         ARC demand data hits:    55     90     61     61
       ARC demand data misses:    20     32     18     17
     ARC demand metadata hits:    69     36     42     40
   ARC demand metadata misses:     9     13     10      9
       ARC prefetch data hits:     0      0      0      0
     ARC prefetch data misses:     6      1      1      1
   ARC prefetch metadata hits:     0      0      0      0
 ARC prefetch metadata misses:     0      0      0      0
                   L2ARC hits:    16     28     14     14
                 L2ARC misses:    19     18     15     14
                  ZFETCH hits:   592   2842   2098   2047
                ZFETCH misses:   308   1326    507    494

Cache efficiency percentage:
                          10s    60s    tot
                  ARC:  73.26  78.03  78.29
      ARC demand data:  73.77  77.22  78.21
  ARC demand metadata:  73.47  80.77  81.63
    ARC prefetch data:   0.00   0.00   0.00
ARC prefetch metadata:   0.00   0.00   0.00
                L2ARC:  60.87  48.28  50.00
               ZFETCH:  68.19  80.54  80.56