Changing /sys/module/zfs/parameters/zfs_arc_max doesn't work anymore (Ubuntu)

[AceSlash]

I'm not sure if this should be an Ubuntu ticket instead, feel free to close that if it's an issue in the ZoL implementation by Canonical.

System information

Type	Version/Name
Distribution Name	Ubuntu
Distribution Version	16.04.3
Linux Kernel	4.4.0-57-generic
Architecture	x64
ZFS Version	0.6.5.6-0ubuntu15
SPL Version	0.6.5.6-0ubuntu4

Describe the problem you're observing

I'm trying to extend the arc size because I have a poor hit ratio (65%) from ~ 47GB to ~128GB:

# echo "1216591429632" > /sys/module/zfs/parameters/zfs_arc_max
# cat /sys/module/zfs/parameters/zfs_arc_max
1216591429632

But the max_arc_size is still at its old value:

# cat /proc/spl/kstat/zfs/arcstats|egrep "^c"
c                               4    48719014608
c_min                           4    33554432
c_max                           4    50691309568

Following advice from IRC, I tried to generate IO, by copying large file between dataset and then using cat to /dev/null. This fills up the arc up to its old max value, but no extension:

# arc_summary -p 1

------------------------------------------------------------------------
ZFS Subsystem Report                            Tue Aug 22 12:21:19 2017
ARC Summary: (HEALTHY)
        Memory Throttle Count:                  0

ARC Misc:
        Deleted:                                2.95b
        Mutex Misses:                           76.06k
        Evict Skips:                            76.06k

ARC Size:                               100.00% 47.21   GiB
        Target Size: (Adaptive)         100.00% 47.21   GiB
        Min Size (Hard Limit):          0.07%   32.00   MiB
        Max Size (High Water):          1510:1  47.21   GiB

ARC Size Breakdown:
        Recently Used Cache Size:       27.63%  13.04   GiB
        Frequently Used Cache Size:     72.37%  34.17   GiB

ARC Hash Breakdown:
        Elements Max:                           10.08m
        Elements Current:               65.43%  6.60m
        Collisions:                             896.19m
        Chain Max:                              6
        Chains:                                 296.45k