ZFS module parameters in Linux (OR kernel parameter in FreeBSD or zfs-in-kernel OS)

# free
             total       used       free     shared    buffers     cached
Mem:      24530656   20768340    3762316          0     115636    2609916
-/+ buffers/cache:   18042788    6487868
Swap:      8388600          0    8388600

# lsmod|grep zfs
zfs                  1195533  1 
zcommon                46278  1 zfs
znvpair                80974  2 zfs,zcommon
zavl                    6925  1 zfs
zunicode              323159  1 zfs
spl                   266655  5 zfs,zcommon,znvpair,zavl,zunicode

# modinfo zfs
filename:       /lib/modules/2.6.32-431.el6.x86_64/extra/zfs.ko
version:        0.6.3-1
license:        CDDL
author:         Sun Microsystems/Oracle, Lawrence Livermore National Laboratory
description:    ZFS
srcversion:     C29A443E3D2B93F605A540B
depends:        spl,znvpair,zcommon,zunicode,zavl
vermagic:       2.6.32-431.el6.x86_64 SMP mod_unload modversions 
parm:           zvol_inhibit_dev:Do not create zvol device nodes (uint)
parm:           zvol_major:Major number for zvol device (uint)
parm:           zvol_threads:Number of threads for zvol device (uint)
parm:           zvol_max_discard_blocks:Max number of blocks to discard (ulong)
parm:           zio_injection_enabled:Enable fault injection (int)
parm:           zio_bulk_flags:Additional flags to pass to bulk buffers (int)
parm:           zio_delay_max:Max zio millisec delay before posting event (int)
parm:           zio_requeue_io_start_cut_in_line:Prioritize requeued I/O (int)
parm:           zfs_sync_pass_deferred_free:Defer frees starting in this pass (int)
parm:           zfs_sync_pass_dont_compress:Don't compress starting in this pass (int)
parm:           zfs_sync_pass_rewrite:Rewrite new bps starting in this pass (int)
parm:           zil_replay_disable:Disable intent logging replay (int)
parm:           zfs_nocacheflush:Disable cache flushes (int)
parm:           zil_slog_limit:Max commit bytes to separate log device (ulong)
parm:           zfs_read_chunk_size:Bytes to read per chunk (long)
parm:           zfs_immediate_write_sz:Largest data block to write to zil (long)
parm:           zfs_flags:Set additional debugging flags (int)
parm:           zfs_recover:Set to attempt to recover from fatal errors (int)
parm:           zfs_expire_snapshot:Seconds to expire .zfs/snapshot (int)
parm:           zfs_vdev_aggregation_limit:Max vdev I/O aggregation size (int)
parm:           zfs_vdev_read_gap_limit:Aggregate read I/O over gap (int)
parm:           zfs_vdev_write_gap_limit:Aggregate write I/O over gap (int)
parm:           zfs_vdev_max_active:Maximum number of active I/Os per vdev (int)
parm:           zfs_vdev_async_write_active_max_dirty_percent:Async write concurrency max threshold (int)
parm:           zfs_vdev_async_write_active_min_dirty_percent:Async write concurrency min threshold (int)
parm:           zfs_vdev_async_read_max_active:Max active async read I/Os per vdev (int)
parm:           zfs_vdev_async_read_min_active:Min active async read I/Os per vdev (int)
parm:           zfs_vdev_async_write_max_active:Max active async write I/Os per vdev (int)
parm:           zfs_vdev_async_write_min_active:Min active async write I/Os per vdev (int)
parm:           zfs_vdev_scrub_max_active:Max active scrub I/Os per vdev (int)
parm:           zfs_vdev_scrub_min_active:Min active scrub I/Os per vdev (int)
parm:           zfs_vdev_sync_read_max_active:Max active sync read I/Os per vdev (int)
parm:           zfs_vdev_sync_read_min_active:Min active sync read I/Os per vdev (int)
parm:           zfs_vdev_sync_write_max_active:Max active sync write I/Os per vdev (int)
parm:           zfs_vdev_sync_write_min_active:Min active sync write I/Osper vdev (int)
parm:           zfs_vdev_mirror_switch_us:Switch mirrors every N usecs (int)
parm:           zfs_vdev_scheduler:I/O scheduler (charp)
parm:           zfs_vdev_cache_max:Inflate reads small than max (int)
parm:           zfs_vdev_cache_size:Total size of the per-disk cache (int)
parm:           zfs_vdev_cache_bshift:Shift size to inflate reads too (int)
parm:           zfs_txg_timeout:Max seconds worth of delta per txg (int)
parm:           zfs_read_history:Historic statistics for the last N reads (int)
parm:           zfs_read_history_hits:Include cache hits in read history (int)
parm:           zfs_txg_history:Historic statistics for the last N txgs (int)
parm:           zfs_deadman_synctime_ms:Expiration time in milliseconds (ulong)
parm:           zfs_deadman_enabled:Enable deadman timer (int)
parm:           spa_asize_inflation:SPA size estimate multiplication factor (int)
parm:           spa_config_path:SPA config file (/etc/zfs/zpool.cache) (charp)
parm:           zfs_autoimport_disable:Disable pool import at module load (int)
parm:           metaslab_debug_load:load all metaslabs during pool import (int)
parm:           metaslab_debug_unload:prevent metaslabs from being unloaded (int)
parm:           zfs_zevent_len_max:Max event queue length (int)
parm:           zfs_zevent_cols:Max event column width (int)
parm:           zfs_zevent_console:Log events to the console (int)
parm:           zfs_top_maxinflight:Max I/Os per top-level (int)
parm:           zfs_resilver_delay:Number of ticks to delay resilver (int)
parm:           zfs_scrub_delay:Number of ticks to delay scrub (int)
parm:           zfs_scan_idle:Idle window in clock ticks (int)
parm:           zfs_scan_min_time_ms:Min millisecs to scrub per txg (int)
parm:           zfs_free_min_time_ms:Min millisecs to free per txg (int)
parm:           zfs_resilver_min_time_ms:Min millisecs to resilver per txg (int)
parm:           zfs_no_scrub_io:Set to disable scrub I/O (int)
parm:           zfs_no_scrub_prefetch:Set to disable scrub prefetching (int)
parm:           zfs_dirty_data_max_percent:percent of ram can be dirty (int)
parm:           zfs_dirty_data_max_max_percent:zfs_dirty_data_max upper bound as % of RAM (int)
parm:           zfs_delay_min_dirty_percent:transaction delay threshold (int)
parm:           zfs_dirty_data_max:determines the dirty space limit (ulong)
parm:           zfs_dirty_data_max_max:zfs_dirty_data_max upper bound in bytes (ulong)
parm:           zfs_dirty_data_sync:sync txg when this much dirty data (ulong)
parm:           zfs_delay_scale:how quickly delay approaches infinity (ulong)
parm:           zfs_prefetch_disable:Disable all ZFS prefetching (int)
parm:           zfetch_max_streams:Max number of streams per zfetch (uint)
parm:           zfetch_min_sec_reap:Min time before stream reclaim (uint)
parm:           zfetch_block_cap:Max number of blocks to fetch at a time (uint)
parm:           zfetch_array_rd_sz:Number of bytes in a array_read (ulong)
parm:           zfs_pd_blks_max:Max number of blocks to prefetch (int)
parm:           zfs_send_corrupt_data:Allow sending corrupt data (int)
parm:           zfs_mdcomp_disable:Disable meta data compression (int)
parm:           zfs_nopwrite_enabled:Enable NOP writes (int)
parm:           zfs_dedup_prefetch:Enable prefetching dedup-ed blks (int)
parm:           zfs_dbuf_state_index:Calculate arc header index (int)
parm:           zfs_arc_min:Min arc size (ulong)
parm:           zfs_arc_max:Max arc size (ulong)
parm:           zfs_arc_meta_limit:Meta limit for arc size (ulong)
parm:           zfs_arc_meta_prune:Bytes of meta data to prune (int)
parm:           zfs_arc_grow_retry:Seconds before growing arc size (int)
parm:           zfs_arc_p_aggressive_disable:disable aggressive arc_p grow (int)
parm:           zfs_arc_p_dampener_disable:disable arc_p adapt dampener (int)
parm:           zfs_arc_shrink_shift:log2(fraction of arc to reclaim) (int)
parm:           zfs_disable_dup_eviction:disable duplicate buffer eviction (int)
parm:           zfs_arc_memory_throttle_disable:disable memory throttle (int)
parm:           zfs_arc_min_prefetch_lifespan:Min life of prefetch block (int)
parm:           l2arc_write_max:Max write bytes per interval (ulong)
parm:           l2arc_write_boost:Extra write bytes during device warmup (ulong)
parm:           l2arc_headroom:Number of max device writes to precache (ulong)
parm:           l2arc_headroom_boost:Compressed l2arc_headroom multiplier (ulong)
parm:           l2arc_feed_secs:Seconds between L2ARC writing (ulong)
parm:           l2arc_feed_min_ms:Min feed interval in milliseconds (ulong)
parm:           l2arc_noprefetch:Skip caching prefetched buffers (int)
parm:           l2arc_nocompress:Skip compressing L2ARC buffers (int)
parm:           l2arc_feed_again:Turbo L2ARC warmup (int)
parm:           l2arc_norw:No reads during writes (int)

# cd /sys/module/zfs/parameters/
# grep ".*" *
l2arc_feed_again:1
l2arc_feed_min_ms:200
l2arc_feed_secs:1
l2arc_headroom:2
l2arc_headroom_boost:200
l2arc_nocompress:0
l2arc_noprefetch:1
l2arc_norw:0
l2arc_write_boost:8388608
l2arc_write_max:8388608
metaslab_debug_load:0
metaslab_debug_unload:0
spa_asize_inflation:24
spa_config_path:/etc/zfs/zpool.cache
zfetch_array_rd_sz:1048576
zfetch_block_cap:256
zfetch_max_streams:8
zfetch_min_sec_reap:2
zfs_arc_grow_retry:5
zfs_arc_max:0
zfs_arc_memory_throttle_disable:1
zfs_arc_meta_limit:0
zfs_arc_meta_prune:1048576
zfs_arc_min:0
zfs_arc_min_prefetch_lifespan:1000
zfs_arc_p_aggressive_disable:1
zfs_arc_p_dampener_disable:1
zfs_arc_shrink_shift:5
zfs_autoimport_disable:0
zfs_dbuf_state_index:0
zfs_deadman_enabled:1
zfs_deadman_synctime_ms:1000000
zfs_dedup_prefetch:1
zfs_delay_min_dirty_percent:60
zfs_delay_scale:500000
zfs_dirty_data_max:2511939174
zfs_dirty_data_max_max:6279847936
zfs_dirty_data_max_max_percent:25
zfs_dirty_data_max_percent:10
zfs_dirty_data_sync:67108864
zfs_disable_dup_eviction:0
zfs_expire_snapshot:300
zfs_flags:1
zfs_free_min_time_ms:1000
zfs_immediate_write_sz:32768
zfs_mdcomp_disable:0
zfs_nocacheflush:0
zfs_nopwrite_enabled:1
zfs_no_scrub_io:0
zfs_no_scrub_prefetch:0
zfs_pd_blks_max:100
zfs_prefetch_disable:0
zfs_read_chunk_size:1048576
zfs_read_history:0
zfs_read_history_hits:0
zfs_recover:0
zfs_resilver_delay:2
zfs_resilver_min_time_ms:3000
zfs_scan_idle:50
zfs_scan_min_time_ms:1000
zfs_scrub_delay:4
zfs_send_corrupt_data:0
zfs_sync_pass_deferred_free:2
zfs_sync_pass_dont_compress:5
zfs_sync_pass_rewrite:2
zfs_top_maxinflight:32
zfs_txg_history:0
zfs_txg_timeout:5
zfs_vdev_aggregation_limit:131072
zfs_vdev_async_read_max_active:3
zfs_vdev_async_read_min_active:1
zfs_vdev_async_write_active_max_dirty_percent:60
zfs_vdev_async_write_active_min_dirty_percent:30
zfs_vdev_async_write_max_active:10
zfs_vdev_async_write_min_active:1
zfs_vdev_cache_bshift:16
zfs_vdev_cache_max:16384
zfs_vdev_cache_size:0
zfs_vdev_max_active:1000
zfs_vdev_mirror_switch_us:10000
zfs_vdev_read_gap_limit:32768
zfs_vdev_scheduler:noop
zfs_vdev_scrub_max_active:2
zfs_vdev_scrub_min_active:1
zfs_vdev_sync_read_max_active:10
zfs_vdev_sync_read_min_active:10
zfs_vdev_sync_write_max_active:10
zfs_vdev_sync_write_min_active:10
zfs_vdev_write_gap_limit:4096
zfs_zevent_cols:80
zfs_zevent_console:0
zfs_zevent_len_max:128
zil_replay_disable:0
zil_slog_limit:1048576
zio_bulk_flags:0
zio_delay_max:30000
zio_injection_enabled:0
zio_requeue_io_start_cut_in_line:1
zvol_inhibit_dev:0
zvol_major:230
zvol_max_discard_blocks:16384
zvol_threads:32

# cd /sys/module/zfs/parameters/
# cat zvol_threads
# modinfo zfs|grep zvol_threads
parm:           zvol_threads:Number of threads for zvol device (uint)
# vi /etc/modprobe.d/zfs.conf
options zfs zvol_threads=64

# modinfo spl|grep spl_kmem_cache_slab_limit
parm:           spl_kmem_cache_slab_limit:Objects less than N bytes use the Linux slab (uint)
# cat /sys/module/spl/parameters/spl_kmem_cache_slab_limit
0
# vi /etc/modprobe.d/spl.conf
options spl spl_kmem_cache_slab_limit=16384

# cat /sys/module/spl/parameters/spl_kmem_cache_slab_limit
16384
# cat /sys/module/zfs/parameters/zvol_threads
64

You can set these like any other module parameter on Linux.  If you want 
them to be persistent the easiest way is to add them to a zfs.conf file 
in /etc/modprobe.d/.  For example something like this.

$ cat /etc/modprobe.d/zfs.conf
options spl spl_kmem_cache_slab_limit=16384
options spl spl_kmem_cache_reclaim=0

Alternately, the ones which are runtime tunable can be set directly 
though /sys/module/zfs/parameters/.  Something like

echo 1 >/sys/module/zfs/parameters/zfs_prefetch_disable

Hope that helps.

Thanks,
Brian