How to change the l2arc_write_max dynamically

ZFS sets the value of the l2arc_write_max to the each cache device of L2ARC only when you add it to a pool of ZFS or restart the OS. If you want to change the value dynamically, you should follow the following steps with the mdb.

• Start the mdb with the root privilege.
  # mdb -kw
• Print the list of the addresses of the actual write max for each device.
  > L2ARC_dev_list::walk list | ::print -a l2arc_dev_t l2ad_write
  30074670a00 l2ad_write = 0x800000
  300ebae8818 l2ad_write = 0x800000
  3014a4269b0 l2ad_write = 0x800000
  30164ae3be0 l2ad_write = 0x800000
• Set a proper value to these addresses.
  > 0x30074670a00/Z c00000
  0x30074670a00: 0x800000 = 0xc00000
  > 0x300ebae8818/Z c00000
  0x300ebae8818: 0x800000 = 0xc00000
  > 3014a4269b0/Z c00000
  0x3014a4269b0: 0x800000 = 0xc00000
  > 30164ae3be0/Z c00000
  0x30164ae3be0: 0x800000 = 0xc00000

Changing the value dynamically may cause unexpected behavior. I don't recomend it though I have never experienced any problem.