illumos/illumos-gate@ff5177ee8bf9a355131ce2cc61ae2da6a5a6fdd6
https://github.com/illumos/illumos-gate/commit/ff5177ee8bf9a355131ce2cc61ae2da6a5a6fdd6

https://www.illumos.org/issues/6569
  The core issue I've found is that there is no throttle for how many
  deletes get assigned to one TXG. As a results when deleting large files
  we end up filling consecutive TXGs with deletes/frees, then write
  throttling other (more important) ops.

  There is an easy test case for this problem. Try deleting several
  large files (at least 1/2 TB) while you do write ops on the same
  pool. What we've seen is performance of these write ops (let's
  call it sideload I/O) would drop to zero.

  More specifically the problem is that dmu_free_long_range_impl()
  can/will fill up all of the dirty data in the pool "instantly",
  before many of the sideload ops can get in. So sideload
  performance will be impacted until all the files are freed.

  The solution we have tested at Nexenta (with positive results)
  creates a relatively simple throttle for how many "free" ops we let
  into one TXG.

  However this solution exposes other problems that should also be
  addressed. If we are to slow down freeing of data that means one
  has to wait even longer (assuming vnode ref count of 1) to get shell
  back after an rm or for NFS thread to finish the free-ing op.
  To avoid this the proposed solution is to call zfs_inactive() async
  for "large" files. Async freeing then begs for the reclaimed space
  to be accounted for in the zpool's "freeing" prop.

  The other issue with having a longer delete is the inability to
  export/unmount for a longer period of time. The proposed solution
  is to interrupt freeing of blocks when a fs is unmounted.

Author: Alek Pinchuk <alek@nexenta.com>
Reviewed by: Matt Ahrens <mahrens@delphix.com>
Reviewed by: Sanjay Nadkarni <sanjay.nadkarni@nexenta.com>
Reviewed by: Pavel Zakharov <pavel.zakharov@delphix.com>
Approved by: Dan McDonald <danmcd@omniti.com>