MFV 312436

MFV 312436

 6569 large file delete can starve out write ops

  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>

Reviewed by:	avg
Differential Revision:	D9008