Commits on Sep 1, 2017

1. 8604 Avoid unnecessary work search in VFS when unmounting snapshots …

   Reviewed by: Matt Ahrens <mahrens@delphix.com>
   Reviewed by: George Wilson <george.wilson@delphix.com>
   
   Every time we want to unmount a snapshot (happens during snapshot
   deletion or renaming) we unnecessarily iterate through all the
   mountpoints in the VFS layer (see zfs_get_vfs).
   
   The current patch completely gets rid of that code and changes
   the approach while keeping the behavior of that code path the
   same. Specifically, it puts a hold on the dataset/snapshot and
   gets its vfs resource reference directly, instead of linearly
   searching for it. If that reference exists we attempt to amount
   it.
   
   With the above change, it became obvious that the nvlist
   manipulations that we do (add_boolean and add_nvlist) take a
   significant amount of time ensuring uniqueness of every new
   element even though they don't have too. Thus, we updated the
   patch so those nvlists are not trying to enforce the uniqueness
   of their elements.
   
   A more complete analysis of the problem solved by this patch
   can be found below:
   https://sdimitro.github.io/post/snap-unmount-perf/
   
   Upstream Bugs: DLPX-53221

   

   sdimitro committed Sep 1, 2017
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