Public bug reported:

Upstream commit:
https://github.com/zfsonlinux/zfs/pull/6989/commits/e0014d56c3c21d53a71c436d54e10a456acb190d

"When the compressed ARC feature was added in commit d3c2ae1
the method of reference counting in the ARC was modified.  As
part of this accounting change the arc_buf_add_ref() function
was removed entirely.

This would have be fine but the arc_buf_add_ref() function
served a second undocumented purpose of updating the ARC access
information when taking a hold on a dbuf.  Without this logic
in place a cached dbuf would not migrate its associated
arc_buf_hdr_t to the MFU list.  This would negatively impact
the ARC hit rate, particularly on systems with a small ARC."

** Affects: zfs-linux (Ubuntu)
     Importance: Medium
     Assignee: Colin Ian King (colin-king)
         Status: In Progress

** Changed in: linux (Ubuntu)
   Importance: Undecided => Medium

** Changed in: linux (Ubuntu)
     Assignee: (unassigned) => Colin Ian King (colin-king)

** Changed in: linux (Ubuntu)
       Status: New => In Progress

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https://bugs.launchpad.net/bugs/1755158

Title:
  Fix ARC hit rate

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