** Also affects: linux (Ubuntu)
   Importance: Undecided
       Status: New

** Also affects: linux (Ubuntu Bionic)
   Importance: Undecided
       Status: New

** Also affects: zfs-linux (Ubuntu Bionic)
   Importance: Medium
     Assignee: Colin Ian King (colin-king)
       Status: Fix Committed

** Changed in: linux (Ubuntu Bionic)
       Status: New => Fix Committed

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

Title:
  Fix ARC hit rate

Status in linux package in Ubuntu:
  Fix Committed
Status in zfs-linux package in Ubuntu:
  Fix Committed
Status in linux source package in Bionic:
  Fix Committed
Status in zfs-linux source package in Bionic:
  Fix Committed

Bug description:
  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."

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