OpenZFS 8025 - dbuf_read() creates unnecessary zio_root() for bonus buf

[behlendorf]

Authored by: Matthew Ahrens mahrens@delphix.com
Reviewed by: Dan Kimmel dan.kimmel@delphix.com
Reviewed by: Pavel Zakharov pavel.zakharov@delphix.com
Reviewed by: Prashanth Sreenivasa pks@delphix.com
Approved by: Robert Mustacchi rm@joyent.com
Ported-by: Brian Behlendorf behlendorf1@llnl.gov

dbuf_read() creates a zio_root() to track and wait for all the zio's
that may happen as part of this call. However, if the blkptr_t for
this buffer is NULL or a hole, we will not create any more zio's, so
this zio_root() is unnecessary. This is always the case when calling
dbuf_read() on a bonus buffer, because it has no blkptr (it's part of
the containing dnode). For workloads that read a lot of bonus buffers
(e.g. file creation and removal), creating and destroying these
unnecessary zio's can decrease performance by around 3%.

The fix is to only create/destroy the zio_root() in dbuf_read() if the
blkptr is not NULL and not a hole.

Porting Notes:

• The error handling for when dbuf_read_impl() fails which was
  originally added in commit 5f6d0b6 has been preserved.

OpenZFS-issue: https://www.illumos.org/issues/8025
OpenZFS-commit: openzfs/openzfs@8ec5c7c

[mention-bot]

@behlendorf, thanks for your PR! By analyzing the history of the files in this pull request, we identified @grwilson, @ahrens and @pcd1193182 to be potential reviewers.