Commits on Jan 13, 2017

1. 7143 dbuf_read() creates unnecessary zio_root() for bonus buf …

   Reviewed by: Dan Kimmel <dan.kimmel@delphix.com>
   Reviewed by: Prashanth Sreenivasa <pks@delphix.com>
   Reviewed by: Pavel Zakharov <pavel.zakharov@delphix.com>
   
   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.

   

   ahrens committed Jan 13, 2017
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