Prefetching of dnodes in dbuf_read() can cause significant mutex
contention for some workloads and isn't very helpful. This is
because we already get 32 dnodes for each block read, and when
iterating over a directory we prefetch the dnodes in the directory.
Disable this prefetching to prevent the lock contention.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Submitted-by: Adam Moss <c@yotes.com>
Submitted-by: Matthew Ahrens <mahrens@delphix.com>
Signed-off-by: Adam Moss <c@yotes.com>
Closes#10877Closes#10953
edd23dba81