MFC r331711: MFV 331710:
9188 increase size of dbuf cache to reduce indirect block decompression

illumos/illumos-gate@268bbb2a2fa79c36d4695d13a595ba50a7754b76

With compressed ARC (6950) we use up to 25% of our CPU to decompress indirect
blocks, under a workload of random cached reads. To reduce this decompression
cost, we would like to increase the size of the dbuf cache so that more
indirect blocks can be stored uncompressed.

If we are caching entire large files of recordsize=8K, the indirect blocks
use 1/64th as much memory as the data blocks (assuming they have the same
compression ratio). We suggest making the dbuf cache be 1/32nd of all memory,
so that in this scenario we should be able to keep all the indirect blocks
decompressed in the dbuf cache. (We want it to be more than the 1/64th that
the indirect blocks would use because we need to cache other stuff in the
dbuf cache as well.)

In real world workloads, this won't help as dramatically as the example
above, but we think it's still worth it because the risk of decreasing
performance is low. The potential negative performance impact is that we
will be slightly reducing the size of the ARC (by ~3%).

Reviewed by: Dan Kimmel <dan.kimmel@delphix.com>
Reviewed by: Prashanth Sreenivasa <pks@delphix.com>
Reviewed by: Paul Dagnelie <pcd@delphix.com>
Reviewed by: Sanjay Nadkarni <sanjay.nadkarni@nexenta.com>
Reviewed by: Allan Jude <allanjude@freebsd.org>
Reviewed by: Igor Kozhukhov <igor@dilos.org>
Approved by: Garrett D'Amore <garrett@damore.org>
Author: George Wilson <george.wilson@delphix.com>