This provides a noticeable write speedup, especially on pools with
less than 30% of free space.
Detailed information (OpenSolaris onnv changesets and Bug IDs):
11146:7e58f40bcb1c
6826241 Sync write IOPS drops dramatically during TXG sync
6869229 zfs should switch to shiny new metaslabs more frequently
11728:59fdb3b856f6
6918420 zdb -m has issues printing metaslab statistics
12047:7c1fcc8419ca
6917066 zfs block picking can be improved
Approved by: delphij (mentor)
Obtained from: OpenSolaris (Bug ID 6826241, 6869229, 6918420, 6917066)
MFC after: 2 weeks
abe5837f7c