[Bug 212132] [Hyper-V]Storage performance tuning for multiple threads on RAID0

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=212132

            Bug ID: 212132
           Summary: [Hyper-V]Storage performance tuning for multiple
                    threads on RAID0
           Product: Base System
           Version: CURRENT
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Many People
          Priority: ---
         Component: kern
          Assignee: freebsd-bugs at FreeBSD.org
          Reporter: honzhan at microsoft.com

The storage performance for FreeBSD on Hyper-V was almost half of Linux on
Hyper-V for FIO multiple threads test. But for single thread, the performance
is almost the same as Linux.

After analysis, there are two potential performance bugs:
(1) IO max request queue is too short: 128 on FreeBSD, but on Linux it is more
than 1000.
(2) The policy of outgoing channel selection has issues. It caused almost all
of the workload to run on some of channels, while the other channels are idle.

Before the fix, the performance comparison with LIS is as follows. For random
write, the performance of BIS is only 1/3 of LIS.

4k 8th  randread                randwrite               read            write   
IOdepth LIS 4k  BIS 4k  LIS 4k  BIS 4k  LIS 4k  BIS 4k  LIS 4k  BIS 4k
'1'     2749    1920.12 4551    2855.11 8100    4263.65 8230    4301.47
'16'    8333    6981.78 6656    4671.84 8161    8242.97 8588    8300.11
'32'    8274    2532.7  6642    1885.82 8160    2559.84 8526    2454.99
'64'    8295    2697.35 6621    2007.61 8172    2666.58 8459    2508.46
'128'   8298    2777.12 6429    2049.26 8197    2578.55 8267    2708.39

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