[RFC PATCH 0/5] md/raid1: improve choose_best_rdev read balance

From: Chen Cheng <redacted>

Prefer a nonrot disk when one can take the read. When pending is
the same, use a real round-robin among the disks we may pick.

fio libaio direct=1, NVMe none, SATA mq-deadline (stock -> this):

- 2x GM9000: 1M QD16 j1 7.031 -> 13.886 GB/s, 50/50; 4k QD1 50/50
- 2x/3x HDWG740: 0.294 -> 0.514 / 0.630 GB/s
- GM9000+HDD seq: 2.468 or 0.280 -> 7.112 GB/s, 100/0 NVMe
- Fanxiang+HDD seq, HDD first: 0.278 -> 0.562 GB/s
- GM9000+HDD 4k QD8/16: clat -47% / -45%


Chen Cheng (5):
  md/raid1: balance reads across non-rotational disks
  md/raid1: do not move nonrot reads onto a rot disk
  md/raid1: do not send random reads to a rot disk
  md/raid1: use rot policy when no nonrot disk is readable
  md/raid1: clarify choose_best_rdev comments

 drivers/md/raid1.c | 122 ++++++++++++++++++++++++++++++++++++++-------
 drivers/md/raid1.h |   1 +
 2 files changed, 104 insertions(+), 19 deletions(-)

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2.55.0