This patch introduces the parameter of 'g_global_tags' so that we can
test this feature by null_blk easiy.
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Not see obvious performance drop with global_tags when the whole hw
depth is kept as same:
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1) no 'global_tags', each hw queue depth is 1, and 4 hw queues
modprobe null_blk queue_mode=3D2 nr_devices=3D4 shared_tags=3D1 global_ta=

gs=3D0

submit_queues=3D4 hw_queue_depth=3D1
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2) 'global_tags', global hw queue depth is 4, and 4 hw queues
modprobe null_blk queue_mode=3D2 nr_devices=3D4 shared_tags=3D1 global_ta=

gs=3D1

submit_queues=3D4 hw_queue_depth=3D4
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3) fio test done in above two settings:
   fio --bs=3D4k --size=3D512G  --rw=3Drandread --norandommap --direct=3D=

1 --

ioengine=3Dlibaio --iodepth=3D4 --runtime=3D$RUNTIME --group_reporting=3D=

1  --

name=3Dnullb0 --filename=3D/dev/nullb0 --name=3Dnullb1 --filename=3D/dev/=

nullb1 --

name=3Dnullb2 --filename=3D/dev/nullb2 --name=3Dnullb3 --filename=3D/dev/=

nullb3

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1M IOPS can be reached in both above tests which is done in one VM.
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I am getting 1.1M IOPS for both cases.

Tested-by: Don Brace <redacted>