> -> read: IOPS=19.1k, BW=149MiB/s (156MB/s)(4473MiB/30001msec)
Isn't this too low? On my non-RAID configuration I get almost 2GB/s with the exact same command. Samsung 980 PRO 1TB.
fio --filename=/dev/md127 --direct=1 --rw=randread --bs=8k --ioengine=io_uring --iodepth=1 --runtime=120 --numjobs=1 --time_based --group_reporting --name=iops-test-job --eta-newline=1 --readonly
i.e. with iodepth=1 and numjobs=1. Because this is what "mimics" index scan (without prefetch) on cold data. More or less.
The command I posted earlier does ~10GB/s on my RAID, which matches your data.
This is an interesting observation but does it really mimic the index scan? This would be essentially a worst case scenario. Submitting IO requests one by one would be a very inefficient way to handle scans, no?