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Damn, I copied the wrong command. I wanted to copy this one:

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.

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> Because this is what "mimics" index scan (without prefetch) on cold data. More or less.

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?

True. Unfortunately it's what index scans in Postgres do right now - it's the last "major" scan type not supporting some sort of prefetch (posix_fadvise or AIO). We're working on it, hopefully it'll get into PG19.

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