> a larger read is generally as fast as multiple smaller one on modern hardware.

Not always if by modern you mean NVMe drives. One synchronous preadv() for 256 KiB gives the kernel/device one big request but 16 independent asynchronous 16 KiB reads can be serviced concurrently. So the latter gives the NVMe controller 16 operations it can schedule in parallel. So depending on the workload and hardware, offsets, filesystem and request sizes that can give you lower aggregate latency or higher throughput.

If you submit 16 contiguous read requests the system will just merge them into one large read request.

Modern SSDs tolerate moderate queue depths very well, but piling on the I/O queue also incurs tail latency jitter unless you're able to ensure the queue depth stays in the moderate range and never goes higher. All else being equal, fewer larger requests is better for I/O latency (though read amplification for the sake of reading more data obviously doesn't help anyone). Though in this scenario, we're mostly comparing the syscall overhead of a single preadv against io_uring bookkeeping for multiple preads, regardless of how you submit the reads they end up being the same operation.

I think that’s only true on paper; in practice it’ll be true only when you have competing reads (at a thousand-foot view) and it might be possible to algorithmically bundle a portion thereof. It originally let SCSI controllers attached to spinning rust HDDs optimize physical manipulation of the disk heads to optimized queued reads of data in a “traveling salesman” sort of way, but modern nand flash can only internally read a full page at a time (which may be much greater than even the apparent physical sector size) anyway and with a strictly constant cost regardless of the “physical location” of the data on the non-existent platter. Old drives had optimization constraints like higher sequential read speeds at the outside of the platter (more bytes per physical rotation) and extremely pathological cases for data written to the innermost tracks of the platter. Individual requests were much finer-grained and the latency was much more varied, so a request from app/thread X for as little as 512 bytes from one location could be cheaply piggy-backed on an existing request from app/thread Y to read multiple megabytes from a physically proximate source that would otherwise have seriously delayed or starved the queued waiting read while the outstanding request was serviced.

In fact, one consistently sees higher bulk IO numbers when using physical media that has been formatted with a large sector size compared to the old 512 byte fixed emulated size. You’d routinely see lower latency and higher IOPs with 4kn (HDDs or SSDs) than you would with 512e disks, even with SCSI or AHCI controllers that featured similar pipelining support to today’s NVME controllers (or even if you place a spinning rust HDD behind NVMe today!).