| There probably is some additional inefficiency when reading pages randomly (compared to sequential reads), but most of the difference is at the storage level. That is, SSDs can handle a lot of random I/O, but it's nowhere close to sequential reads. For example, I have a RAID0 with 4 SSDs (Samsung 990 PRO, so consumer, but quite good for reads). And this is what fio says: # random reads, 8K, direct IO, depth=1 fio --filename=device name --direct=1 --rw=randread --bs=4k --ioengine=libaio --iodepth=256 --runtime=120 --numjobs=4 --time_based --group_reporting --name=iops-test-job --eta-newline=1 --readonly -> read: IOPS=19.1k, BW=149MiB/s (156MB/s)(4473MiB/30001msec) # sequential reads, 8K, direct IO, depth=1 fio --filename=/dev/md127 --direct=1 --rw=read --bs=8k --ioengine=io_uring --iodepth=1 --runtime=30 --numjobs=1 --time_based --group_reporting --name=random-1 --eta-newline=1 --readonly -> read: IOPS=85.5k, BW=668MiB/s (700MB/s)(19.6GiB/30001msec) With buffered I/O, random read stay at ~19k IOPS, while sequential reads get to ~1M IOPS (thanks to read-ahead, either at the OS level, or in the SSD). So part of this is sequential reads benefiting from implicit "prefetching", which reduces the observed cost of a page. But for random I/O there's no such thing, and so it seems more expensive. It's more complex (e.g. sequential reads allow issuing larger reads), of course. |