Hi, On 2025-08-14 01:11:07 +0200, Tomas Vondra wrote: > On 8/13/25 23:57, Peter Geoghegan wrote: > > On Wed, Aug 13, 2025 at 5:19 PM Tomas Vondra <to...@vondra.me> wrote: > >> It's also not very surprising this happens with backwards scans more. > >> The I/O is apparently much slower (due to missing OS prefetch), so we're > >> much more likely to hit the I/O limits (max_ios and various other limits > >> in read_stream_start_pending_read). > > > > But there's no OS prefetch with direct I/O. At most, there might be > > some kind of readahead implemented in the SSD's firmware. > > > > Good point, I keep forgetting direct I/O means no OS read-ahead. Not > sure if there's a good way to determine if the SSD can do something like > that (and how well). I wonder if there's a way to do backward sequential > scans in fio ..
In theory, yes, in practice, not quite: https://github.com/axboe/fio/issues/1963 So right now it only works if you skip over some blocks. For that there rather significant performance differences on my SSDs. E.g. andres@awork3:~/src/fio$ fio --directory /srv/fio --size=$((1024*1024*1024)) --name test --bs=4k --rw read:8k --buffered 0 2>&1|grep READ READ: bw=179MiB/s (188MB/s), 179MiB/s-179MiB/s (188MB/s-188MB/s), io=341MiB (358MB), run=1907-1907msec andres@awork3:~/src/fio$ fio --directory /srv/fio --size=$((1024*1024*1024)) --name test --bs=4k --rw read:-8k --buffered 0 2>&1|grep READ READ: bw=70.6MiB/s (74.0MB/s), 70.6MiB/s-70.6MiB/s (74.0MB/s-74.0MB/s), io=1024MiB (1074MB), run=14513-14513msec So on this WD Red SN700 there's a rather substantial performance difference. On a Samsung 970 PRO I don't see much of a difference. Nor on a ADATA SX8200PNP. Greetings, Andres Freund
