Re: index prefetching

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 <tomas(at)vondra(dot)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(at)awork3:~/src/fio$ fio --directory /srv/fio --size=$((1024*1024*1024)) --name test --bs=4k --rw read:8k --buffered 0 2>&1|grep READ
READ: bw=179MiB/s (188MB/s), 179MiB/s-179MiB/s (188MB/s-188MB/s), io=341MiB (358MB), run=1907-1907msec
andres(at)awork3:~/src/fio$ 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