Re: Skip prefetch for block references that follow a FPW or WILL_INIT of the same block

Hi Jeremy,

I think I missed reading your github link yesterday night. I agree it shows
IOPS difference between primary and replica.

After digging in a bit more (thanks to your blog[1]), I noticed the current
prefetcher
always calls FADVISE before pread, which looks like bypass OS readahead for
sequential access.

In the replay path, this may happen right after the primary finishes a
large
sequential write (e.g., vacuum or a full-table update). But I am not sure
if these workload are
common because most of the time vacuum can skip pages and the write is not
purely
sequential and full-table update is not that common as well.

But I agree this is a problem we need to fix.

> This is exactly the problem. Because of that ratio.....

I think repeated fadvise calls on continuous blocks would not trigger OS
prefetch and we'd be turning
sequential I/O into random I/O, burning more IOPS while leaving bandwidth
underutilized.....

[1]
https://ardentperf.com/2026/10/20/postgres-recovery_prefetch-considered-harmful-and-causing-outages/

Thanks,
Shihao