On Thu, Jul 02, 2020 at 03:09:29PM +1200, Thomas Munro wrote:
On Sat, Jun 6, 2020 at 12:36 PM Stephen Frost <sfr...@snowman.net> wrote:
* Tomas Vondra (tomas.von...@2ndquadrant.com) wrote:
> I wonder if we can collect some stats to measure how effective the
> prefetching actually is. Ultimately we want something like cache hit
> ratio, but we're only preloading into page cache, so we can't easily
> measure that. Perhaps we could measure I/O timings in redo, though?
That would certainly be interesting, particularly as this optimization
seems likely to be useful on some platforms (eg, zfs, where the
filesystem block size is larger than ours..) and less on others
(traditional systems which have a smaller block size).
I know one way to get information about cache hit ratios without the
page cache fuzz factor: if you combine this patch with Andres's
still-in-development AIO prototype and tell it to use direct IO, you
get the undiluted truth about hits and misses by looking at the
"prefetch" and "skip_hit" columns of the stats view. I'm hoping to
have a bit more to say about how this patch works as a client of that
new magic soon, but I also don't want to make this dependent on that
(it's mostly orthogonal, apart from the "how deep is the queue" part
which will improve with better information).
FYI I am still trying to reproduce and understand the problem Tomas
reported; more soon.
Any luck trying to reproduce thigs? Should I try again and collect some
additional debug info?
regards
--
Tomas Vondra http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
