Greetings, * Robert Haas (robertmh...@gmail.com) wrote: > On Thu, Aug 27, 2020 at 2:51 PM Stephen Frost <sfr...@snowman.net> wrote: > > > Hm? At least earlier versions didn't do prefetching for records with an > > > fpw, and only for subsequent records affecting the same or if not in s_b > > > anymore. > > > > We don't actually read the page when we're replaying an FPW though..? > > If we don't read it, and we entirely write the page from the FPW, how is > > pre-fetching helping..? > > Suppose there is a checkpoint. Then we replay a record with an FPW, > pre-fetching nothing. Then the buffer gets evicted from > shared_buffers, and maybe the OS cache too. Then, before the next > checkpoint, we again replay a record for the same page. At this point, > pre-fetching should be helpful.
Sure- but if we're talking about 25GB of WAL, on a server that's got 32GB, then why would those pages end up getting evicted from memory entirely? Particularly, enough of them to end up with such a huge difference in replay time.. I do agree that if we've got more outstanding WAL between checkpoints than the system's got memory then that certainly changes things, but that wasn't what I understood the case to be here. > Admittedly, I don't quite understand whether that is what is happening > in this test case, or why SDD vs. HDD should make any difference. But > there doesn't seem to be any reason why it doesn't make sense in > theory. I agree that this could be a reason, but it doesn't seem to quite fit in this particular case given the amount of memory and WAL. I'm suspecting that it's something else and I'd very much like to know if it's a general "this applies to all (most? a lot of?) SSDs because the hardware has a larger than 8KB page size and therefore the kernel has to read it", or if it's something odd about this particular system and doesn't apply generally. Thanks, Stephen
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