| From: | Robert Haas <robertmhaas(at)gmail(dot)com> |
|---|---|
| To: | Stephen Frost <sfrost(at)snowman(dot)net> |
| Cc: | Andres Freund <andres(at)anarazel(dot)de>, Sait Talha Nisanci <Sait(dot)Nisanci(at)microsoft(dot)com>, Thomas Munro <thomas(dot)munro(at)gmail(dot)com>, Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>, Dmitry Dolgov <9erthalion6(at)gmail(dot)com>, David Steele <david(at)pgmasters(dot)net>, Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: [EXTERNAL] Re: WIP: WAL prefetch (another approach) |
| Date: | 2020-08-27 20:19:23 |
| Message-ID: | CA+TgmoYSnACj2ouDKVO+vJ=3r_=ChhmE2mvy2DGOKbpDeTYG+Q@mail.gmail.com |
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| Lists: | pgsql-hackers |
On Thu, Aug 27, 2020 at 2:51 PM Stephen Frost <sfrost(at)snowman(dot)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.
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.
--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company
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