| From: | SATYANARAYANA NARLAPURAM <satyanarlapuram(at)gmail(dot)com> |
|---|---|
| To: | Simon Riggs <simon(dot)riggs(at)enterprisedb(dot)com> |
| Cc: | Tomas Vondra <tomas(dot)vondra(at)enterprisedb(dot)com>, Thomas Munro <thomas(dot)munro(at)gmail(dot)com>, Julien Rouhaud <rjuju123(at)gmail(dot)com>, Ashutosh Sharma <ashu(dot)coek88(at)gmail(dot)com>, Daniel Gustafsson <daniel(at)yesql(dot)se>, Stephen Frost <sfrost(at)snowman(dot)net>, Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>, Andres Freund <andres(at)anarazel(dot)de>, David Steele <david(at)pgmasters(dot)net>, Dmitry Dolgov <9erthalion6(at)gmail(dot)com>, Jakub Wartak <Jakub(dot)Wartak(at)tomtom(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: WIP: WAL prefetch (another approach) |
| Date: | 2022-04-12 18:04:41 |
| Message-ID: | CAHg+QDdokr2ifE4m5on=ynLX+5gs=sTu5q67JWKw0=qDjLWrMQ@mail.gmail.com |
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| Lists: | pgsql-hackers |
> Other way around. FPWs make prefetch unnecessary.
> Therefore you would only want prefetch with FPW=off, AFAIK.
>
A few scenarios I can imagine page prefetch can help are, 1/ A DR replica
instance that is smaller instance size than primary. Page prefetch can
bring the pages back into memory in advance when they are evicted. This
speeds up the replay and is cost effective. 2/ Allows larger
checkpoint_timeout for the same recovery SLA and perhaps improved
performance? 3/ WAL prefetch (not pages by itself) can improve replay by
itself (not sure if it was measured in isolation, Tomas V can comment on
it). 4/ Read replica running analytical workload scenario Tomas V mentioned
earlier.
>
> Or put this another way: when is it safe and sensible to use
> recovery_prefetch != off?
>
When checkpoint_timeout is set large and under heavy write activity, on a
read replica that has working set higher than the memory and receiving
constant updates from primary. This covers 1 & 4 above.
> --
> Simon Riggs http://www.EnterpriseDB.com/
>
>
>
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