From: | Thomas Munro <thomas(dot)munro(at)gmail(dot)com> |
---|---|
To: | Heikki Linnakangas <hlinnaka(at)iki(dot)fi> |
Cc: | Andres Freund <andres(at)anarazel(dot)de>, Noah Misch <noah(at)leadboat(dot)com>, Peter Smith <smithpb2250(at)gmail(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com>, vignesh C <vignesh21(at)gmail(dot)com>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>, Melanie Plageman <melanieplageman(at)gmail(dot)com> |
Subject: | Re: Relation bulk write facility |
Date: | 2024-02-24 22:37:50 |
Message-ID: | CA+hUKGLuG2s4-+4DH+JDJh0CiSGU8SbGfK1ZXYWkMQS7iHdGxw@mail.gmail.com |
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On Sun, Feb 25, 2024 at 11:16 AM Thomas Munro <thomas(dot)munro(at)gmail(dot)com> wrote:
> On Sun, Feb 25, 2024 at 11:06 AM Heikki Linnakangas <hlinnaka(at)iki(dot)fi> wrote:
> > Regarding the issue at hand, perhaps we should define PG_IO_ALIGN_SIZE as 16 on AIX, if that's the best the linker can do on that platform.
>
> You'll probably get either an error or silently fall back to buffered
> I/O, if direct I/O is enabled and you try to read/write a badly
> aligned buffer. That's documented (they offer finfo() to query it,
> but it's always 4KB for the same sort of reasons as it is on every
> other OS).
I guess it's the latter ("to work efficiently" sounds like it isn't
going to reject the request):
https://www.ibm.com/docs/en/aix/7.3?topic=tuning-direct-io
If you make it < 4KB then all direct I/O would be affected, not just
this one place, so then you might as well just not allow direct I/O on
AIX at all, to avoid giving a false impression that it does something.
(Note that if we think the platform lacks O_DIRECT we don't make those
assertions about alignment).
FWIW I'm aware of one other thing that is wrong with our direct I/O
support on AIX: it should perhaps be using a different flag. I
created a wiki page to defer thinking about any AIX issues
until/unless at least one real, live user shows up, which hasn't
happened yet: https://wiki.postgresql.org/wiki/AIX
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