Re: Alignment of disk-I/O from userland.

From: Poul-Henning Kamp (phk_at_phk.freebsd.dk)
Date: 10/07/03

  • Next message: Greg 'groggy' Lehey: "Re: Alignment of disk-I/O from userland." 
    To: Sam Leffler <sam@errno.com>
Date: Tue, 07 Oct 2003 08:29:17 +0200

    In message <200310061753.28562.sam@errno.com>, Sam Leffler writes:
    >On Monday 06 October 2003 04:11 pm, Poul-Henning Kamp wrote:
    >> In message <20031006163218.L55190@pooker.samsco.home>, Scott Long writes:
    > ...stuff deleted...
    >> >As for returning an error code for a buffer that we (arbitrarily) believe
    >> >to be too big to align, [...]
    >>
    >> I have never advocated returning an error based on "alignment and size",
    >> only based on alignment alone.
    >
    >Imposing this restriction is a major semantic change that I consider a very
    >bad idea. You are basically imposing the semantics of O_DIRECT on all i/o
    >operations going to a device. I think it is important to give best effort to
    >support unaligned operations `by default. I can imagine restricting this to
    >some upper size bound but existing applications, regardless of how well you
    >consider them to be written, must continue to work.

    Now now, you are missing two of the finer points:

    1: Not "on all i/o operations going to a device", but rather "on i/o
        operations which take the physread/write fast-path to avoid a copyin/out
        overhead." (disks and tapes mostly). Ttys, /dev/null and all the
        "typical" devices are unaffected.

    2: Right now we _do_ impose this restriction, but our error-reporting
        is wildly inaccurate. 

    -- 
Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG         | TCP/IP since RFC 956
FreeBSD committer       | BSD since 4.3-tahoe    
Never attribute to malice what can adequately be explained by incompetence.
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  • Next message: Greg 'groggy' Lehey: "Re: Alignment of disk-I/O from userland."

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    • Re: Alignment of disk-I/O from userland.
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