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

From: Greg 'groggy' Lehey (grog_at_FreeBSD.org)
Date: 10/07/03

  • Next message: Harti Brandt: "Re: Alignment of disk-I/O from userland." 
    Date: Tue, 7 Oct 2003 16:24:51 +0930
To: Poul-Henning Kamp <phk@phk.freebsd.dk>

    On Tuesday, 7 October 2003 at 8:29:17 +0200, Poul-Henning Kamp wrote:
    > 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).

    Ah. That's a whole different story.

    > Ttys, /dev/null and all the "typical" devices are unaffected.

    This is what I thought you meant at first, but your example using
    stdin suggested differently.

    Yes, that's not such a serious restriction.

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

    And this is a pretty good indication that we're not going to break a
    lot of things with it.

    If this only applies to special devices, I don't see a problem with
    it.

    Greg 

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    Relevant Pages

    • Re: Alignment of disk-I/O from userland.
      ... In message, Sam Leffler writes: ... >> only based on alignment alone. ... >Imposing this restriction is a major semantic change that I consider a very ...
      (freebsd-arch)
    • Re: Alignment of disk-I/O from userland.
      ... > only based on alignment alone. ... Imposing this restriction is a major semantic change that I consider a very ... support unaligned operations `by default. ...
      (freebsd-arch)