Date: Tue, 07 Oct 2003 08:29:17 +0200 From: "Poul-Henning Kamp" <phk@phk.freebsd.dk> To: Sam Leffler <sam@errno.com> Cc: arch@freebsd.org Subject: Re: Alignment of disk-I/O from userland. Message-ID: <32324.1065508157@critter.freebsd.dk> In-Reply-To: Your message of "Mon, 06 Oct 2003 17:53:28 PDT." <200310061753.28562.sam@errno.com>
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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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