Alignment of disk-I/O from userland.

[Poul-Henning Kamp]

In message <200310061753.28562.sam at errno.com>, Sam Leffler writes:
>On Monday 06 October 2003 04:11 pm, Poul-Henning Kamp wrote:
>> In message <20031006163218.L55190 at 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
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