Re: write alignment matters?

[Robert Elz <kre%munnari.OZ.AU@localhost>]

    Date:        Wed, 22 Jun 2011 19:30:55 -0400 (EDT)
    From:        der Mouse <mouse%Rodents-Montreal.ORG@localhost>
    Message-ID:  <201106222330.TAA28359%Sparkle.Rodents-Montreal.ORG@localhost>

  | But the interface is much older than that, and, even if it's not
  | codified, there's a lot of history behind the notion that userland
  | alignment of write() buffers affects, at most, performance, to the
  | point where I consider it part of the interface.

Not on access to raw devices it isn't, and never was - what Erik Fair
said (Message-id: <5F005E6A-5441-4BEC-BB3C-4A9B79584BFB%netbsd.org@localhost>)
was 100% correct - if you're using a raw device, it is up to the
application to meet whatever the requirements of that particular device
are, because one of the properties of raw devices is that they don't
do any kind of rebuffering of data (and the driver must not - that is
a part of the interface contract).

What the rules are vary from device to device, if you don't like this,
don't use raw devices.   If you want to function on a large subset
(possibly all) raw devices you need to make your code extremely
pessimistic about what it can do (align to 4K or so boundary, use
sizes a multiple of 512, and no bigger than 64KB).

kre