Re: write alignment matters?

[Matt Thomas <matt%3am-software.com@localhost>]

On Jun 23, 2011, at 4:41 AM, Thor Lancelot Simon wrote:

> On Thu, Jun 23, 2011 at 03:36:25PM +0700, Robert Elz wrote:
>>    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).
> 
> That doesn't seem like it can really be right.  There are plenty of
> systems where devices cannot DMA from user addresses.

Really?  That seems unlikely.