Re: A problem about DIRECT IO on ext3

[Jens Axboe <axboe@suse.de>]

On Mon, Oct 17 2005, Grzegorz Kulewski wrote:
> On Mon, 17 Oct 2005, Jens Axboe wrote:
> >>how to correct this problem ?
> >
> >See your buffer address, it's not aligned. You need to align that as
> >well. This is needed because the hardware will dma directly to the user
> >buffer, and to be on the safe side we require the same alignment as the
> >block layer will normally generate for file system io.
> >
> >So in short, just align your read buffer to the same as your block size
> >and you will be fine. Example:
> >
> >#define BS      (4096)
> >#define MASK    (BS - 1)
> >#define ALIGN(buf)      (((unsigned long) (buf) + MASK) & ~(MASK))
> >
> >char *ptr = malloc(BS + MASK);
> >char *buf = (char *) ALIGN(ptr);
> >
> >read(fd, buf, BS);
> 
> Shouldn't one use posix_memalign(3) for that?

Dunno if one 'should', one 'can' if one wants to. I prefer to do it
manually so I don't have to jump through #define hoops to get at it
(which, btw, still doesn't expose it on this machine).

-- 
Jens Axboe