Re: A problem about DIRECT IO on ext3

[Grzegorz Kulewski <kangur@polcom.net>]

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?


Grzegorz Kulewski