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    SubjectRe: A problem about DIRECT IO on ext3
    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

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      Last update: 2005-10-17 11:19    [W:0.062 / U:1.992 seconds]
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