Messages in this thread | | | Date | Mon, 17 Oct 2005 11:17:10 +0200 | From | Jens Axboe <> | Subject | Re: A problem about DIRECT IO on ext3 |
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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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