Jeff, On Thu, Mar 28, 2013 at 6:28 PM, Jeff Moyer <jmoyer@xxxxxxxxxx> wrote: > Hi, > > The alignment text for O_DIRECT is slightly misleading: > > Under Linux 2.4, transfer sizes, and the alignment of the user buffer > and the file offset must all be multiples of the logical block size of > the file system. Under Linux 2.6, alignment to 512-byte boundaries > suffices. > > The last sentence is incorrect. You cannot perform O_DIRECT I/O in > sizes smaller than the underlying logical block size (for block devices > and block-based file systems; nfs is a different beast). I've attached > my proposed change. Comments and word-smithing are most welcome. Could you provide some detail on how you determined, verified, or tested this detail? I'm not saying you're wrong, but that text has been there a long time (since before I was maintainer), in the days when there was no VCS and changelog. It's even possible that I sent the patch to Andries that added this piece, since I have an idea that I tested this detail on Linux 2.6 at some point in the distant past. Indeed, I just now did a (very) quick test using an old program that suggest that you can do O_DIRECT I/O with block sizes smaller than the LBS. Thanks, Michael > Signed-off-by: Jeff Moyer <jmoyer@xxxxxxxxxx> > > diff --git a/man2/open.2 b/man2/open.2 > index e518c1f..df92d6a 100644 > --- a/man2/open.2 > +++ b/man2/open.2 > @@ -787,7 +787,8 @@ operation in > Under Linux 2.4, transfer sizes, and the alignment of the user buffer > and the file offset must all be multiples of the logical block size > of the file system. > -Under Linux 2.6, alignment to 512-byte boundaries suffices. > +Under Linux 2.6, alignment to the underlying block device's logical > +block size suffices. > .LP > .B O_DIRECT > I/Os should never be run concurrently with the -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Author of "The Linux Programming Interface"; http://man7.org/tlpi/ -- To unsubscribe from this list: send the line "unsubscribe linux-man" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
