[patch] open.2: Update note about the alignment of the user buffer and the file offset for O_DIRECT flag

[Peter Schiffer <pschiffe-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>]

[-- Attachment #1: Type: text/plain, Size: 377 bytes --]

Hello Michael,

the sentence in open(2) man page in notes for O_DIRECT flag:
"Under Linux 2.6, alignment to 512-byte boundaries suffices."

is not universally correct. The alignment is a property of the storage, 
for example, 4k-sector drives with no 512 byte sector emulation will be 
unable to perform 512-byte direct I/O.

The patch clarifies this sentence.

Thanks,

peter

[-- Attachment #2: open.2.patch --]
[-- Type: text/x-patch, Size: 695 bytes --]

diff --git a/man2/open.2 b/man2/open.2
index 389945f..c80686d 100644
--- a/man2/open.2
+++ b/man2/open.2
@@ -1322,7 +1322,16 @@ 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 filesystem.
-Under Linux 2.6, alignment to 512-byte boundaries suffices.
+Under Linux 2.6 and newer, alignment to the logical block size of the
+underlying storage (typically 512 bytes) suffices.
+Logical block size can be determined with
+.BR ioctl (2)
+.B BLKSSZGET
+system call or
+.BR blockdev (8)
+command with
+.B --getss
+parameter.
 .LP
 .B O_DIRECT
 I/Os should never be run concurrently with the