statfs.2: Add notes on statfs64() and fstatfs64()

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man2/statfs.2

@@ -23,7 +23,7 @@
 .\" Modified 2003-08-17 by Walter Harms
 .\" Modified 2004-06-23 by Michael Kerrisk <mtk.manpages@gmail.com>
 .\"
-.TH STATFS 2 2010-09-04 "Linux" "Linux Programmer's Manual"
+.TH STATFS 2 2010-11-21 "Linux" "Linux Programmer's Manual"
 .SH NAME
 statfs, fstatfs \- get file system statistics
 .SH SYNOPSIS
@@ -192,13 +192,27 @@ The Linux
 was inspired by the 4.4BSD one
 (but they do not use the same structure).
 .SH NOTES
-The kernel has system calls
-.BR statfs (),
-.BR fstatfs (),
-.BR statfs64 (),
+The original Linux
+.BR statfs ()
+and
+.BR fstatfs ()
+system calls were not designed with extremely large file sizes in mind.
+Subsequently, Linux 2.6
+added new
+.BR statfs64 ()
 and
 .BR fstatfs64 ()
-to support this library call.
+system calls that employ a new structure,
+.IR statfs64 .
+The new structure contains the same fields as the original
+.I statfs
+structure, but the sizes of various fields are increased,
+to accommodate large file sizes.
+The glibc
+.BR statfs ()
+and
+.BR fstatfs ()
+wrapper functions transparently deal with the kernel differences.
 
 Some systems only have \fI<sys/vfs.h>\fP, other systems also have
 \fI<sys/statfs.h>\fP, where the former includes the latter.