The sub-second timestamps available on many filesystems are preserved when requested across copies, but aren't used to determine file differences.

If a file exists at both origin and destination and its contents the same size in each place, and the timestamps only differ in the sub-second resolution, rsync will treat the files as the same (unless you use --checksum).

So if a file is created, and then a snapshot of its dir is taken, then the origin file is modified (but the size is preserved) within the same second, an attempt to update that snapshot using rsync will fail to copy the change.

Here's a script that reproduces the issue with high reliability for me:

#!/bin/bash                                                                                                                                                                                              

set -x

DIR=$(mktemp -d -p $(pwd))

mkdir $DIR/d1
mkdir $DIR/d2

echo dummy > $DIR/d1/dummy
echo dummy > $DIR/d2/dummy

echo one > $DIR/d1/afile
sleep 0.1
echo two > $DIR/d2/afile

/usr/bin/stat $DIR/d1/afile | grep Mod
/usr/bin/stat $DIR/d2/afile | grep Mod

~/packages/rsync/rsync --delete -a -HAX -vii $DIR/d2/ $DIR/d1

diff -r $DIR/d1 $DIR/d2

/usr/bin/stat $DIR/d1/afile | grep Mod
/usr/bin/stat $DIR/d2/afile | grep Mod



If the diff shows a difference, then the rsync didn't copy afile's contents over. However, note the stat info from the last two lines - the updated modify timestamp *will* be synced, making an inconsistent sync.

The following patch adds a check of the high-res timestamp to unchanged_file. This solves the problem for me, and I've guarded it so it shouldn't break on systems with no high-res timestamp. Please let me know if I can be helpful in testing it further or making it more robust.



diff --git a/generator.c b/generator.c
index 3a4504f..2f64f5d 100644
--- a/generator.c
+++ b/generator.c
@@ -588,7 +588,11 @@ int unchanged_file(char *fn, struct file_struct *file, STRUCT_STAT *st)
        if (ignore_times)
                return 0;
 
-       return cmp_time(st->st_mtime, file->modtime) == 0;
+       return cmp_time(st->st_mtime, file->modtime) == 0
+#ifdef ST_MTIME_NSEC
+               && st->ST_MTIME_NSEC == F_MOD_NSEC(file)
+#endif
+               ;
 }