[Bug 11521] rsync does not use high-resolution timestamps to determine file differences

[f-rsync at media.mit.edu]

    > Date: Sun, 24 Jan 2016 15:43:20 -0800
    > From: Wayne Davison <wayne at thedavisons.net>

A couple questions below; please bear with me.

    > No, if you do a ext4 -> ext4 copy, rsync has set the matching ns info for
    > transferred files since 3.1.0. There was a case prior to rsync 3.1.2 where
    > a brand-new file transferred in the same second it was created wouldn't get
    > the right ns value because rsync was optimizing away the time-set if the
    > file's mod-time matched in the integer part (3.1.2 fixed that).

Oh, I see what happened.  My problem is that no Ubuntu LTS before
14.04 had rsync 3.1.0 or newer, and the original capability took more
than four years to make it into a released rsync version, if I'm
reading the release notes correctly.*  Unfortunately, that means
the vast majority of my machine base predates the fix, including the
machine hosting the backups.  I can obviously install newer rsyncs,
but that gives me a big installed base of pre-fix data that I'm going
to have to fix, and no more rsync security updates unless I track them
manually.  Yet I'd rather do this now, so I'm future-proofed, than
be badly surprised some years down the road when this rsync behavior
becomes the default, and to keep the problem from continuing to get worse.

* I may have tried 3.1.0 at some point and then realized the problems
  it'd give me for backups and didn't install it everywhere, pending
  a better fix; this is starting to ring bells.  I'm really surprised
  that the initial patch of Sep 7, 2009 never made it into a released
  rsync until 3.1.0 of Sep 28 2013; that's a four-year delay, and
  explains why I obviously never tried this out until perhaps an
  experiment with 3.1.0, and no doubt I didn't want to run a private
  version that wouldn't get security updates.  Ubuntu rsync versions:
  10.04 has 3.0.7 proto 30; 12.04 has 3.0.9 p 30; 14.04 has 3.1.0 p 31.)

So what it looks like is that the capability to transfer ns times at
all existed in CVS but not released since 2009, in released since
2013, and in an Ubuntu LTS since 2014.  And the current patch -seems-
to be an optimization that avoids -comparison- if the ns times match,
but that only affects speed---it doesn't change what gets written in
any event, just how fast.  Right?  But actually I think it changes
behavior besides that---see my test case below.

    > Beginning with this patch you can run rsync -aiv --checksum - at -1 and have
    > it fix the full mod-time on any matching files it finds. But for most newer
    > backups, the ns time will already be set correctly (as long as it was
    > created using a new enough rsync and protocol 31). If someone has a large
    > link-dest hierarchy that predates 3.1.0, then you could be sharing
    > hard-linked matching files from back before the ns info was included (the
    > older files would all have 0 for the ns value).

Wow, that -c really hurts.  If one wanted to live dangerously---with
the assumption that two files that otherwise match in all metadata
(including obviously length :) but whose timestamps differ in that one
has integer seconds and the other has the same integer seconds but
also nanoseconds, can rsync readjust the dates, without doing a full
checksum?  If not, I may write such a tool, or do it the (very) slow
way and have rsync re-checksum a few terabytes of my backups... :)
[Might find some bitrot that way, of course.]

Also, I actually -can't- use that command to fix my snapshots,
because (if I understand correctly), it will -alter- my existing
snapshots to match the -current- contents of files, destroying them
---I'll no longer be able to go back in time to a previous version.
I only want to update ns times on files in the older snapshots
if and only if changing integer times to ns times would be the
only modification.  I think rsync -ac - at -1 will do far more, yes?

As for - at -1, that introduces a surprising change in behavior when I
try it.  I'm unsure if it's intended, though I think it is.  But it
will -definitely- break my hardlinks and bloat the backups if I try
it without readjusting the dates in --link-dest directories (e.g.,
previous snapshots).  I find that specifying - at -1 copies the ns
timestamp from the source to the destination even if the --link-dest
directory has an integer timestamp, and so I assume this is part of
the purpose of the patch?  Not just an optimization, but a change in
the way --link-dest might work.  Observe:

22:57:22 ~$ mkdir T
22:57:25 ~$ cd T
22:57:26 ~/T$ mkdir 1 2 3 4 5 6
22:57:30 ~/T$ lat() { ls -alF -i --full-time "$@"; }
22:57:49 ~/T$ touch 1/foo
22:57:53 ~/T$ ln 1/foo 1/bar
22:57:56 ~/T$ lat */*
1321175 -rw-r--r-- 2 user user 0 2016-01-24 22:57:53.013689572 -0500 1/bar
1321175 -rw-r--r-- 2 user user 0 2016-01-24 22:57:53.013689572 -0500 1/foo
22:57:59 ~/T$ rsync -aviH 1/ 2/
sending incremental file list
.d..t...... ./
>f+++++++++ foo
hf+++++++++ bar => foo

sent 139 bytes  received 53 bytes  384.00 bytes/sec
total size is 0  speedup is 0.00
22:58:13 ~/T$ lat */*
1321175 -rw-r--r-- 2 user user 0 2016-01-24 22:57:53.013689572 -0500 1/bar
1321175 -rw-r--r-- 2 user user 0 2016-01-24 22:57:53.013689572 -0500 1/foo
1321176 -rw-r--r-- 2 user user 0 2016-01-24 22:57:53.013689572 -0500 2/bar
1321176 -rw-r--r-- 2 user user 0 2016-01-24 22:57:53.013689572 -0500 2/foo
22:58:17 ~/T$ rsync -aviH --link-dest=../2 1/ 3/
sending incremental file list
.d..t...... ./

sent 89 bytes  received 19 bytes  216.00 bytes/sec
total size is 0  speedup is 0.00
22:59:05 ~/T$ lat */*
1321175 -rw-r--r-- 2 user user 0 2016-01-24 22:57:53.013689572 -0500 1/bar
1321175 -rw-r--r-- 2 user user 0 2016-01-24 22:57:53.013689572 -0500 1/foo
1321176 -rw-r--r-- 4 user user 0 2016-01-24 22:57:53.013689572 -0500 2/bar
1321176 -rw-r--r-- 4 user user 0 2016-01-24 22:57:53.013689572 -0500 2/foo
1321176 -rw-r--r-- 4 user user 0 2016-01-24 22:57:53.013689572 -0500 3/bar
1321176 -rw-r--r-- 4 user user 0 2016-01-24 22:57:53.013689572 -0500 3/foo
22:59:10 ~/T$ touch --date="2016-01-24 22:57:53 -0500" 2/bar
23:00:07 ~/T$ lat */*
1321175 -rw-r--r-- 2 user user 0 2016-01-24 22:57:53.013689572 -0500 1/bar
1321175 -rw-r--r-- 2 user user 0 2016-01-24 22:57:53.013689572 -0500 1/foo
1321176 -rw-r--r-- 4 user user 0 2016-01-24 22:57:53.000000000 -0500 2/bar
1321176 -rw-r--r-- 4 user user 0 2016-01-24 22:57:53.000000000 -0500 2/foo
1321176 -rw-r--r-- 4 user user 0 2016-01-24 22:57:53.000000000 -0500 3/bar
1321176 -rw-r--r-- 4 user user 0 2016-01-24 22:57:53.000000000 -0500 3/foo
23:00:09 ~/T$ rsync -aviH --link-dest=../2 1/ 4/
sending incremental file list
.d..t...... ./

sent 89 bytes  received 19 bytes  216.00 bytes/sec
total size is 0  speedup is 0.00
23:00:36 ~/T$ lat */*
1321175 -rw-r--r-- 2 user user 0 2016-01-24 22:57:53.013689572 -0500 1/bar
1321175 -rw-r--r-- 2 user user 0 2016-01-24 22:57:53.013689572 -0500 1/foo
1321176 -rw-r--r-- 6 user user 0 2016-01-24 22:57:53.000000000 -0500 2/bar
1321176 -rw-r--r-- 6 user user 0 2016-01-24 22:57:53.000000000 -0500 2/foo
1321176 -rw-r--r-- 6 user user 0 2016-01-24 22:57:53.000000000 -0500 3/bar
1321176 -rw-r--r-- 6 user user 0 2016-01-24 22:57:53.000000000 -0500 3/foo
1321176 -rw-r--r-- 6 user user 0 2016-01-24 22:57:53.000000000 -0500 4/bar
1321176 -rw-r--r-- 6 user user 0 2016-01-24 22:57:53.000000000 -0500 4/foo
23:00:38 ~/T$ ~/rsync-HEAD-20160124-1917GMT/rsync -aviH --link-dest ../2/ 1/ 5/
sending incremental file list
.d..t...... ./

sent 89 bytes  received 19 bytes  216.00 bytes/sec
total size is 0  speedup is 0.00
23:00:56 ~/T$ lat */*
1321175 -rw-r--r-- 2 user user 0 2016-01-24 22:57:53.013689572 -0500 1/bar
1321175 -rw-r--r-- 2 user user 0 2016-01-24 22:57:53.013689572 -0500 1/foo
1321176 -rw-r--r-- 8 user user 0 2016-01-24 22:57:53.000000000 -0500 2/bar
1321176 -rw-r--r-- 8 user user 0 2016-01-24 22:57:53.000000000 -0500 2/foo
1321176 -rw-r--r-- 8 user user 0 2016-01-24 22:57:53.000000000 -0500 3/bar
1321176 -rw-r--r-- 8 user user 0 2016-01-24 22:57:53.000000000 -0500 3/foo
1321176 -rw-r--r-- 8 user user 0 2016-01-24 22:57:53.000000000 -0500 4/bar
1321176 -rw-r--r-- 8 user user 0 2016-01-24 22:57:53.000000000 -0500 4/foo
1321176 -rw-r--r-- 8 user user 0 2016-01-24 22:57:53.000000000 -0500 5/bar
1321176 -rw-r--r-- 8 user user 0 2016-01-24 22:57:53.000000000 -0500 5/foo
23:00:59 ~/T$ ~/rsync-HEAD-20160124-1917GMT/rsync - at -1 -aviH --link-dest ../2/ 1/ 6/
sending incremental file list
.d..t...... ./
>f..t...... bar
hf+++++++++ foo => bar

sent 136 bytes  received 50 bytes  372.00 bytes/sec
total size is 0  speedup is 0.00
23:01:13 ~/T$ lat */*
1321175 -rw-r--r-- 2 user user 0 2016-01-24 22:57:53.013689572 -0500 1/bar
1321175 -rw-r--r-- 2 user user 0 2016-01-24 22:57:53.013689572 -0500 1/foo
1321176 -rw-r--r-- 8 user user 0 2016-01-24 22:57:53.000000000 -0500 2/bar
1321176 -rw-r--r-- 8 user user 0 2016-01-24 22:57:53.000000000 -0500 2/foo
1321176 -rw-r--r-- 8 user user 0 2016-01-24 22:57:53.000000000 -0500 3/bar
1321176 -rw-r--r-- 8 user user 0 2016-01-24 22:57:53.000000000 -0500 3/foo
1321176 -rw-r--r-- 8 user user 0 2016-01-24 22:57:53.000000000 -0500 4/bar
1321176 -rw-r--r-- 8 user user 0 2016-01-24 22:57:53.000000000 -0500 4/foo
1321176 -rw-r--r-- 8 user user 0 2016-01-24 22:57:53.000000000 -0500 5/bar
1321176 -rw-r--r-- 8 user user 0 2016-01-24 22:57:53.000000000 -0500 5/foo
1321177 -rw-r--r-- 2 user user 0 2016-01-24 22:57:53.013689572 -0500 6/bar
1321177 -rw-r--r-- 2 user user 0 2016-01-24 22:57:53.013689572 -0500 6/foo
23:01:16 ~/T$ rsync --version
rsync  version 3.1.0  protocol version 31
Copyright (C) 1996-2013 by Andrew Tridgell, Wayne Davison, and others.
Web site: http://rsync.samba.org/
Capabilities:
    64-bit files, 64-bit inums, 64-bit timestamps, 64-bit long ints,
    socketpairs, hardlinks, symlinks, IPv6, batchfiles, inplace,
    append, ACLs, xattrs, iconv, symtimes, prealloc

rsync comes with ABSOLUTELY NO WARRANTY.  This is free software, and you
are welcome to redistribute it under certain conditions.  See the GNU
General Public Licence for details.
23:07:25 ~/T$ ~/rsync-HEAD-20160124-1917GMT/rsync --version
rsync  version 3.1.3dev  protocol version 31
Copyright (C) 1996-2015 by Andrew Tridgell, Wayne Davison, and others.
Web site: http://rsync.samba.org/
Capabilities:
    64-bit files, 64-bit inums, 64-bit timestamps, 64-bit long ints,
    socketpairs, hardlinks, symlinks, IPv6, batchfiles, inplace,
    append, no ACLs, xattrs, iconv, symtimes, prealloc

rsync comes with ABSOLUTELY NO WARRANTY.  This is free software, and you
are welcome to redistribute it under certain conditions.  See the GNU
General Public Licence for details.
23:07:32 ~/T$ cat /etc/issue
Ubuntu 14.04.3 LTS \n \l

23:08:21 ~/T$