[Bug 7249] Add an option to use O_NOATIME

[samba-bugs at samba.org]

https://bugzilla.samba.org/show_bug.cgi?id=7249

--- Comment #6 from don311 at gmx.com ---
+1 from here as well.

I understand the arguments about the limited usefulness of the atime field in
general, and I agree with much of it, but to me there are more use cases than
what's mentioned in what I've read so far - such as providing helpful clues for
certain kinds of "forensic" debugging.  atime has been around for a very long
time (every *NIX variant I've worked on, going back to the 1980's), so even
though its utility may be a bit "specialized", there are clearly people who are
surprised to see it ignored by a tool like rsync.  IMO back-up/replication/sync
tools should strive to record or replicate the "originals" as faithfully as
possible[*], with minimal impact on the contents or meta-data of files being
backed up or replicated.

That is to say, one should ideally be able to use rsync to do things like
moving a directory tree to a new place (file system, machine), or
copying/sync'ing it to a back-up location, and later restoring the originals,
without the user of those files being able to discern any differences in them.

I built rsync with Nicolas George's O_NOATIME patch on the latest Git, and IMO
this change brings rsync one step closer to that ideal (on Linux at least).  It
not only avoids clobbering the atime values for the original files as they're
being read during a first copy operation, but also when they're being
checksummed (on both sides) if one uses the "-c" option in a later re-sync.

Martin von Gagern: That's the main reason that I think "--noatime" should be
sent to the remote side.  I agree that in some/many cases, rsync users would
not care about atimes being preserved in the copy.  For backups, supposing
atimes had been copied to the backup machine[*] (perhaps not part of the
scenario you had in mind), they would need to be preserved during re-syncs in
order to be able to available when sync'd back to the original place during a
restore.

One problem with unconditionally sending a new option to the remote side is
that if it isn't supported on the remote side, it fails, whereas I would still
see benefit (for some use cases) in using O_NOATIME on the local side only in
that case.  Not sure how easy it would be to handle that situation.

Aside from that, I would really like to see this enhancement be integrated as
soon as possible.  Since it's an option, and doesn't affect the protocol etc,
it doesn't seem as though it would have any negative impact when it isn't being
used.  Seems like a win-win. :)  Could it be integrated before the next
release?


[*] Which brings us to the --atime patch -- but that's another topic.

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