Re: pg_upgrade and rsync

On Tue, Jan 27, 2015 at 09:44:51PM -0500, David Steele wrote:
> On 1/27/15 9:32 PM, Bruce Momjian wrote
> > Now, this isn't actually a problem for the first time that file is
> > backed up- the issue is if that file isn't changed again.  rsync won't
> > re-copy it, but that change that rsync missed won't be in the WAL
> > history for the *second* backup that's done (only the first), leading to
> > a case where that file would end up corrupted.
> > Interesting problem, but doesn't rsync use sub-second accuracy?
> >
> According to my empirical testing on Linux and OSX the answer is no:
> rsync does not use sub-second accuracy.  This seems to be true even on
> file systems like ext4 that support millisecond mod times, at least it
> was true on Ubuntu 12.04 running ext4.
> 
> Even on my laptop there is a full half-second of vulnerability for
> rsync.  Faster systems may have a larger window.

OK, bummer.  Well, I don't think we ever recommend to run rsync without
checksums, but the big problem is that rsync doesn't do checksums by
default.  :-(

pg_upgrade recommends using two rsyncs:
  To make a valid copy of the old cluster, use rsync</> to create  a dirty copy of the old cluster while the server is running, then shut  down the old server and run rsync</> again to update the copy  with any changes to make it consistent.  You might want to exclude some

I am afraid that will not work as it could miss changes, right?  When
would the default mod-time checking every be safe?

--  Bruce Momjian          http://momjian.us EnterpriseDB                             http://enterprisedb.com
 + Everyone has their own god. +