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 <command>rsync</> to create
a dirty copy of the old cluster while the server is running, then shut
down the old server and run <command>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 <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com

+ Everyone has their own god. +