Re: Use of rsync for data directory copying

On Mon, Jul 9, 2012 at 08:00:48PM -0700, David Kerr wrote:
>
> On Jul 9, 2012, at 7:48 PM, Bruce Momjian wrote:
>
> > Rsync is popular with Postgres users, but I don't understand how they
> > are using the default check mode (file size, modification time) to
> > synchronize shut-down data directories? It seems they would have to use
> > --checksum because it is too easy for files to change in the same
> > second, and for a backend to write into the middle of a file.
> >
> > Is everyone who is using rsync with Postgres also using --checksum mode?
>
>
> I must be missing something, if they're shut down you can't write to them. =)
>
> I do use rsync though for resyncing my mirror's, it's been working great so far. I assume
> that the WAL fixes anything that gets goofed up in the copy. (hopefully I've been assuming correctly.)

If two writes happens in the middle of a file in the same second, it
seems one might be missed. Yes, I suppose the WAL does fix that during
replay, though if both servers were shut down cleanly, WAL would not be
replayed.

If you using it for a hot backup, and WAL would clean that up.

--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com

+ It's impossible for everything to be true. +