Re: Differential backup

Simon Riggs <simon@2ndQuadrant.com> wrote:
> Thinking about allowing a backup to tell which files have changed
> in the database since last backup. This would allow an external
> utility to copy away only changed files.
> 
> Now there's a few ways of doing this and many will say this is
> already possible using file access times.
Who would say otherwise?  Under what circumstances would PostgreSQL
modify a file without changing the "last modified" timestamp or the
file size?  If you're concerned about the converse, with daemon-
based rsync you can copy just the modified portions of a file on
which the directory information has changed.  Or is this targeting
platforms which don't have rsync?
> An explicit mechanism where Postgres could authoritatively say
> which files have changed would make many feel safer, especially
> when other databases also do this.
Why?  I must be missing something, because my feeling is that if you
can't trust your OS to cover something like this, how can you trust
any application *running* under that OS to do it?
> Is this route worthwhile?
I'm not seeing it, but I could be missing something.  Can you
describe a use case where this would be beneficial?
-Kevin