Re: Differential backup

On Tue, 2010-04-27 at 09:50 -0400, Alvaro Herrera wrote:
> Simon Riggs 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.
> >
> > An explicit mechanism where Postgres could authoritatively say which
> > files have changed would make many feel safer, especially when other
> > databases also do this.
> >
> > We keep track of which files require fsync(), so we could also keep
> > track of changed files using that same information.
>
> Why file level? Seems a bit too coarse (particularly if you have large
> file support enabled). Maybe we could keep block-level last change info
> in a separate fork.

Block-level is mostly available by using LSN, you just need to scan the
file. So block level seems not useful enough for the extra overhead.

File-level would be sufficient for most purposes. If you wanted to go
finer grained you can then scan just the files that have changed.

--
Simon Riggs www.2ndQuadrant.com