Re: file cloning in pg_upgrade and CREATE DATABASE

On Tue, Oct 02, 2018 at 11:07:06PM -0300, Alvaro Herrera wrote:
> I see. Peter is proposing to have a fourth mode, essentially
> --transfer-mode=clone-or-copy.

Yes, a mode which depends on what the file system supports. Perhaps
"safe" or "fast" could be another name, in the shape of the fastest
method available which does not destroy the existing cluster's data.

> Thinking about a generic tool that wraps pg_upgrade (say, Debian's
> wrapper for it) this makes sense: just use the fastest non-destructive
> mode available. Which ones are available depends on what the underlying
> filesystem is, so it's not up to the tool's writer to decide which to
> use ahead of time.

This could have merit. Now, it seems to me that we have two separate
concepts here, which should be addressed separately:
1) cloning file mode, which is the new actual feature.
2) automatic mode, which is a subset of the copy mode and the new clone
mode. What I am not sure when it comes to that stuff is if we should
consider in some way the link mode as being part of this automatic
selection concept..
--
Michael