Re: file cloning in pg_upgrade and CREATE DATABASE

I have made a major revision of this patch.

I have removed all the changes to CREATE DATABASE. That was too
contentious and we got lost in unrelated details there. The real
benefit is for pg_upgrade.

Another point was that for pg_upgrade use a user would like to know
beforehand whether reflinking would be used, which was not possible with
the copy_file_range() API. So here I have switched to using the ioctl()
call directly.

So the new interface is that pg_upgrade has a new option
--reflink={always,auto,never}. (This option name is adapted from GNU
cp.) From the documentation:

<para>
The setting <literal>always</literal> requires the use of relinks. If
they are not supported, the <application>pg_upgrade</application> run
will abort. Use this in production to limit the upgrade run time.
The setting <literal>auto</literal> uses reflinks when available,
otherwise it falls back to a normal copy. This is the default. The
setting <literal>never</literal> prevents use of reflinks and always
uses a normal copy. This can be useful to ensure that the upgraded
cluster has its disk space fully allocated and not shared with the old
cluster.
</para>

Also, pg_upgrade --check will check whether the selected option would work.

--
Peter Eisentraut http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services