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