Re: [HACKERS] [patch] reorder tablespaces in basebackup tar streamfor backup_label

Hi,

sorry, it took me a while to get back to this.

Am Freitag, den 03.03.2017, 15:44 +0900 schrieb Michael Paquier:
> On Wed, Feb 22, 2017 at 9:23 PM, Bernd Helmle <mailings@oopsware.de> wrote:
> > The comment in the code says explicitely to add the base directory to
> > the end of the list, not sure if that is out of a certain reason.
> >
> > I'd say this is an oversight in the implementation. I'm currently
> > working on a tool using the streaming protocol directly and i've
> > understood it exactly the way, that the default tablespace is the first
> > one in the stream.
> >
> > So +1 for the patch.
> 
> Commit 507069de has switched the main directory from the beginning to
> the end of the list, and the thread about this commit is here:
> https://www.postgresql.org/message-id/AANLkTikgmZRkBuQ%2B_hcwPBv7Cd7xW48Ev%3DUBHA-k4v0W%40mail.gmail.com
> 
> +       /* Add a node for the base directory at the beginning.  This way, the
> +        * backup_label file is always the first file to be sent. */
>         ti = palloc0(sizeof(tablespaceinfo));
>         ti->size = opt->progress ? sendDir(".", 1, true, tablespaces,
> true) : -1;
> -       tablespaces = lappend(tablespaces, ti);
> +       tablespaces = lcons(ti, tablespaces);
> So, the main directory is located at the end on purpose. When using
> --wal-method=fetch the WAL segments are part of the main tarball, so
> if you send the main tarball first you would need to generate a second
> tarball with the WAL segments that have been generated between the
> moment the main tarball has finished until the end of the last
> tablespace taken if you want to have a consistent backup. 

Ah, thanks for pointing that out, I've missed that in my testing.

> Your patch would work with the stream mode though.

Or, if not requesting the "WAL" option of the replication protocol's
BASE_BACKUP command.

I agree it doesn't make sense to start messing with fetch mode, but I
don't think we guarantee any ordering of tablespaces (to wit, Bernd was
pretty sure it was the other way around all the time), neither do I
think having the main tablespace be the first for non-WAL/stream and the
last for WAL/fetch would be confusing personally, though I understand
this is debatable.

So I've updated the patch to only switch the main tablespace to be first
in case WAL isn't included, please find it attached.


Michael

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