Re: Tablespaces

Gavin Sherry wrote:
> On Tue, 2 Mar 2004, Bruce Momjian wrote:
> 
> > Gavin Sherry wrote:
> > > Actually, I think that's a pretty good idea :-). I'd solves a bunch of
> > > issues in the backend (postmaster start up can recurse through
> > > $PGDATA/tablespaces looking for postmaster.pid files) and will also assist
> > > admins with complex configurations (perhaps).
> >
> > Why are you asking about postmaster.pid files.  That file goes in the
> > top level /data directory, no?
> 
> I was trying to be paranoid about users who have multiple postmasters on
> the same machine and want to share a table space while both systems are
> live. There'd be no mechanism to test for that situation if we didn't have
> something like a postmaster.pid file. Is this being a little too paranoid?

Oh, yikes, I see.  Right now we have the interlock on the /data
directory, but once you start moving stuff out from under /data using
tablespaces, we do perhaps loose the interlock.  However, I assume the
CREATE TABLESPACE is going to create the tablespace directory, so I
don't see how two postmasters could both create the directory.

For example, if you say 
CREATE TABLESPACE tb IN '/var/tb1'

I assume you have to create:
/var/tb1/pgsql_tablespace

and then
/var/tb1/pgsql_tablespace/oid1/var/tb1/pgsql_tablespace/oid2

or something like that, and set the proper permissions on
pgsql_tablespace.  We will have write permission on the directory they
pass to us, but we might not have permissions to change the mode of the
directory they pass, so we have to create a subdirectory anyway, and
that is our interlock.

For example:# run as root$ chmod a+w .$ ls -ld .drwxrwxrwx  2 root  wheel  512 Mar  2 23:51 .# run as the postmaster$
mkdirnew$ ls -ld newdrwxr-xr-x  2 postgres  wheel  512 Mar  2 23:52 new$ chmod 700 new$ chmod 700 .chmod: .: Operation
notpermittedchmod: .: Operation not permitted
 

As you can see, I have permission to create the /new directory, but no
ability to set its mode, so we have to create a directory that matches
the permissions of /data:
drwx------  6 postgres  postgres  512 Mar  2 12:48 /u/pg/data/


We could require the admin to create a directory that we own instead of
just one that we have write permission in, but why bother when we can
use the new directory as an interlock from multiple postmasters anyway.

Right now we do require the directory used as /data be one where we can
create a /data subdirectory, so this seems similar.  We don't put the
data directly in the passed directory, but in /data under that.  In
fact, we could just call it /var/tb1/data instead of
/var/tb1/pgsql_tablespace.

--  Bruce Momjian                        |  http://candle.pha.pa.us pgman@candle.pha.pa.us               |  (610)
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