Re: Moving pg_xlog - Mailing list pgsql-performance

Keith Worthington wrote:

>On Wed, 01 Jun 2005 12:19:40 -0400, Tom Lane wrote
>
>
>>"Keith Worthington" <keithw@narrowpathinc.com> writes:
>>
>>
>>>I have been reading about increasing PostgreSQL performance
>>>by relocating the pg_xlog to a disk other than the one
>>>where the database resides.  I have the following pg_xlogs
>>>on my system.
>>>
>>>/raid02/databases/pg_xlog
>>>/raid02/rhdb_databases/pg_xlog
>>>/raid02/databases-8.0.0/pg_xlog
>>>/var/lib/pgsql/data/pg_xlog
>>>
>>>I have no idea why the forth entry is there.  It is in the PostgreSQL
>>>installation directory.
>>>
>>>
>>It's there because the RPM sets up a database under /var/lib/pgsql/data.
>>
>>
>>
>>>1) stop the postmaster
>>>2) rm -rf /var/lib/pgsql/data/pg_xlog
>>>3) mv /raid02/databases/pg_xlog /var/lib/pgsql/data/pg_xlog
>>>4) ln -s /var/lib/pgsql/data/pg_xlog /raid02/databases/pg_xlog
>>>5) start postmaster
>>>
>>>
>>Put the xlog anywhere BUT there!!!!!!!!!
>>
>>
>>
>>>If I can do that and place the pg_xlog in the installation
>>>directory will I create any installation issues the next
>>>time I upgrade PostgreSQL?
>>>
>>>
>>Oh, the installation will be just fine ... but your database will not
>>be after the upgrade wipes out your WAL.  Put the xlog under some
>>non-system-defined directory.
>>
>>            regards, tom lane
>>
>>---------------------------(end of broadcast)---------------------------
>>TIP 7: don't forget to increase your free space map settings
>>
>>
>
>Thanks Tom.  I am glad I asked before I leaped. 8-0
>
>Is there a convention that most people follow.  It would seem that anywhere in
>the installation directory is a bad idea.  From what I have read on other
>threads it does not want to be in the database directory since in most cases
>that would put it on the same disk as the database.
>
>
>
We tend to use somthing that associates the WAL with the appropriate
cluster, like

/var/lib/CLUSTER for the data
/var/lib/CLUSTER_WAL for WAL files.

>I am assuming due to lack of reaction that the symbolic link is not an issue.
> Is there a cleaner or more appropriate way of moving the pg_xlog.
>
>
>

A symbolic link is the standard way to do it.

>Finally, am I correct in assuming that as long as the postmaster is shut down
>moving the log is safe?
>
>

You are correct.  Moving the WAL files with the postmaster running would
be a very bad thing.

--
Brad Nicholson  416-673-4106
Database Administrator, Afilias Canada Corp.