Re: Multiple WALs for PITR ?

Goulet, Dick wrote:
> As an Oracle DBA primarily, moving more towards PostGreSQL as time
> passes, the "alter system switch logfile" command is very often used at
> the end of a hot backup to force a logfile switch and consequently
> provide a full archive log set.  The command causes the database engine
> to cease using the current online redo log and have it archived to the
> archive log destination.  This is the only way to be sure that all
> required archive logs are present in a backup to reconstitute the
> database at another location.
>
>
> Dick Goulet
> Senior Oracle DBA
> Oracle Certified 8i DBA
> -----Original Message-----
> From: simon@2ndquadrant.com [mailto:simon@2ndquadrant.com]
> Sent: Wednesday, October 06, 2004 5:36 PM
> To: mkl@webde-ag.de
> Cc: pgsql-admin@postgresql.org
> Subject: Re: [ADMIN] Multiple WALs for PITR ?
>
>
> Michael Kleiser <mkl@webde-ag.de> wrote on 06.10.2004, 16:29:08:
>  > I wan`t to use Point in Time Recovery
>  > I allread read
>  > http://developer.postgresql.org/docs/postgres/backup-online.html#BACKU
>  > P-ARCHIVING-WAL
>  >
>
> Good!
>
>  > But I still wonder, if it is possible for PostgreSQL 8.0 to write
>  > multiple, redundant WAL-Files like Oracles with its redo-log-groups ?
>
> Not currently.
>
> That was something I considered, though in the end lacked both
> implementation time and justification for.
>
> If you can explain why you think that might be necessary, I'd be happy
> to consider it for v8.1.
>
> RAID-10 seems a good solution for me...

I heared about some RAIDs saying "yes your data is on disk"
to the oracle-database, but it wasn't so.
So Oracle can't do a complete restore.
The solution was to store the redo-log-files (=WALS) to local attached disk,
but of course redundant on multiple disks.
But other oracle-dba use RAIDs to store the redo-logs with no problems.

>
>  >
>  > Is threre an equivalent to Oracles "ALTER SYSTEM SWITCH LOGFILE" ?
>  >
>
> Not currently.
>
> Again, if you can say why we might want that, it can also be added -
> other than "they have it, so we want it too" :-)

In the online-backup-strategie described in
http://developer.postgresql.org/docs/postgres/backup-online.html#BACKUP-ARCHIVING-WAL
thre must be a special attention fir active WAL, if one want to backup it also.
With  "ALTER SYSTEM SWITCH LOGFILE" one could make it to a not active WAL.
This would make it easier to backup it, especially if one want to use
backups-programms like amanda.
If there where multiple redundant WALs, one could compare them as a consistency-check.
But this is only possible if nobody writes to the compared files.

>
> This is a likely addition for 8.1, since we will most likely want to
> archive xlog files every N minutes, whatever the transaction rate,
> allowing people to specify a maximum period of data loss - but this
> seemed like an enhancement rather than a definite feature for 8.0
>
Great!

Regards
   Michael Kleiser