Обсуждение: On-line backup and point-in-time recovery (PITR)

Hi All,

I have a couple of questions regarding On-line backup and point-in-time
recovery (PITR) in the postgres documentation:

1.How is it that we can create a tar file of the PG Data directory
whilst postgres is running?

2.What happens with any DB inserts, updates, deletes etc that occur
between the SELECT pg_start_backup() and the end of the tar?
They would be already logged in the WAL files and already applied in the
tarball, do they then get duplicated during log replay in the recover
process?

Thanks In Advance


Enzo D'Addario...


pieNETWORK's Ltd...

Enzo D'addario wrote:
> Hi All,
>
> I have a couple of questions regarding On-line backup and point-in-time
> recovery (PITR) in the postgres documentation:
>
> 1.How is it that we can create a tar file of the PG Data directory
> whilst postgres is running?

Whe pg_xlog archived files contain information to recover any
inconsistency in the backup tar file.

> 2.What happens with any DB inserts, updates, deletes etc that occur
> between the SELECT pg_start_backup() and the end of the tar?
> They would be already logged in the WAL files and already applied in the
> tarball, do they then get duplicated during log replay in the recover
> process?

No, the recovery is just like crash recovery and everything comes back
clean, but you do have to recover to a time _after_ the end of the
backup.  If you want to recover to a time during the backup you have to
use the previous backup and xlog files.

--
  Bruce Momjian                        |  http://candle.pha.pa.us
  pgman@candle.pha.pa.us               |  (610) 359-1001
  +  If your life is a hard drive,     |  13 Roberts Road
  +  Christ can be your backup.        |  Newtown Square, Pennsylvania 19073

On Thu, Aug 18, 2005 at 11:01:27PM -0400, Bruce Momjian wrote:
> Enzo D'addario wrote:

> > 2.What happens with any DB inserts, updates, deletes etc that occur
> > between the SELECT pg_start_backup() and the end of the tar?
> > They would be already logged in the WAL files and already applied in the
> > tarball, do they then get duplicated during log replay in the recover
> > process?
>
> No, the recovery is just like crash recovery and everything comes back
> clean, but you do have to recover to a time _after_ the end of the
> backup.  If you want to recover to a time during the backup you have to
> use the previous backup and xlog files.

One important detail is that the system is able to detect whether a
change registered in WAL was already applied to the data files or not.
So if it's already applied, it doesn't need to be applied again.

--
Alvaro Herrera (<alvherre[a]alvh.no-ip.org>)
"La naturaleza, tan frágil, tan expuesta a la muerte... y tan viva"