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Re: snapshot backup with WAL files

From: "Kevin Grittner" <Kevin(dot)Grittner(at)wicourts(dot)gov>
To: <isabella(dot)ghiurea(at)nrc-cnrc(dot)gc(dot)ca>, "pgsql-admin(at)postgresql(dot)org" <pgsql-admin(at)postgresql(dot)org>
Subject: Re: snapshot backup with WAL files
Date: 2011-07-29 15:53:40
Message-ID: 4E329134020000250003F8D9@gw.wicourts.gov (view raw or flat)
Thread:
Lists: pgsql-admin
Isabella Ghiurea <isabella(dot)ghiurea(at)nrc-cnrc(dot)gc(dot)ca> wrote:
 
> I would like to configure  our Pg db for OS  file system snapshot
> backup with Pg  WAL files, need to know if would be enough in pg
> db to call :
> |*SELECT* *pg_start_backup*('pitr_backup');
> and next take the snapshot  and after
> pg_stop the backup and close the WAL files . During this time  the
> full pg db read/writes will continue.
> Are any loss of data we should may be aware during this backup
> procedure?
> env :RHES-64 bits with LVM directories, PG 9.0, the db size >150GB
 
I'm not clear what you mean by "snapshot".  If you're talking about
something which takes an *atomic* snapshot of the entire PostgreSQL
data space, including WAL files, then just taking the snapshot is
enough without any other actions.  Assuming that you are using safe
settings for fsync, full_page_writes, and synchronous_commit, it
will recover all successfully committed transactions and roll back
all others.
 
If you're talking about tar, cpio, or some other software which is
not atomic, you should follow the instructions in the documentation
for one of the supported techniques completely and carefully:  For
PITR-style backups, that means ensuring that you have a working flow
of WAL files being captured *before* you run pg_start_backup().
 
http://www.postgresql.org/docs/9.0/interactive/backup.html
 
-Kevin

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