Re: Snapshot backups - Mailing list pgsql-general
Thanks Magnus,
Could you elaborate a bit more on this?
I've been having a look at do_pg_start_backup() and I can't really see anything apart from enabling full page writes and running a checkpoint to avoid getting a torn page. I could be missing something easily though, as I'm not familiar with the codebase.
do_pg_stop_backup() isn't really of consequence, as the backup is taken before this - so any restore is to a point in time before this as well.
I was under the impression a restore was (more or less) the same as a crash recovery, and logically it seems like PGDATA snapshot is equivalent to a crash/restart (disk at a discrete point in time).
I can understand if log replay might take longer, but I am struggling to see how it could result in an inconsistent state?
As I said I know this isn't best practice, but just want to understand how it works.
Cheers,
James Sewell
Solutions Architect
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On Thu, Jun 20, 2013 at 6:34 PM, Magnus Hagander <magnus@hagander.net> wrote:
On Thu, Jun 20, 2013 at 8:45 AM, James Sewell <james.sewell@lisasoft.com> wrote:Hey All,This is a message to confirm my thoughts / validate a possible approach.In a situation where PGDATA and {XLOG, ARCHIVELOG} are on different SAN/NAS volumes and a backup is to be initiated do pg_start_backup and pg_stop_backup need to be used?I am using snapshots of each volume for backup.My thinking is that they are not needed (although I realise it is good practice).As far as I can tell all they are doing is something like:pg_start_backup:- create backup label- trigger checkpointpg_stop_backup- remove backup label file- creates backup history file- trigger log switchThere is nothing in here that is *required* from a backup point of view. Am I missing anything?The backup functions also set internal state in the database, so you can't just replace it with doing those operations manually. You do need to call those functions.--
Magnus Hagander
Me: http://www.hagander.net/
Work: http://www.redpill-linpro.com/
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