Re: Snapshot backups

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 checkpoint

pg_stop_backup

  - remove backup label file

  - creates backup history file

  - trigger log switch

There 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.

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 Magnus Hagander
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