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(at)hagander(dot)net>wrote:

>
> On Thu, Jun 20, 2013 at 8:45 AM, James Sewell <james(dot)sewell(at)lisasoft(dot)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.
>
> --
> Magnus Hagander
> Me: http://www.hagander.net/
> Work: http://www.redpill-linpro.com/
>

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