Re: Snapshot backups

I understand what you are saying, and I understand how the backup_label
works - but I still don't understand why the pg_start and pg_stop commands
are REQUIRED when doing a snapshot backup to ensure data integrity.

Surely not using them and restoring a snapshot is the same as starting
after a crash, and will result in log replay to get to the latest possible
consistent state?

I thought PostgreSQL guaranteed a consistent state after a crash, am I
mistaken about this?

James

James Sewell
PostgreSQL Team Lead / Solutions Architect
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On Mon, Jul 29, 2013 at 4:54 PM, Amit Langote <amitlangote09(at)gmail(dot)com>wrote:

>
> On Mon, Jul 29, 2013 at 3:32 PM, James Sewell <james(dot)sewell(at)lisasoft(dot)com>wrote:
>
>> Hey all,
>>
>> I understand that I have already been given an answer here, but I am
>> still curious as to why this is the case (perhaps I should ask this on the
>> hackers list though, if so let me know).
>>
>> More importantly I'd like to understand why I would need to use the
>> start/stop backup commands to ensure a valid backup when using filesystem
>> snapshots (assuming I get the order correct)- worst case scenario wouldn't
>> it be the same as a crash and cause an automatic roll-forward?
>>
>>
>>
> pg_start_backup('backup_label') and pg_stop_backup(), if I understand it
> correctly, write to the 'backup_label' file the information necessary to
> recover "consistently" from that backup. For example, backup_label file
> contains the checkpoint location and its REDO location (identified as "START
> WAL LOCATION:" field in the backup_label file.) While you are reading the
> code, you can read the comment above the function read_backup_label()
> in src/backend/access/transam/xlog.c
>
>
> --
> Amit Langote
>

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