Re: Problems with postgres online backup - restore

Hello Richard,

OK, understood it and looks to me a good system with whole block overwrite
and then the deltas.

Thnx.

Ciao,
Gerhard

--
http://www.wiesinger.com/

On Fri, 26 Jun 2009, Richard Huxton wrote:

> Gerhard Wiesinger wrote:
>> Hello,
>>
>> OK, what's then the difference doing a pg_start_backup() or just doing the
>> backup?
>
> pg_start_backup() forces a checkpoint (and logs a label for your backup too).
>
>> Isn't that a problem that the datablock are very inconsistent, even so
>> inconsistent that they are corrupt:
>>
>> E.g. A part of a datablock is written when e.g. the file is tarred. =>
>> Datablock on backup is corrupt => An then even the WAL can't be applied.
>>
>> Why does it work correctly? Or is there some design problem?
>
> It works because the WAL doesn't hold a list of row updates ("update row
> 12345 set field 4 = true") but block updates. Any update to a disk block is
> recorded - table or index. The WAL doesn't really know anything about
> tables, columns, primary keys etc - just disk blocks.
>
> One small optimisation is that the first time a block is touched after a
> checkpoint the value of the whole block is written to WAL and after that only
> updates.
>
> So - if you have a checkpointed system (all updates guaranteed written to
> disk) and a complete set of WAL files from that point on you can always
> recreate the writes to any point in time after that.
>
> http://www.postgresql.org/docs/8.3/static/continuous-archiving.html
>
> --
> Richard Huxton
> Archonet Ltd
>
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