Re: explanation of some configs

Matthew Wakeling wrote:
> On Sat, 7 Feb 2009, justin wrote:
>> In a big databases a checkpoint could get very large before time had
>> elapsed and if server cashed all that work would be rolled back.
>
> No. Once you commit a transaction, it is safe (unless you play with
> fsync or asynchronous commit). The size of the checkpoint is irrelevant.
>
> You see, Postgres writes the data twice. First it writes the data to
> the end of the WAL. WAL_buffers are used to buffer this. Then Postgres
> calls fsync on the WAL when you commit the transaction. This makes the
> transaction safe, and is usually fast because it will be sequential
> writes on a disc. Once fsync returns, Postgres starts the (lower
> priority) task of copying the data from the WAL into the data tables.
> All the un-copied data in the WAL needs to be held in memory, and that
> is what checkpoint_segments is for. When that gets full, then Postgres
> needs to stop writes until the copying has freed up the checkpoint
> segments again.
>
> Matthew
>
Well then we have conflicting instructions in places on
wiki.postgresql.org which links to this
http://www.varlena.com/GeneralBits/Tidbits/annotated_conf_e.html