Re: Proposal for 9.1: WAL streaming from WAL buffers

On 06/11/2010 04:31 PM, Tom Lane wrote:
> Fujii Masao<masao(dot)fujii(at)gmail(dot)com> writes:
>> In 9.0, walsender reads WAL always from the disk and sends it to the standby.
>> That is, we cannot send WAL until it has been written (and flushed) to the disk.
>
> I believe the above statement to be incorrect: walsender does *not* wait
> for an fsync to occur.
>
> I agree with the idea of trying to read from WAL buffers instead of the
> file system, but the main reason why is that the current behavior makes
> FADVISE_DONTNEED for WAL pretty dubious. It'd be a good idea to still
> (artificially) limit replication to not read ahead of the written-out
> data.
>
>> ... Since we can write and send WAL simultaneously, in synchronous
>> replication, a transaction commit has only to wait for either of them. So the
>> performance would significantly increase.
>
> That performance claim, frankly, is ludicrous. There is no way that
> round trip network delay plus write+fsync on the slave is faster than
> local write+fsync. Furthermore, I would say that you are thinking
> exactly backwards about the requirements for synchronous replication:
> what that would mean is that transaction commit waits for *both*,
> not whichever one finishes first.

hmm not sure that is what fujii tried to say - I think his point was
that in the original case we would have serialized all the operations
(first write+sync on the master, network afterwards and write+sync on
the slave) and now we could try parallelizing by sending the wal before
we have synced locally.

Stefan