Re: Proposal for 9.1: WAL streaming from WAL buffers

On 06/11/2010 04:47 PM, Tom Lane wrote:
> Stefan Kaltenbrunner<stefan@kaltenbrunner.cc>  writes:
>> 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.
>
> Well, we're already not waiting for fsync, which is the slowest part.
> If there's a performance problem, it may be because FADVISE_DONTNEED
> disables kernel buffering so that we're forced to actually read the data
> back from disk before sending it on down the wire.

hmm ok - but assuming sync rep we would end up with something like the 
following(hypotetically assuming each operation takes 1 time unit):

originally:

write 1
sync 1
network 1
write 1
sync 1

total: 5

whereas in the new case we would basically have the write+sync compete 
with network+write+sync in parallel(total 3 units) and we would only 
have to wait for the slower of those two sets of operations instead of 
the total time of both or am I missing something.


Stefan