Re: Streaming Replication patch for CommitFest 2009-09

Fujii Masao wrote:
> On Thu, Sep 24, 2009 at 7:57 PM, Heikki Linnakangas
> <heikki(dot)linnakangas(at)enterprisedb(dot)com> wrote:
>>>> - I know I said we should have just asynchronous replication at first,
>>>> but looking ahead, how would you do synchronous?
>>> As the previous patch did, I'm going to make walsender read the latest
>>> XLOG from wal_buffers, introduce the signaling between a backend
>>> and walsender, and keep a backend waiting until the specified XLOG
>>> has been written or fsynced in the standby.
>> Ok. I don't think walsender needs to access wal_buffers even then,
>> though. Once the backend has written the WAL, walsender can well read it
>> from disk (it will surely be in OS cache still).
>
> I think that walsender should not delay sending the XLOG until it has
> been written by the backend, for performance improvement. Otherwise,
> XLOG write and send are performed in serial, which would increase a
> response time. Should those be performed in parallel?

Well, sure, performance is good, but let's keep it simple for now. The
write() to disk should normally be absorbed by the OS cache and return
quickly, so it's not a big delay.

--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com