Re: Sync Rep: First Thoughts on Code

Hi,

Thanks for the helpful comments!

On Wed, Dec 17, 2008 at 8:50 PM, Simon Riggs <simon@2ndquadrant.com> wrote:
>
> On Wed, 2008-12-17 at 12:07 +0900, Fujii Masao wrote:
>
>> OK. I will extend synchronous_replication, make walsender send XLOG
>> with synchronization mode flag and make walreceiver perform according
>> to the flag.
>
> Sounds good.
>
>> > My perspective is that synchronous_replication specifies how long to
>> > wait. Current settings are "off" (don't wait) or "on" (meaning wait
>> > until point #3). So I think we should change this to a list of options
>> > to allow people to more carefully select how much waiting is required.
>>
>> In the latest patch, "off" keeps us waiting for replication in some
>> cases, e.g. forceSyncCommit = true. This is analogous to the way
>> synchronous_commit works. When "off" keeps us waiting for
>> replication, which option (#1-#6) should we choose? Should it be
>> user-configurable (though the parameter values are doubled)?
>> hardcode #3? "off" always should not keep us waiting for
>> replication?
>
> I would hard code #4, i.e. make it fsync, so that DDL changes are
> regarded as "high value transactions".
>
> A parameter sounds like overkill. We'd need to explain what
> forceSyncCommit does to users then, which is easier to avoid.

Agreed, I also think that hard code is better. But I'm nervous that "off"
keeps us waiting for replication in cases other than DDL, e.g. flush
buffer, truncate clog, checkpoint.. etc. synchronous_replication = off
is quite similar to synchronous_commit = off. If we would hard code #4,
the performance might degrade although it's asynchronous replication.
So, I'd like to hard code #3. What is your opinion?

Regards,

-- 
Fujii Masao
NIPPON TELEGRAPH AND TELEPHONE CORPORATION
NTT Open Source Software Center