Re: Parallelize stream replication process

On 2020/09/15 13:41, Bharath Rupireddy wrote:
> On Tue, Sep 15, 2020 at 9:27 AM Li Japin <japinli(at)hotmail(dot)com> wrote:
>>
>> For now, postgres use single process to send, receive and replay the WAL when we use stream replication,
>> is there any point to parallelize this process? If it does, how do we start?
>>
>> Any thoughts?

Probably this is another parallelism than what you're thinking,
but I was thinking to start up walwriter process in the standby server
and make it fsync the streamed WAL data. This means that we leave
a part of tasks of walreceiver process to walwriter. Walreceiver
performs WAL receive and write, and walwriter performs WAL flush,
in parallel. I'm just expecting that this change would improve
the replication performance, e.g., reduce the time to wait for
sync replication.

Without this change (i.e., originally), only walreceiver performs
WAL receive, write and flush. So wihle walreceiver is fsyncing WAL data,
it cannot receive newly-arrived WAL data. If WAL flush takes a time,
which means that the time to wait for sync replication in the primary
would be enlarged.

Regards,

--
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION