Re: Parallel copy

On Wed, Oct 14, 2020 at 6:51 PM vignesh C <vignesh21(at)gmail(dot)com> wrote:
>
> On Fri, Oct 9, 2020 at 11:01 AM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> >
> > I am not able to properly parse the data but If understand the wal
> > data for non-parallel (1116 | 0 | 3587203) and parallel (1119
> > | 6 | 3624405) case doesn't seem to be the same. Is that
> > right? If so, why? Please ensure that no checkpoint happens for both
> > cases.
> >
>
> I have disabled checkpoint, the results with the checkpoint disabled
> are given below:
> | wal_records | wal_fpi | wal_bytes
> Sequential Copy | 1116 | 0 | 3587669
> Parallel Copy(1 worker) | 1116 | 0 | 3587669
> Parallel Copy(4 worker) | 1121 | 0 | 3587668
> I noticed that for 1 worker wal_records & wal_bytes are same as
> sequential copy, but with different worker count I had noticed that
> there is difference in wal_records & wal_bytes, I think the difference
> should be ok because with more than 1 worker the order of records
> processed will be different based on which worker picks which records
> to process from input file. In the case of sequential copy/1 worker
> the order in which the records will be processed is always in the same
> order hence wal_bytes are the same.
>

Are all records of the same size in your test? If so, then why the
order should matter? Also, even the number of wal_records has
increased but wal_bytes are not increased, rather it is one-byte less.
Can we identify what is going on here? I don't intend to say that it
is a problem but we should know the reason clearly.

--
With Regards,
Amit Kapila.